CA1254610A - High frequency gas discharge lamp dimming ballast - Google Patents
High frequency gas discharge lamp dimming ballastInfo
- Publication number
- CA1254610A CA1254610A CA000488850A CA488850A CA1254610A CA 1254610 A CA1254610 A CA 1254610A CA 000488850 A CA000488850 A CA 000488850A CA 488850 A CA488850 A CA 488850A CA 1254610 A CA1254610 A CA 1254610A
- Authority
- CA
- Canada
- Prior art keywords
- terminals
- current
- circuit
- electronic ballast
- gas discharge
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
- H05B41/00—Circuit arrangements or apparatus for igniting or operating discharge lamps
- H05B41/14—Circuit arrangements
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
- H05B41/00—Circuit arrangements or apparatus for igniting or operating discharge lamps
- H05B41/14—Circuit arrangements
- H05B41/36—Controlling
- H05B41/38—Controlling the intensity of light
- H05B41/39—Controlling the intensity of light continuously
- H05B41/392—Controlling the intensity of light continuously using semiconductor devices, e.g. thyristor
- H05B41/3921—Controlling the intensity of light continuously using semiconductor devices, e.g. thyristor with possibility of light intensity variations
- H05B41/3924—Controlling the intensity of light continuously using semiconductor devices, e.g. thyristor with possibility of light intensity variations by phase control, e.g. using a triac
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/04—Modifications for accelerating switching
- H03K17/042—Modifications for accelerating switching by feedback from the output circuit to the control circuit
- H03K17/0424—Modifications for accelerating switching by feedback from the output circuit to the control circuit by the use of a transformer
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/51—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used
- H03K17/56—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used by the use, as active elements, of semiconductor devices
- H03K17/567—Circuits characterised by the use of more than one type of semiconductor device, e.g. BIMOS, composite devices such as IGBT
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
- H05B41/00—Circuit arrangements or apparatus for igniting or operating discharge lamps
- H05B41/14—Circuit arrangements
- H05B41/24—Circuit arrangements in which the lamp is fed by high frequency ac, or with separate oscillator frequency
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10S315/00—Electric lamp and discharge devices: systems
- Y10S315/04—Dimming circuit for fluorescent lamps
Abstract
HIGH FREQUENCY GAS DISCHARGE LAMP DIMMING BALLAST
ABSTRACT OF THE DISCLOSURE
A high frequency gas discharge lamp dimming bal-last with good line power factor is disclosed. The unit is controlled by a current sensing loop which senses control switch current rather than lamp current and employs simple circuitry with sufficiently rapid control response neces-sary to maintain high line power factor and stable lamp operating characteristics over a very wide range of light output. An isolated output transformer provides protection against most common miswire conditions and accurately regu-lates lamp energy over a wide range of input and lamp arc voltage conditions. A pulse width modulated control input provides an isolated control input for easier wiring and reduces the susceptibility of the control to noise inter-ference.
ABSTRACT OF THE DISCLOSURE
A high frequency gas discharge lamp dimming bal-last with good line power factor is disclosed. The unit is controlled by a current sensing loop which senses control switch current rather than lamp current and employs simple circuitry with sufficiently rapid control response neces-sary to maintain high line power factor and stable lamp operating characteristics over a very wide range of light output. An isolated output transformer provides protection against most common miswire conditions and accurately regu-lates lamp energy over a wide range of input and lamp arc voltage conditions. A pulse width modulated control input provides an isolated control input for easier wiring and reduces the susceptibility of the control to noise inter-ference.
Description
HIGH FREQUENCY GAS DISCHARGE LAMP DIMMING BALLAS~
BACKGROUND OF ~HE INVEN~ION
~ his invention relates to electronic ballasts for ~as discharge lamp loads and more particularly to a ballast which permits lamp dimming, has a high power factor and is inexpensive.
Electronic ballasting circuits for fluorescent and similar ~as discharge lamps are well known. Most electronic ballasts operate lamps at a fixed output level generally similar to the level which may be obtained from the lamp when operated from normal line frequency ballast circuits.
~he advantage in such application is obtained from the higher lamp efficiency at frequencies significantly higher than normal power frequencies of 50 or 60 Hz. High fre-quency operation can therefore provide equal light output with less power input to the lamp than is possible at line frequency. Alternatively, it is possible to obtain greater light output with the same electrical power input. If suitably low losses can be achieved in the high frequency ballast circuit, significant energy savings can result, particularly in the case of fluorescent lamps where the reduced system energy can result in system energy savings of greater than 20%.
BACKGROUND OF ~HE INVEN~ION
~ his invention relates to electronic ballasts for ~as discharge lamp loads and more particularly to a ballast which permits lamp dimming, has a high power factor and is inexpensive.
Electronic ballasting circuits for fluorescent and similar ~as discharge lamps are well known. Most electronic ballasts operate lamps at a fixed output level generally similar to the level which may be obtained from the lamp when operated from normal line frequency ballast circuits.
~he advantage in such application is obtained from the higher lamp efficiency at frequencies significantly higher than normal power frequencies of 50 or 60 Hz. High fre-quency operation can therefore provide equal light output with less power input to the lamp than is possible at line frequency. Alternatively, it is possible to obtain greater light output with the same electrical power input. If suitably low losses can be achieved in the high frequency ballast circuit, significant energy savings can result, particularly in the case of fluorescent lamps where the reduced system energy can result in system energy savings of greater than 20%.
- 2 -Electronic ballasts which also permit dimming or control of gas discharge lamp output, are also known. ~he ability to control lamp output, coupled with the inherently more efficient lamp operation possible at high frequencies, S can provide very significant energy savings when applied with suitable automatic controls. ~he savings possible with such dimmable high frequency gas discharge lamp bal-lasts can easily exceed 50% compared to uncontrolled line fre~uency ballasted systems.
Energy savings of this magnitude make such systems very desirable, yet they still have not gone into signifi-cant commercial use as compared to the number of lamps that continue to use uncontrolled line frequency ballasts. ~he reasons for this include: cost, susceptibility to damage due to miswire errors or accidental turn on of control de-vices, complex magnetic structures, poor power factor and limited control range.
High cost is a significant reason which has inhib-ited the introduction of electronic dimming ballasts. Line frequency ballasts have been manufactured for nearly 50 years and are highly optimized from a cost standpoint.
Further, fewer individual components are needed in line frequency ballasts compared to existing frequency ballasts.
~herefore, the high frequency electronic ballast has been inherently more costly than the line frequency ballast.
~ he large number of components and their suscep-tibility to damage reduces the reliabillty of electronic ballasts. Line frequency ballasts are better able to with-stand extraordinary stresses which may be applied, particu-larly if a miswire condition occurs during installation~While a line frequency ballast can withstand a shorted out-~ut for many minutes, electronic ballasts usually fail im-mediately under the same conditions. ~his not only redu-ces system reliability, but also makes it difficult to meet ;10 safety requirements, such as those specified by Underwri-ters Laboratories.
Many prior art ballasts use two or more power semi-conductor devices in their inverter circuits. Since these devices often dissipate significant amounts of power, they run at elevated temperatures, which substantially reduces their reliability. Also, these devices are relatively cost-ly, both to purchase and to mount in the unit. It is ob-viously desirable to minimize the number of such devices to obtain maximum reliability and lower costs. Often, the use of two such devices introduces the possibility of instant-aneous catastrophic failure modes if, for example, both devices were to conduct current simultaneously, instead of alternately, as is usually intended. Since it is quite ~ossible for elec$ronic noise, or other unanticipated events, to momentarily initiate simultaneous conduction, this represents a further reliability risk in circuits using multiple power devices.
Another common shortcoming of prior art electronic ballasts is the use of many and/or complex magnetic struc-tures. ~hese are needed to improve deficiencies such as poor power factor, or to reduce the total number of elec-tronic components required. While these results are desir-able, the introduction of complex magnetic components redu-ces the manufacturability of the unit. Although the com-ponents themselves are smaller at high frequencles, their small size requires greater preclsion in manufacturing.
~he high frequencies at which they are used often require the use of special materials and core shapes, all of which make the unit more difficult to manufacture and therefore more expensive.
An additional common shortcoming of prior art electronic ballasts is poor power factor. ~his is due to the use of a heavy filtered full wave bridge power supply ~s'~o using a large electrolytic filter capacitor to provide a d-c volta~e to the high frequency inverter found in all such ballasts. Heavily filtered supplies of this type draw a relatively large capacitive current from the power line, so that in many cases, a branch circuit which can supply 90 fluorescent lamps using line frequency ballasts, may be limited to less than 70 lamps using high frequency bal-lasts. ~he filter capacitors are also physically large compared to the other components and carry relatively high ripple currents. ~herefore they increase the size and cost of the unit. Also, because of their electrolytic construc-tion and significant power dissipation, they reduce the overall reliability of the system.
In the case of electronic dimming ballasts, addi-tional input wires are needed to provide control informa-tion to the ballast and this complicates installation.
~hese control leads are invariably referenced to the a-c line power input circuit. Many lighting systems are con-nected to three-phase a-c circuits, and it is common prac-tice to supply adjacent fixtures from dissimilar phases toreduce the perceived flicker level in the discharge lamp output. ~he control wires which are galvanically refer-enced to the a-c power input are commonly connected in par-allel to cause all ballasts to follow the same control vol-tage. All ballasts having such parallel control leads mustthen be operated from the same a-c voltage phase to avoid a phase-to-phase short circuit of the a-c source through the ballasts and their parallel control leads, which would cause the destruction of the ballasts. Avoiding this con-dition results in additional complications in installingwiring for such a lighting system, whereby minor wiring mistakes can result in widespread destruction of ballast units.
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Also, in a dimming ballast, it is common practice to use a variable voltage level on the control inputs to provide the ballast with information regarding the desired level of light output. Since the ballast is a high fre-~uency power supply, it can easily induce noise componentsinto the control leads which disturb the operation of the lamps. ~his is particularly true if the control voltage is near zero volts when the lamps are at their lowest output setting. Under these conditions, the lamps are most sensi-tive to disturbances and any noise picked up on the controlleads is most significant when compared to the relatively low control voltage at low light output levels.
Finally, most prior art dimming ballasts are lim-ited in their control range and vary the light output of their lamps over a ratio of only 10 to 1, or even less.
~hile this is adequate for energy management applications, it is insufficient for most kinesthetic or architectural dimming applications where a range of 100 to 1 is more com-monly required. ~he additional flexibility provided by such a wide range control capability significantly in-creases the functional and marketable value of the high frequency unit compared to the standard uncontrolled line frequency ballast. ~hus, since the high frequency ballast is inherently more costly, it is necessary to offer signif-icant performance advantages to justify the unavoidablyhigher unit cost.
Several prior art references demonstrate many of the above pOilltS. U.S. Patent 4,414,~91 dated November 8, 1~83 discloses a non-dimming electronic ballast with rel-atively few components, but using two power semiconductordevices, a lar~e filter capacitor and a relatively complex ma~netic structure. Further, the magnetic structure shown is built of relatively expensive material.
A fiimilar device is disclosed in U.S. Patent 4,3~2,087 dated July 5, 1983 which uses two power semicon-ductor devices, complex magnetics and a large filter capac-i tor. Dimming is obtained by voltage reduction or pulse width modulation of the power devices, but the dimming range cannot exceed a 10 to 1 ratio with standard fluores-cent lamps.
U.S. Patent 4,358,716 dated November 9, 1982 dis-closes an electronic ballast which can be dimmed by duty cycle control of high frequency pulse bursts for gas dis-charge lamps. ~his unit also includes two power devices lO and a large filter capacitor previously discussed.
~he circuit shown in U.S. Patent 4,392,086 dated July 5, 1983 improves power factor by removing the large capacitor to a small auxiliary supply which is used to keep the lamp arc struck during periods of 60 Hz line power 15 cycle when the voltage is too low for proper lamp opera-tion. However, two power devices and several magnetic structures are used and the dimming control range is rela-tively limited.
U.S. Patent 4,277,728 dated July 7, 1981 also uses 20 a relatively small d-c filter capacitor in combination with an active switching regulator to improve power factor.
~hree semiconductor power devices are used and a large num-ber of complex magnetic elements is required to implement the circuit. A control range of only 10 to 1 is availab]e.
In all of the preceding prior art patents which teach dimming, the dimming control leads would be galvan-ically referenced to the a-c power leads. ~his gives rise to the aforementioned wiring complications and the possi-bility of catastrophic miswire conditions.
U.S. Patents 3,619,716 and 3,731,142, assigned to Lutron Electronics Co., Inc. and U.S. Patent 3,265,930 dis-close a single power switching device in an electronic bal-last, In particular, patents 3,619,716 and 3,731,142 teach control of ~as discharge lamps by use of` a single power switching device and a pulse forming network connected across the lamp. By keeping the "on" time of the power device short compared to the lamp arc time constant, lamp current runaway is avoided and the pulse forming network stored energy is allowed to circulate through the arc when the power switch is in the "off" state. ~he use of a sin-gle switch eliminates the cost and reliability problems described above, and the stored energy in the pulse forming network allows wide range dimming control. ~hese princi-ples were applied commercially in 1974 in an electronicdimming ballast sold by Lutron Electronics Co., Inc. under the trademark "Hi-Lume".
~ he Hi-Lume ballast circuit uses a simple output inductor as a pulse forming network, and a current sensing resistor is placed in series with the lamp arc current.
~he control circuitry rectifies and filters the voltage dropped across the current sensing resistor, which is pro-portional to lamp arc current. ~his value is compared to the dimming control voltage input, and the duty cycle of the sin~le power switching device (a switching transistor) is adJusted until the lamp arc current is stable at the level commanded by the magnitude of the dimming control voltage. A large filter capacitor supplies smooth d-c vol-ta~e to the inverter portion of the circuit. ~he use of an accurate servo feedback loop which directly monitors the lamp arc current results in very stable dimming capability for the Hi-Lume circuit over a range in excess of tO0 to 1 li~ht output ratio, with no striations or flicker effects visible in the lamps.
~oday, 10 years after its introduction, the Hi-Lume system is still the best quality fluorescent dimming control commercially available. However, to achieve this level of performance, some compromises were required.
~hus, the large filter capacitor causes a relatively poor 1~5~6~
power ~actor and branch circuit lamp capacities must be de-rated. The control circuitry is relatively complex in or-der to stabilize the internal servo loop and results in high cost, even though a single power device and one rela-tively simple magnetic structure is required. Inherentreliability is very good due to the single power device structure, but miswiring the lamp leads can result in fail-ures since they are directly connected to the power switch and control circuitry. Also, the dimming control leads are referenced directly to the a-c line through the bridge rec-tifier. ~his means increased costs since isolation ampli-fier circuitry and current surge reduction circuitry must be used in conjunction with any significant numbers of these devices.
15 BRIEF DESCRIP~ION OF ~HE INVEN~ION
~ he present invention retains the simple basic structure and excellent dimming performance of the Hi-Lume product, while improving significantly such areas as con-trol circuit simplicity, power factor, miswire protection, control circuit isolation and noise immunity. ~hus, the circuit of the present invention still uses only one power switch and a single magnetic structure. However, the cur-rent sense resistor is placed in series with the power switch device, rather than the lamp. Also, the lamp is 25 galvanically isolated from the switching circuitry. ~hese precautions practically eliminate the possibility of cir-cuit failure due to miswiring of the lamp leads, since such errors can no lon~er disable the current sensing circuitry or cause direct shorts to ground from the a-c ]ine.
Also, the filter capacitor may now have a rela-tively small value, which serves to provide a high fre-~uency short at the power supply terminals, but does not si~nificantly affect the 60 Hz line power factor at full lamp output. When the inverter is delivering full lamp o power, the current drain serves to discharge the filter capacitor rapidly and the d-c bus voltage has a typical unfiltered full wave rectified voltage waveform. I'his pro-vides excellent line power factor at full lamp output. At 5 very low lamp output levels, the capacitor voltage is smoothed, and does not follow the line voltage waveform, so power factor is reduced. However, at low output levels, a low power factor has no significant disadvantages and the smoother d-c waveform serves to improve lamp stabi li ty and 10 dimming performance at the very low end of the range.
~ he control circuitry shown above also operates substantially differently from prior art devices. In U.S.
Patent 3,265,930 referred to above, the lamp arc current is monitored and directly controls the conduction state of the 15 series power switching device. ~he device is turned on and remains on until the arc current reaches a predetermined upper limit, and the power device is opened and left open until the arc current falls below a second limit. In the present invention, lamp arc current is not measured but 20 power switch current is used instead, to avoid the miswire problems already described. Further, in the present inven-tion the power switch remains on until an upper current limit is exceeded, but turns off and remains off until an internal osci llator signals it to turn back on to start 25 another cycle. ~his fixes the operating frequency of the circuit to a known value, instead of making it a function of lamp parameters, as in U.S. Patent 3,265,930. ~his makes the lamp light output less dependent on variations in line volta~e and lamp arc voltage, and prevents operation 30 in the audible frequency range, where an annoying "singing"
noise may be noticed by users of the area illuminated.
~ he circuit of U.S. Patent 3,265,930 also turns off the power switch at a fixed predetermined value of cur-rent. ~he present invention makes this limit a function of 35 the instantaneous line voltage. ~his type of control re-sults in a nearly sinusoidal line current waveshape which is in phase with the line voltage. ~herefore, the line power factor, as mentioned above, is very good. ~his rapid variation in control current level to follow the 120 Hz fullwave envelope is not feasible in the present Hi-Lume device, since filtered current feedback signals used there respond too slowly to follow a line frequency correction function such as this. ~he rapid response of the present invention current loop also provides an inherently stable control loop and greatly reduces the complexity of the con-trol circuitry. Finally, the rapid response prevents the power device from seeing current surges above its normal design levels, even under unusual operating conditions such as miswires.
~he circuit of the present invention also employs a dielectrically isolated coupling device between the dim-ming control leads and the control circuitry. Instead of a variable d-c volta~e input, the coupler is supplied with a variable duty cycle square wave input. As mentioned ear-lier, this allows easier wiring layout, since one no longer needs to be concerned with assuring that units with paral-lel control leads must always be fed from the same a-c phase line. Noise immunity is also improved.
~he invention also provides a novel method for causing multiple lamps operated from a single ballast cir-cuit to share equally the arc current supplied by the bal-last. Since gas discharge lamps have a negative resistance arc voltage/current characteristic, operating such lamps in parallel always results in one lamp conducting all the cur-rent and the other going completely out. ~here are twoknown approaches to solving this problem. The most common is to operate lamps in series. While this works well with several lamps at 60 Hz and with two lamps at high frequen-cies, the higher voltages required can cause problems in 1~5~ 3 in meeting U.L. Safety Standards. Furthermore, in a wide range dimming control, capacitive leakage currents become very significant at less than 1% of full rated output, and series operation of three or more lamps results in serious unbalance in lamp output at low light levels.
A second known approach to this problem is to op-erate the lamps in parallel, using small but expensive cur-rent balancing transformers to keep one lamp from stealing all the available arc current. In a two lamp unit only one balancer is required, but a four lamp pack requires three balancers and the e~pense and complexity are quite signifi-cant.
~ he use of an isolated output transformer in the present invention along with the control of the power switch current, enables a new method of forcing lamp cur-rent sharing in multiple lamp units. As was already men-tioned, two series lamps work well, but more than two cre-ates problems. ~he present invention uses a separate energy storage inductor and two output transformers with their primaries connected in series. The load on each transformer is one or more, preferably two, series connec-ted lamps. ~his achieves the effect of four series lamps with inherent current sharing properties, but the voltages at the transformer secondaries are no higher than in the two lamp case. In this scheme, there are three separate ma~netic elements. When a single output transformer is used, the storage inductor may easily he incorporated as part of the transformer structure itself, so that only one magnetic element is required. Similarly, the three e]ement structure described above may be integrated into a single magnetic element. Although such an element is relatively complex, the ability to share all the rest of the circuitry among four lamps instead of two is cost effective for the overall unit.
As pointed out above, an important feature of the invention is the use of a single power switch. Note, how-ever, that the single power switch can be implemented in a novel manner by a high voltage bipolar device and a low voltage power MOSFE~ device connected in cascode circuit relationship. ~he high voltage bipolar device provides the necessary high voltage characteristics for the switch, while the low voltage power MOSFE~ provides e~tremely high speed operation. In the preferred embodiment of the inven-tion, a NPN bipolar device is connected in series with thedrain and source electrodes of a low voltage power ~OSFE~.
Included in the connection between the bipolar transistor emitter and the drain of the power MOSFE~, is the primary winding of a current transformer. ~he secondary winding is then connected between the base of the bipolar device and the drain o the MOSFE~. ~he base of the bipolar device is also connected to a small zener diode which has its other terminal connected to the power MOSFET source terminal.
~his novel control and switching circuit, for what is, in effect, a single power switching device, provides high vol-tage, high speed turn off and is extremely easy to drive.
~he circuit arrangement also prevents second breakdown of the bi~olar transistor.
BKIEF DESCRIP~ION OF ~E DRAWINGS
Figure 1 is a circuit diagram of a prior art Hi-Lume dimming device.
Fi~ure 2 is a circuit diagram of the novel circuit of the present invention.
Figure 3 shows the output voltage on the fi]ter capacitor of Figure 2 when the inverter is delivering full lamp output.
Figure 4 shows the voltage on the filter capacitor of Figure 2 at reduced lamp power condition.
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Figure 5 is a circuit diagram of a novel isolated output transformer arrangement employing plural lamps which are forced to share current equally.
Figure 6 is a circuit diagram of an embodiment of the novel single power switch and electrical drive circui-try therefor.
DE~AILED DESCRIPl`ION OF ~HE DRAWINGS
Referring first to the circuit of Figure 1, there is shown therein a prior art Hi-Lume electronic ballast which is capable of a wide degree of regulation and which has been manufactured and sold by the assignee of the pre-sent invention for many years. ~he circuit is operated from an a-c source 10, which can be a conventional, commer-cially available source of voltage, i.e. 110, 220 or 277 volts at 50 or 60 Hz. ~he output of the source 10 is ap-plied to the conventional single phase full wave bridge connected rectifier 11, which delivers a rectified output across a relatively large filter capacitor 12, which is an electrolytic capacitor. ~he positive terminal of bridge 11 is also connected to the collector of the schematically illustrated bipolar switching transistor 13, which in turn is connected to the inductive ballast 14 and gas discharge lamp 15, which can be any desired gas discharge lamp, i.e.
a fluorescent lamp. ~aps 16 and 17 on the ballast 14 pro-vide a source of filament voltage for the filaments of thegas discharge lamp 15. A current sensing resistor 18 is connected in series with the lamp 15 and produces an output voltage which is applied to control circuitry 19. Control circui-try l9 can be of any desired type and receives an inputdimming control voltage on leads 20 and 21 which control, in combination with the current sensing resistor 18, the control signal applied to the base of transistor 13 and tends to regulate the output to keep the current through current sensing resistor 18 to a value set by the dimming voltage control signal at lines 20 and 21.
l~S~ o ~ he circuit o~ Figure 1 is a relatively simple structure having excellent dimming performance. ~he cir-cuit uses the simple output inductor 14 as a pulse forming network and the current sensing resistor 18 is in series with the lamp arc current. ~he control circuitry 19 is arranged to rectify and filter the voltage across resistor 1~, which is proportional to lamp arc current. ~his moni-tored voltage value is then compared to the dimming control voltage input and the duty cycle of the switching transis-tor 13 is then adjusted until the lamp arc current is sta-ble at the level commanded by the magnitude of the dimming volta~e control applied to leads 20 and 21. ~he large fil-ter capacitor 12 simply supplies smooth d-c voltage to the inverter portion of the circuit.
rhe circuit of Figure 1 employs an accurate servo feedback loop, which results in very stable dimming capa-bility. ~his, however, is at the expense of a large filter capacitor which reduces the power factor of the circuit.
Moreover, the control circuitry 19 is relatively complex in order to provide a stabilized internal servo loop and is relatively expensive. ~hus, the entire circuit is rela-tively expensive, even though only a single power switching device and a relatively simple magnetic structure is used.
Miswiring of the lamp leads for the lamp 15 can also result in failures since the leads are directly connected to the power switch 13 and control circuitry 19. ~lso, the dim-ming control leads are necessarily referenced directly to the a-c line through the bridge connected rectifier 11 (this connection is not shown). ~herefore, there are in-creased costs since isolation amplifier circuitry and cur-rent surge reduction circuitry must be used in conjunction with any significant numbers of the circuits of Figure 1.
Figure 2 is a circuit diagram of the present in-vention which retains the simple basic structure and excel-lent dimming performance of the prior art circuit of Figure 1, while significantly simplifying the control circuit andimproving power factor, miswire protection, control circuit isolation and noise immunity. In Figure 2, components similar to those of the circuit of Figure 1 have similar identifying numberals. In Figure 2, however~ the capacitor 12 is much smaller than that of Figure 1. For example, in the - 14a -6 ~ 0 circuit of Figure 1, capacitor 12 is about 300 microfarads, whereas in the circuit of Figure 2 it is about 3 microfar-ads. Depending on the size of the lamp load, prior art circuits have used capacitances of as low as 35 microfar-ads, for low lamp loads. However, the invention contem-plates the use of capacitance values less than about 30 microfarads, even for high lamp loads.
As further shown in Figure 2, the ballast trans-former consists of the two winding transformer 30 having a primary winding 31 which is dielectrically isolated from and magnetically coupled to the secondary winding 32. ~he secondary winding 32 contains the taps 16 and 17 for oper-ating the filament of the gas discharge lamp 15.
~he switching transistor 13 in Figure 2 is then connected in series with winding 31 and in series with the current sensing resistor 18. Significantly, the current sensing resistor 18 of Figure 2 is connected in series with the transistor 13, rather than in series with the actual lamp current of lamp 15 as in Figure 1.
A further significant change in the circuit of Figure 1 is the employment of a standard optoisolator 35 for coupling the input signal at the control input lines 20 and 21 to the control circuitry 19. ~he optoisolator 35 consists of any conventional internal arrangement, such as a light emitting diode which is optically coupled to, but dielectrically insulated from, a light sensitive transis-tor, which are the schematically lllustrated components in Figure 2.
~he arrangement shown for the lamp 15, which is operated from an isolating transformer 30, permits galvanic isolation of the lamp from the switching circuitry. ~his precaution practically eliminates the possibility of cir-cuit failure due to miswiring of the lamp leads since such errors can no longer disable the current sensing circuitry, or cause direct shorts to ground from the a-c line.
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~ he filter capacitor i2 is made much smaller in the circuit of Figure 2 and acts as a high frequency short at the power supply terminals of the a-c line 10. However, the capacitor does not significantly affect the line power factor at full lamp output. ~hus, when the inverter is delivering full lamp power, the current drain serves to discharge the capacitor 12 rapidly so that the d-c bus vol-ta~e has the typical unfiltered full wave rectified voltage waveform shown in Figure 3. ~hus, there is an excellent line power factor at full lamp output. When, however, the lamp output current is relatively low, the capacitor vol-tage across capacitor 12 does not follow the line voltage waveform but appears, for example, as shown in Figure 4.
~herefore, at low output levels, power factor is reduced.
rhis, however, has no significant disadvantage at the low current level, and the smoother d-c waveform improves lamp stability and dimming performance at the low end of the dimming range.
As pointed out above, it is also significant in the novel circuit of Figure 2 that lamp arc current is not measured by the current sensing resistor 18, as in the case of Figure 1, but instead the power switch current is mea-sured. ~his connection removes the current sensing resis-tor 18 from the lamp terminals and thus helps avoid miswire problems. Moreover, in the use of the circuit of the in-vention, the power switching device 13 remains on until an upper current limit is exceeded and then turns o~f and re-mains off until an lnternal oscillator (not shown) within the control circuitry 19 turns it back on to start a new cycle. ~his then fixes the operating frequency of the cir-cuit to a known value, rather than making it a function of lam~ parameter, as in the prior art. ~his in turn makes the lamp light output less dependent on variations in line voltage and lamp arc voltage, and prevents operation in an audio frequency range where an annoying "hum" may be pro-duced.
rhe circuit of Figure 2 also provides a control current loop ~vhich is inherently stable, provides rapid response and greatly reduces the complexity of the control circuitry. Such rapid response prevents the power switch-5 ing transistor 13 from exposure to current surges which areabove its normal design levels, even under unusual operat-ing conditions such as those caused by miswiring.
~ he use of the optoisolator 35 permits a more dir-ect connection from the dimming control leads to the con-lO trol circuitry l9. rhus, in the past a variable d-c vol-tage input has been used. rhe optoisolator 35 permits the use of a variable duty cycle square wave input to the con-trol leads 20 and 21 for more direct control of the system.
rhis, again, allows easier wiring, since multiple ballasts 15 wi th paralleled control leads no longer need be fed from the same a-c phase line. rhe optocoupler also provides improved noise immunity for the system.
~ he present invention also provides a novel arrangement for operating multiple lamps from a single bal-20 last circuit, and to force the multiple lamps to equallyshare the arc current provided by the ballast. rhis novel current dividing circuit is shown in Figure 5, ~herein com-ponents similar to those of Figure 2 have been given simi-lar identifying numerals. rhus, in Figure 5 the nodes 40 25 and 41 correspond to the nodes 40 and 41 in Figure 2. A
single ballast inductor 42 is connected in parallel with primary windings 43 and 44 o E transEormers 45 and 46, which have respective secondary windings 47 and 48. rhe turns ratio of wlndings 43 and 44 to secondary windings 47 and 48 30 rnay be 1:2. Each of windings 47 and 48 is connected to drive two series connected lamps 49-50 and 5l-52, respec-tively. Winding taps 53 and 54 are connected to one fila-ment of each of lamps 49 and 50, and a central winding tap 55 is connected to the other filaments of each of lamps 49 35 and 50 as shown in Figure 5. A similar arrangement is pro-vided for transformer 46 and lamps 51 and 52.
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It will be observed that any desired number of transformers 45 and 46 could have been used to produce any desired number of lamps in the particular bank described in Fi~ure 5. Each set of lamps consists of only two lamps in series, thus limiting the maximum voltage necessary in the ballast. rhis novel method of balancing the current be-tween a plurality of lamps is made possible through the use of of the isolated output transformer, and with the remote disposition of the current sensing resistor. Inductor 42 of Figure 5 acts as a separate energy storage inductor, while the output transformers 45 and 46 carry the same pri-mary current, thus insuring equal division of current be-tween the lamps 49 through 52.
~he scheme shown in Figure 5 requires three separ-ate magnetic elements. rhus, the ballast becomes rela-tively complex, but the ability to force sharing of current between four lamps, instead of two, is very cost effective for the overall unit.
Another feature of the present invention involves a novel imple~entation for the single power switching transistor 13 of the foregoing Figures along with a novel control mode therefor. More specifically, and as disclosed in Figure 6, the single, high voltage switching device 18 can be formed of a high voltage NPN transistor 70, which is connected in series with a low voltage power MOSFEr 71.
rhese components are connected in the well-known cascode circuit arrangement. rhe PNP transistor 70 may be a type MJE 13007A (Motorola) device and the MOSEEr 71 may be a BUZ
710 (Siemens) device. rhe bi~olar transistor 70 produces the necessary high voltage withstandability required for the switch 13, while the MOSFEr 71 provides the desirable high speed operation for the device.
A novel control circuit is also employed in Figure 6, which insures cooperation in the operation of transis-tors 70 and 71, while avoiding second breakdown of the bi-125~6~0 polar transistor 70. ~hus, a current transformer 72 isprovided, which has 24 turns on a ferrite core. A four turn tap section 73 is connected in series between the emitter of transistor 70 and the drain of transistor 71 as shown. ~he remainder of the winding is then connected to the base of transistor 70. A zener diode is then connected between the base of transistor 70 and the source of trans-istor 71 as shown. ~he control circuit 19 of Figure 2, for example, is then connected to the gate of MOSFE~ 71 in or-der to switch on and off the switching structure 13.
In operation, assuming that the switch 13 is inconduction and a signal is produced to turn off the switch 13, the MOSFE~ 71 shuts off extremely rapidly, thereby causing a sharp drop in the emitter current of the bipolar transistor 70. ~he bipolar collector current which still flows during turn off will pass through the zener diode and to the source lead of the MOSFE~ to fully turn off bipolar transistor 70. ~hus, high speed turn off can be obtained while the MOSFET is still well protected.
Note that during conduction of the switch 13, emitter current passes through the four turn section 73 of transformer 72. Consequently, transformer 72 acts as a current transformer and forces one-fifth of the full emit-ter current into the base thereby generating the base drive for transistor 70. ~he ferrite core will be designed so that it will not saturate during this operation.
As a result, the above described circuit of Figure 6 acts to provide high speed turn off and is very easy to drive, Moreover, second breakdown of the bipolar transistor 70 is prevented to make the bipolar transistor more rugged.
Although the present invention has been described in connection with preferred embodiments thereof, many var-iations and modifications will now become apparent to those skilled in the art. It is preferred therefore, that the ~resent invention be limited not by the specific disclosure herein, but only by the appended claims.
Energy savings of this magnitude make such systems very desirable, yet they still have not gone into signifi-cant commercial use as compared to the number of lamps that continue to use uncontrolled line frequency ballasts. ~he reasons for this include: cost, susceptibility to damage due to miswire errors or accidental turn on of control de-vices, complex magnetic structures, poor power factor and limited control range.
High cost is a significant reason which has inhib-ited the introduction of electronic dimming ballasts. Line frequency ballasts have been manufactured for nearly 50 years and are highly optimized from a cost standpoint.
Further, fewer individual components are needed in line frequency ballasts compared to existing frequency ballasts.
~herefore, the high frequency electronic ballast has been inherently more costly than the line frequency ballast.
~ he large number of components and their suscep-tibility to damage reduces the reliabillty of electronic ballasts. Line frequency ballasts are better able to with-stand extraordinary stresses which may be applied, particu-larly if a miswire condition occurs during installation~While a line frequency ballast can withstand a shorted out-~ut for many minutes, electronic ballasts usually fail im-mediately under the same conditions. ~his not only redu-ces system reliability, but also makes it difficult to meet ;10 safety requirements, such as those specified by Underwri-ters Laboratories.
Many prior art ballasts use two or more power semi-conductor devices in their inverter circuits. Since these devices often dissipate significant amounts of power, they run at elevated temperatures, which substantially reduces their reliability. Also, these devices are relatively cost-ly, both to purchase and to mount in the unit. It is ob-viously desirable to minimize the number of such devices to obtain maximum reliability and lower costs. Often, the use of two such devices introduces the possibility of instant-aneous catastrophic failure modes if, for example, both devices were to conduct current simultaneously, instead of alternately, as is usually intended. Since it is quite ~ossible for elec$ronic noise, or other unanticipated events, to momentarily initiate simultaneous conduction, this represents a further reliability risk in circuits using multiple power devices.
Another common shortcoming of prior art electronic ballasts is the use of many and/or complex magnetic struc-tures. ~hese are needed to improve deficiencies such as poor power factor, or to reduce the total number of elec-tronic components required. While these results are desir-able, the introduction of complex magnetic components redu-ces the manufacturability of the unit. Although the com-ponents themselves are smaller at high frequencles, their small size requires greater preclsion in manufacturing.
~he high frequencies at which they are used often require the use of special materials and core shapes, all of which make the unit more difficult to manufacture and therefore more expensive.
An additional common shortcoming of prior art electronic ballasts is poor power factor. ~his is due to the use of a heavy filtered full wave bridge power supply ~s'~o using a large electrolytic filter capacitor to provide a d-c volta~e to the high frequency inverter found in all such ballasts. Heavily filtered supplies of this type draw a relatively large capacitive current from the power line, so that in many cases, a branch circuit which can supply 90 fluorescent lamps using line frequency ballasts, may be limited to less than 70 lamps using high frequency bal-lasts. ~he filter capacitors are also physically large compared to the other components and carry relatively high ripple currents. ~herefore they increase the size and cost of the unit. Also, because of their electrolytic construc-tion and significant power dissipation, they reduce the overall reliability of the system.
In the case of electronic dimming ballasts, addi-tional input wires are needed to provide control informa-tion to the ballast and this complicates installation.
~hese control leads are invariably referenced to the a-c line power input circuit. Many lighting systems are con-nected to three-phase a-c circuits, and it is common prac-tice to supply adjacent fixtures from dissimilar phases toreduce the perceived flicker level in the discharge lamp output. ~he control wires which are galvanically refer-enced to the a-c power input are commonly connected in par-allel to cause all ballasts to follow the same control vol-tage. All ballasts having such parallel control leads mustthen be operated from the same a-c voltage phase to avoid a phase-to-phase short circuit of the a-c source through the ballasts and their parallel control leads, which would cause the destruction of the ballasts. Avoiding this con-dition results in additional complications in installingwiring for such a lighting system, whereby minor wiring mistakes can result in widespread destruction of ballast units.
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Also, in a dimming ballast, it is common practice to use a variable voltage level on the control inputs to provide the ballast with information regarding the desired level of light output. Since the ballast is a high fre-~uency power supply, it can easily induce noise componentsinto the control leads which disturb the operation of the lamps. ~his is particularly true if the control voltage is near zero volts when the lamps are at their lowest output setting. Under these conditions, the lamps are most sensi-tive to disturbances and any noise picked up on the controlleads is most significant when compared to the relatively low control voltage at low light output levels.
Finally, most prior art dimming ballasts are lim-ited in their control range and vary the light output of their lamps over a ratio of only 10 to 1, or even less.
~hile this is adequate for energy management applications, it is insufficient for most kinesthetic or architectural dimming applications where a range of 100 to 1 is more com-monly required. ~he additional flexibility provided by such a wide range control capability significantly in-creases the functional and marketable value of the high frequency unit compared to the standard uncontrolled line frequency ballast. ~hus, since the high frequency ballast is inherently more costly, it is necessary to offer signif-icant performance advantages to justify the unavoidablyhigher unit cost.
Several prior art references demonstrate many of the above pOilltS. U.S. Patent 4,414,~91 dated November 8, 1~83 discloses a non-dimming electronic ballast with rel-atively few components, but using two power semiconductordevices, a lar~e filter capacitor and a relatively complex ma~netic structure. Further, the magnetic structure shown is built of relatively expensive material.
A fiimilar device is disclosed in U.S. Patent 4,3~2,087 dated July 5, 1983 which uses two power semicon-ductor devices, complex magnetics and a large filter capac-i tor. Dimming is obtained by voltage reduction or pulse width modulation of the power devices, but the dimming range cannot exceed a 10 to 1 ratio with standard fluores-cent lamps.
U.S. Patent 4,358,716 dated November 9, 1982 dis-closes an electronic ballast which can be dimmed by duty cycle control of high frequency pulse bursts for gas dis-charge lamps. ~his unit also includes two power devices lO and a large filter capacitor previously discussed.
~he circuit shown in U.S. Patent 4,392,086 dated July 5, 1983 improves power factor by removing the large capacitor to a small auxiliary supply which is used to keep the lamp arc struck during periods of 60 Hz line power 15 cycle when the voltage is too low for proper lamp opera-tion. However, two power devices and several magnetic structures are used and the dimming control range is rela-tively limited.
U.S. Patent 4,277,728 dated July 7, 1981 also uses 20 a relatively small d-c filter capacitor in combination with an active switching regulator to improve power factor.
~hree semiconductor power devices are used and a large num-ber of complex magnetic elements is required to implement the circuit. A control range of only 10 to 1 is availab]e.
In all of the preceding prior art patents which teach dimming, the dimming control leads would be galvan-ically referenced to the a-c power leads. ~his gives rise to the aforementioned wiring complications and the possi-bility of catastrophic miswire conditions.
U.S. Patents 3,619,716 and 3,731,142, assigned to Lutron Electronics Co., Inc. and U.S. Patent 3,265,930 dis-close a single power switching device in an electronic bal-last, In particular, patents 3,619,716 and 3,731,142 teach control of ~as discharge lamps by use of` a single power switching device and a pulse forming network connected across the lamp. By keeping the "on" time of the power device short compared to the lamp arc time constant, lamp current runaway is avoided and the pulse forming network stored energy is allowed to circulate through the arc when the power switch is in the "off" state. ~he use of a sin-gle switch eliminates the cost and reliability problems described above, and the stored energy in the pulse forming network allows wide range dimming control. ~hese princi-ples were applied commercially in 1974 in an electronicdimming ballast sold by Lutron Electronics Co., Inc. under the trademark "Hi-Lume".
~ he Hi-Lume ballast circuit uses a simple output inductor as a pulse forming network, and a current sensing resistor is placed in series with the lamp arc current.
~he control circuitry rectifies and filters the voltage dropped across the current sensing resistor, which is pro-portional to lamp arc current. ~his value is compared to the dimming control voltage input, and the duty cycle of the sin~le power switching device (a switching transistor) is adJusted until the lamp arc current is stable at the level commanded by the magnitude of the dimming control voltage. A large filter capacitor supplies smooth d-c vol-ta~e to the inverter portion of the circuit. ~he use of an accurate servo feedback loop which directly monitors the lamp arc current results in very stable dimming capability for the Hi-Lume circuit over a range in excess of tO0 to 1 li~ht output ratio, with no striations or flicker effects visible in the lamps.
~oday, 10 years after its introduction, the Hi-Lume system is still the best quality fluorescent dimming control commercially available. However, to achieve this level of performance, some compromises were required.
~hus, the large filter capacitor causes a relatively poor 1~5~6~
power ~actor and branch circuit lamp capacities must be de-rated. The control circuitry is relatively complex in or-der to stabilize the internal servo loop and results in high cost, even though a single power device and one rela-tively simple magnetic structure is required. Inherentreliability is very good due to the single power device structure, but miswiring the lamp leads can result in fail-ures since they are directly connected to the power switch and control circuitry. Also, the dimming control leads are referenced directly to the a-c line through the bridge rec-tifier. ~his means increased costs since isolation ampli-fier circuitry and current surge reduction circuitry must be used in conjunction with any significant numbers of these devices.
15 BRIEF DESCRIP~ION OF ~HE INVEN~ION
~ he present invention retains the simple basic structure and excellent dimming performance of the Hi-Lume product, while improving significantly such areas as con-trol circuit simplicity, power factor, miswire protection, control circuit isolation and noise immunity. ~hus, the circuit of the present invention still uses only one power switch and a single magnetic structure. However, the cur-rent sense resistor is placed in series with the power switch device, rather than the lamp. Also, the lamp is 25 galvanically isolated from the switching circuitry. ~hese precautions practically eliminate the possibility of cir-cuit failure due to miswiring of the lamp leads, since such errors can no lon~er disable the current sensing circuitry or cause direct shorts to ground from the a-c ]ine.
Also, the filter capacitor may now have a rela-tively small value, which serves to provide a high fre-~uency short at the power supply terminals, but does not si~nificantly affect the 60 Hz line power factor at full lamp output. When the inverter is delivering full lamp o power, the current drain serves to discharge the filter capacitor rapidly and the d-c bus voltage has a typical unfiltered full wave rectified voltage waveform. I'his pro-vides excellent line power factor at full lamp output. At 5 very low lamp output levels, the capacitor voltage is smoothed, and does not follow the line voltage waveform, so power factor is reduced. However, at low output levels, a low power factor has no significant disadvantages and the smoother d-c waveform serves to improve lamp stabi li ty and 10 dimming performance at the very low end of the range.
~ he control circuitry shown above also operates substantially differently from prior art devices. In U.S.
Patent 3,265,930 referred to above, the lamp arc current is monitored and directly controls the conduction state of the 15 series power switching device. ~he device is turned on and remains on until the arc current reaches a predetermined upper limit, and the power device is opened and left open until the arc current falls below a second limit. In the present invention, lamp arc current is not measured but 20 power switch current is used instead, to avoid the miswire problems already described. Further, in the present inven-tion the power switch remains on until an upper current limit is exceeded, but turns off and remains off until an internal osci llator signals it to turn back on to start 25 another cycle. ~his fixes the operating frequency of the circuit to a known value, instead of making it a function of lamp parameters, as in U.S. Patent 3,265,930. ~his makes the lamp light output less dependent on variations in line volta~e and lamp arc voltage, and prevents operation 30 in the audible frequency range, where an annoying "singing"
noise may be noticed by users of the area illuminated.
~ he circuit of U.S. Patent 3,265,930 also turns off the power switch at a fixed predetermined value of cur-rent. ~he present invention makes this limit a function of 35 the instantaneous line voltage. ~his type of control re-sults in a nearly sinusoidal line current waveshape which is in phase with the line voltage. ~herefore, the line power factor, as mentioned above, is very good. ~his rapid variation in control current level to follow the 120 Hz fullwave envelope is not feasible in the present Hi-Lume device, since filtered current feedback signals used there respond too slowly to follow a line frequency correction function such as this. ~he rapid response of the present invention current loop also provides an inherently stable control loop and greatly reduces the complexity of the con-trol circuitry. Finally, the rapid response prevents the power device from seeing current surges above its normal design levels, even under unusual operating conditions such as miswires.
~he circuit of the present invention also employs a dielectrically isolated coupling device between the dim-ming control leads and the control circuitry. Instead of a variable d-c volta~e input, the coupler is supplied with a variable duty cycle square wave input. As mentioned ear-lier, this allows easier wiring layout, since one no longer needs to be concerned with assuring that units with paral-lel control leads must always be fed from the same a-c phase line. Noise immunity is also improved.
~he invention also provides a novel method for causing multiple lamps operated from a single ballast cir-cuit to share equally the arc current supplied by the bal-last. Since gas discharge lamps have a negative resistance arc voltage/current characteristic, operating such lamps in parallel always results in one lamp conducting all the cur-rent and the other going completely out. ~here are twoknown approaches to solving this problem. The most common is to operate lamps in series. While this works well with several lamps at 60 Hz and with two lamps at high frequen-cies, the higher voltages required can cause problems in 1~5~ 3 in meeting U.L. Safety Standards. Furthermore, in a wide range dimming control, capacitive leakage currents become very significant at less than 1% of full rated output, and series operation of three or more lamps results in serious unbalance in lamp output at low light levels.
A second known approach to this problem is to op-erate the lamps in parallel, using small but expensive cur-rent balancing transformers to keep one lamp from stealing all the available arc current. In a two lamp unit only one balancer is required, but a four lamp pack requires three balancers and the e~pense and complexity are quite signifi-cant.
~ he use of an isolated output transformer in the present invention along with the control of the power switch current, enables a new method of forcing lamp cur-rent sharing in multiple lamp units. As was already men-tioned, two series lamps work well, but more than two cre-ates problems. ~he present invention uses a separate energy storage inductor and two output transformers with their primaries connected in series. The load on each transformer is one or more, preferably two, series connec-ted lamps. ~his achieves the effect of four series lamps with inherent current sharing properties, but the voltages at the transformer secondaries are no higher than in the two lamp case. In this scheme, there are three separate ma~netic elements. When a single output transformer is used, the storage inductor may easily he incorporated as part of the transformer structure itself, so that only one magnetic element is required. Similarly, the three e]ement structure described above may be integrated into a single magnetic element. Although such an element is relatively complex, the ability to share all the rest of the circuitry among four lamps instead of two is cost effective for the overall unit.
As pointed out above, an important feature of the invention is the use of a single power switch. Note, how-ever, that the single power switch can be implemented in a novel manner by a high voltage bipolar device and a low voltage power MOSFE~ device connected in cascode circuit relationship. ~he high voltage bipolar device provides the necessary high voltage characteristics for the switch, while the low voltage power MOSFE~ provides e~tremely high speed operation. In the preferred embodiment of the inven-tion, a NPN bipolar device is connected in series with thedrain and source electrodes of a low voltage power ~OSFE~.
Included in the connection between the bipolar transistor emitter and the drain of the power MOSFE~, is the primary winding of a current transformer. ~he secondary winding is then connected between the base of the bipolar device and the drain o the MOSFE~. ~he base of the bipolar device is also connected to a small zener diode which has its other terminal connected to the power MOSFET source terminal.
~his novel control and switching circuit, for what is, in effect, a single power switching device, provides high vol-tage, high speed turn off and is extremely easy to drive.
~he circuit arrangement also prevents second breakdown of the bi~olar transistor.
BKIEF DESCRIP~ION OF ~E DRAWINGS
Figure 1 is a circuit diagram of a prior art Hi-Lume dimming device.
Fi~ure 2 is a circuit diagram of the novel circuit of the present invention.
Figure 3 shows the output voltage on the fi]ter capacitor of Figure 2 when the inverter is delivering full lamp output.
Figure 4 shows the voltage on the filter capacitor of Figure 2 at reduced lamp power condition.
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Figure 5 is a circuit diagram of a novel isolated output transformer arrangement employing plural lamps which are forced to share current equally.
Figure 6 is a circuit diagram of an embodiment of the novel single power switch and electrical drive circui-try therefor.
DE~AILED DESCRIPl`ION OF ~HE DRAWINGS
Referring first to the circuit of Figure 1, there is shown therein a prior art Hi-Lume electronic ballast which is capable of a wide degree of regulation and which has been manufactured and sold by the assignee of the pre-sent invention for many years. ~he circuit is operated from an a-c source 10, which can be a conventional, commer-cially available source of voltage, i.e. 110, 220 or 277 volts at 50 or 60 Hz. ~he output of the source 10 is ap-plied to the conventional single phase full wave bridge connected rectifier 11, which delivers a rectified output across a relatively large filter capacitor 12, which is an electrolytic capacitor. ~he positive terminal of bridge 11 is also connected to the collector of the schematically illustrated bipolar switching transistor 13, which in turn is connected to the inductive ballast 14 and gas discharge lamp 15, which can be any desired gas discharge lamp, i.e.
a fluorescent lamp. ~aps 16 and 17 on the ballast 14 pro-vide a source of filament voltage for the filaments of thegas discharge lamp 15. A current sensing resistor 18 is connected in series with the lamp 15 and produces an output voltage which is applied to control circuitry 19. Control circui-try l9 can be of any desired type and receives an inputdimming control voltage on leads 20 and 21 which control, in combination with the current sensing resistor 18, the control signal applied to the base of transistor 13 and tends to regulate the output to keep the current through current sensing resistor 18 to a value set by the dimming voltage control signal at lines 20 and 21.
l~S~ o ~ he circuit o~ Figure 1 is a relatively simple structure having excellent dimming performance. ~he cir-cuit uses the simple output inductor 14 as a pulse forming network and the current sensing resistor 18 is in series with the lamp arc current. ~he control circuitry 19 is arranged to rectify and filter the voltage across resistor 1~, which is proportional to lamp arc current. ~his moni-tored voltage value is then compared to the dimming control voltage input and the duty cycle of the switching transis-tor 13 is then adjusted until the lamp arc current is sta-ble at the level commanded by the magnitude of the dimming volta~e control applied to leads 20 and 21. ~he large fil-ter capacitor 12 simply supplies smooth d-c voltage to the inverter portion of the circuit.
rhe circuit of Figure 1 employs an accurate servo feedback loop, which results in very stable dimming capa-bility. ~his, however, is at the expense of a large filter capacitor which reduces the power factor of the circuit.
Moreover, the control circuitry 19 is relatively complex in order to provide a stabilized internal servo loop and is relatively expensive. ~hus, the entire circuit is rela-tively expensive, even though only a single power switching device and a relatively simple magnetic structure is used.
Miswiring of the lamp leads for the lamp 15 can also result in failures since the leads are directly connected to the power switch 13 and control circuitry 19. ~lso, the dim-ming control leads are necessarily referenced directly to the a-c line through the bridge connected rectifier 11 (this connection is not shown). ~herefore, there are in-creased costs since isolation amplifier circuitry and cur-rent surge reduction circuitry must be used in conjunction with any significant numbers of the circuits of Figure 1.
Figure 2 is a circuit diagram of the present in-vention which retains the simple basic structure and excel-lent dimming performance of the prior art circuit of Figure 1, while significantly simplifying the control circuit andimproving power factor, miswire protection, control circuit isolation and noise immunity. In Figure 2, components similar to those of the circuit of Figure 1 have similar identifying numberals. In Figure 2, however~ the capacitor 12 is much smaller than that of Figure 1. For example, in the - 14a -6 ~ 0 circuit of Figure 1, capacitor 12 is about 300 microfarads, whereas in the circuit of Figure 2 it is about 3 microfar-ads. Depending on the size of the lamp load, prior art circuits have used capacitances of as low as 35 microfar-ads, for low lamp loads. However, the invention contem-plates the use of capacitance values less than about 30 microfarads, even for high lamp loads.
As further shown in Figure 2, the ballast trans-former consists of the two winding transformer 30 having a primary winding 31 which is dielectrically isolated from and magnetically coupled to the secondary winding 32. ~he secondary winding 32 contains the taps 16 and 17 for oper-ating the filament of the gas discharge lamp 15.
~he switching transistor 13 in Figure 2 is then connected in series with winding 31 and in series with the current sensing resistor 18. Significantly, the current sensing resistor 18 of Figure 2 is connected in series with the transistor 13, rather than in series with the actual lamp current of lamp 15 as in Figure 1.
A further significant change in the circuit of Figure 1 is the employment of a standard optoisolator 35 for coupling the input signal at the control input lines 20 and 21 to the control circuitry 19. ~he optoisolator 35 consists of any conventional internal arrangement, such as a light emitting diode which is optically coupled to, but dielectrically insulated from, a light sensitive transis-tor, which are the schematically lllustrated components in Figure 2.
~he arrangement shown for the lamp 15, which is operated from an isolating transformer 30, permits galvanic isolation of the lamp from the switching circuitry. ~his precaution practically eliminates the possibility of cir-cuit failure due to miswiring of the lamp leads since such errors can no longer disable the current sensing circuitry, or cause direct shorts to ground from the a-c line.
1~2S~
~ he filter capacitor i2 is made much smaller in the circuit of Figure 2 and acts as a high frequency short at the power supply terminals of the a-c line 10. However, the capacitor does not significantly affect the line power factor at full lamp output. ~hus, when the inverter is delivering full lamp power, the current drain serves to discharge the capacitor 12 rapidly so that the d-c bus vol-ta~e has the typical unfiltered full wave rectified voltage waveform shown in Figure 3. ~hus, there is an excellent line power factor at full lamp output. When, however, the lamp output current is relatively low, the capacitor vol-tage across capacitor 12 does not follow the line voltage waveform but appears, for example, as shown in Figure 4.
~herefore, at low output levels, power factor is reduced.
rhis, however, has no significant disadvantage at the low current level, and the smoother d-c waveform improves lamp stability and dimming performance at the low end of the dimming range.
As pointed out above, it is also significant in the novel circuit of Figure 2 that lamp arc current is not measured by the current sensing resistor 18, as in the case of Figure 1, but instead the power switch current is mea-sured. ~his connection removes the current sensing resis-tor 18 from the lamp terminals and thus helps avoid miswire problems. Moreover, in the use of the circuit of the in-vention, the power switching device 13 remains on until an upper current limit is exceeded and then turns o~f and re-mains off until an lnternal oscillator (not shown) within the control circuitry 19 turns it back on to start a new cycle. ~his then fixes the operating frequency of the cir-cuit to a known value, rather than making it a function of lam~ parameter, as in the prior art. ~his in turn makes the lamp light output less dependent on variations in line voltage and lamp arc voltage, and prevents operation in an audio frequency range where an annoying "hum" may be pro-duced.
rhe circuit of Figure 2 also provides a control current loop ~vhich is inherently stable, provides rapid response and greatly reduces the complexity of the control circuitry. Such rapid response prevents the power switch-5 ing transistor 13 from exposure to current surges which areabove its normal design levels, even under unusual operat-ing conditions such as those caused by miswiring.
~ he use of the optoisolator 35 permits a more dir-ect connection from the dimming control leads to the con-lO trol circuitry l9. rhus, in the past a variable d-c vol-tage input has been used. rhe optoisolator 35 permits the use of a variable duty cycle square wave input to the con-trol leads 20 and 21 for more direct control of the system.
rhis, again, allows easier wiring, since multiple ballasts 15 wi th paralleled control leads no longer need be fed from the same a-c phase line. rhe optocoupler also provides improved noise immunity for the system.
~ he present invention also provides a novel arrangement for operating multiple lamps from a single bal-20 last circuit, and to force the multiple lamps to equallyshare the arc current provided by the ballast. rhis novel current dividing circuit is shown in Figure 5, ~herein com-ponents similar to those of Figure 2 have been given simi-lar identifying numerals. rhus, in Figure 5 the nodes 40 25 and 41 correspond to the nodes 40 and 41 in Figure 2. A
single ballast inductor 42 is connected in parallel with primary windings 43 and 44 o E transEormers 45 and 46, which have respective secondary windings 47 and 48. rhe turns ratio of wlndings 43 and 44 to secondary windings 47 and 48 30 rnay be 1:2. Each of windings 47 and 48 is connected to drive two series connected lamps 49-50 and 5l-52, respec-tively. Winding taps 53 and 54 are connected to one fila-ment of each of lamps 49 and 50, and a central winding tap 55 is connected to the other filaments of each of lamps 49 35 and 50 as shown in Figure 5. A similar arrangement is pro-vided for transformer 46 and lamps 51 and 52.
l~S~ O
It will be observed that any desired number of transformers 45 and 46 could have been used to produce any desired number of lamps in the particular bank described in Fi~ure 5. Each set of lamps consists of only two lamps in series, thus limiting the maximum voltage necessary in the ballast. rhis novel method of balancing the current be-tween a plurality of lamps is made possible through the use of of the isolated output transformer, and with the remote disposition of the current sensing resistor. Inductor 42 of Figure 5 acts as a separate energy storage inductor, while the output transformers 45 and 46 carry the same pri-mary current, thus insuring equal division of current be-tween the lamps 49 through 52.
~he scheme shown in Figure 5 requires three separ-ate magnetic elements. rhus, the ballast becomes rela-tively complex, but the ability to force sharing of current between four lamps, instead of two, is very cost effective for the overall unit.
Another feature of the present invention involves a novel imple~entation for the single power switching transistor 13 of the foregoing Figures along with a novel control mode therefor. More specifically, and as disclosed in Figure 6, the single, high voltage switching device 18 can be formed of a high voltage NPN transistor 70, which is connected in series with a low voltage power MOSFEr 71.
rhese components are connected in the well-known cascode circuit arrangement. rhe PNP transistor 70 may be a type MJE 13007A (Motorola) device and the MOSEEr 71 may be a BUZ
710 (Siemens) device. rhe bi~olar transistor 70 produces the necessary high voltage withstandability required for the switch 13, while the MOSFEr 71 provides the desirable high speed operation for the device.
A novel control circuit is also employed in Figure 6, which insures cooperation in the operation of transis-tors 70 and 71, while avoiding second breakdown of the bi-125~6~0 polar transistor 70. ~hus, a current transformer 72 isprovided, which has 24 turns on a ferrite core. A four turn tap section 73 is connected in series between the emitter of transistor 70 and the drain of transistor 71 as shown. ~he remainder of the winding is then connected to the base of transistor 70. A zener diode is then connected between the base of transistor 70 and the source of trans-istor 71 as shown. ~he control circuit 19 of Figure 2, for example, is then connected to the gate of MOSFE~ 71 in or-der to switch on and off the switching structure 13.
In operation, assuming that the switch 13 is inconduction and a signal is produced to turn off the switch 13, the MOSFE~ 71 shuts off extremely rapidly, thereby causing a sharp drop in the emitter current of the bipolar transistor 70. ~he bipolar collector current which still flows during turn off will pass through the zener diode and to the source lead of the MOSFE~ to fully turn off bipolar transistor 70. ~hus, high speed turn off can be obtained while the MOSFET is still well protected.
Note that during conduction of the switch 13, emitter current passes through the four turn section 73 of transformer 72. Consequently, transformer 72 acts as a current transformer and forces one-fifth of the full emit-ter current into the base thereby generating the base drive for transistor 70. ~he ferrite core will be designed so that it will not saturate during this operation.
As a result, the above described circuit of Figure 6 acts to provide high speed turn off and is very easy to drive, Moreover, second breakdown of the bipolar transistor 70 is prevented to make the bipolar transistor more rugged.
Although the present invention has been described in connection with preferred embodiments thereof, many var-iations and modifications will now become apparent to those skilled in the art. It is preferred therefore, that the ~resent invention be limited not by the specific disclosure herein, but only by the appended claims.
Claims (36)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An electronic ballast for a gas discharge lamp; said electronic ballast being dimmable over a large range; said ballast comprising: an a-c input circuit; a full wave rectifier having a-c terminals connected to said a-c input circuit and having d-c output ter-minals; a filter capacitor connected across said d-c output terminals; a transformer means having dielectrically isolated primary and secondary windings; a gas discharge lamp connected across the terminals of said secondary wind-ing; a single semiconductor switching means having a con-trol electrode and first and second power terminals; a cur-rent sensing means; a control circuit connected to said control electrode and operable to turn said switching means on and off at a controlled duty cycle rate; a dimming level set circuit connected to said control circuit to set said duty cycle to a value related to a given degree of dimming of said lamp; said current sensing means being connected to said control circuit and operable to adjust said duty cycle to a value which maintains the current through said current sensing means at a value related to that called for by said dimming level set circuit; characterized in that said d-c output terminals, said primary winding, said single semiconductor switch means and said current sensing means are connected in closed series relation.
2. The electornic ballast of claim 1 which is further characterized in that said filter capacitor has a value less than about 30 microfarads.
3. The electronic ballast of claim 1 which is further characterized in that said secondary winding has heater tap sections connected to the heaters of the fila-ments of said gas discharge lamp.
4. The electronic ballast of claim 1, which is further characterized in including a second transformer having dielec-trically isolated primary and secondary windings; said primary windings of said transformer and of said second transformer con-nected in series in said closed series circuit; an inductor means connected in parallel with said series connected primary wind-ings; and a second gas discharge lamp connected across the ter-minals of said secondary winding.
5. The electronic ballast of claim 4, which is further characterized in that at least two series connected gas discharge lamps are connected across the terminals of each of said secon-dary windings.
6. The electronic ballast of claim 1, 2 or 3, which is further characterized in that an optoisolator component couples said dimming level set circuit to said control circuit.
7. The electronic ballast of claim 1, 2 or 3, which is further characterized in that said single semi-conductor swit-ching means is a bipolar transistor.
8. The electronic ballast of claim 1, which is further characterized in that said single semi-conductor switching means comprises a high voltage bipolar transistor and a low voltage power MOSFET connected in cascade relation.
9. The electronic ballast of claim 8, which is further characterized in including current transformer means having a tap terminal means and end terminals; said end terminals connected to the base of said bipolar transistor and the drain of said MOSFET, respectively;
said tap terminal connected to the emitter of said bipolar transistor; the source of said MOSFET and the collector of said bipolar transistor connected in said series circuit;
and a zener diode connected between said base of said bipolar transistor and said source of said MOSFET.
said tap terminal connected to the emitter of said bipolar transistor; the source of said MOSFET and the collector of said bipolar transistor connected in said series circuit;
and a zener diode connected between said base of said bipolar transistor and said source of said MOSFET.
10. An electronic ballast for a gas discharge lamp;
said electronic ballast being dimmable over a large range; said ballast comprising; an a-c input circuit, a rectifier having a-c terminals connected to said a-c input circuit and having d-c out-put terminals; a filter capacitor connected across said d-c out-put terminals, said filter capacitor having a value less than about 30 microfarads; a transformer means having primary and sec-ondary windings; a gas discharge lamp connected across the termi-nals of said secondary winding; a single semi-conductor switching means having a control electrode and first and second power ter-minals; a current sensing means; a control circuit connected to said control electrode and operable to turn said switching means on and off at a controlled duty cycle rate; a dimming level set circuit connected to said control circuit to set said duty cycle to a value related to a given degree of dimming of said lamp;
said current sensing means being connected to said control cir-cuit and operable to adjust said duty cycle to a value which maintains the current through said current sensing means at a value related to that called for by said dimming level set cir-cuit; said d-c output terminals, said primary winding, said single semi-conductor switch means and said current sensing means being connected in closed series relation.
said electronic ballast being dimmable over a large range; said ballast comprising; an a-c input circuit, a rectifier having a-c terminals connected to said a-c input circuit and having d-c out-put terminals; a filter capacitor connected across said d-c out-put terminals, said filter capacitor having a value less than about 30 microfarads; a transformer means having primary and sec-ondary windings; a gas discharge lamp connected across the termi-nals of said secondary winding; a single semi-conductor switching means having a control electrode and first and second power ter-minals; a current sensing means; a control circuit connected to said control electrode and operable to turn said switching means on and off at a controlled duty cycle rate; a dimming level set circuit connected to said control circuit to set said duty cycle to a value related to a given degree of dimming of said lamp;
said current sensing means being connected to said control cir-cuit and operable to adjust said duty cycle to a value which maintains the current through said current sensing means at a value related to that called for by said dimming level set cir-cuit; said d-c output terminals, said primary winding, said single semi-conductor switch means and said current sensing means being connected in closed series relation.
11. The electronic ballast of claim 10, wherein said secondary winding has heater tap sections connected to the heaters of the filaments of said gas discharge lamp.
12. The electronic ballast of claim 10, which includes a second transformer having primary and secondary windings; said primary windings of said transformer and of said second trans-former connected in series in said closed series circuit; and inductor means connected in parallel with said series connected primary windings; and a second gas discharge lamp connected across the terminals of said secondary winding of said second transformer.
13. The electronic ballast of claim 10, wherein at least two series connected gas discharge lamps are connected across the terminals of each of said secondary windings.
14. The electronic ballast of claim 10, which further includes a dielectrically isolated coupling device for coupling said dimming level set circuit to said control circuit.
15. The electronic ballast of claim lo, wherein said single semi-conductor switching means comprises a bipolar tran-sistor.
16. An electronic ballast for a gas discharge lamp;
said electronic ballast being dimmable over a large range; said ballast comprising: an a-c input circuit; a bridge connected rec-tifier having a-c terminals connected to said a-c input circuit and having d-c output terminals; a filter capacitor connected across said d-c output terminals, said filter capacitor having a value less than about 30 microfarads; an energy storage element having first and second terminals; a gas discharge lamp induc-tively coupled to said energy storage element, a single semi-con-ductor switching means which comprises a high voltage bipolar transistor and low voltage power FET connected in cascode rela-tion; and having a control electrode and first and second power terminals; a current sensing means; a control circuit connected to said control electrode and operable to turn said switching means on and off at a controlled duty cycle rate; a dimming level set circuit connected to said control circuit to set said duty cycle to a value related to a given degree of dimming of said lamp; said current sensing means being connected to said control circuit and operable to adjust said duty cycle to a value which maintains the current through said current sensing means at a value related to that called for by said dimming level set cir-cuit; said d-c output terminals, said first and second terminals of said energy storage element, said single semi-conductor switch means and said current sensing means being connected in close series relation.
said electronic ballast being dimmable over a large range; said ballast comprising: an a-c input circuit; a bridge connected rec-tifier having a-c terminals connected to said a-c input circuit and having d-c output terminals; a filter capacitor connected across said d-c output terminals, said filter capacitor having a value less than about 30 microfarads; an energy storage element having first and second terminals; a gas discharge lamp induc-tively coupled to said energy storage element, a single semi-con-ductor switching means which comprises a high voltage bipolar transistor and low voltage power FET connected in cascode rela-tion; and having a control electrode and first and second power terminals; a current sensing means; a control circuit connected to said control electrode and operable to turn said switching means on and off at a controlled duty cycle rate; a dimming level set circuit connected to said control circuit to set said duty cycle to a value related to a given degree of dimming of said lamp; said current sensing means being connected to said control circuit and operable to adjust said duty cycle to a value which maintains the current through said current sensing means at a value related to that called for by said dimming level set cir-cuit; said d-c output terminals, said first and second terminals of said energy storage element, said single semi-conductor switch means and said current sensing means being connected in close series relation.
17. The electronic ballast of claim 16, which further includes a dielectrically isolated coupling device for coupling said dimming level set circuit to said control circuit.
18. The electronic ballast of claim 16, wherein said single semi-conductor switching means comprises a transistor.
19. An electronic ballast for a gas discharge lamp;
said ballast comprising: an a-c input circuit; a full wave bridge connected rectifier having a-c terminals connected to said a-c input circuit and having d-c output terminals; a filter capacitor connected across said d-c output terminals, said filter capacitor having a value less than about 30 microfarads; a transformer means having primary and secondary windings; a gas discharge lamp connected across the terminals of said secondary winding; a single semi-conductor switching means having a control electrode and first and second power terminals; a current sensing means; a control circuit connected to said control electrode and operable to turn said switching means on and off at a controlled duty cycle rate; a current level set circuit connected to said control circuit to set said duty cycle to a value related to a given value of current in said lamp; said current sensing means being connected to said control circuit and operable to adjust said duty cycle to a value which maintains the current through said current sensing means at a value related to that called for by said current level set circuit; said d-c output terminals, said primary winding, said single semi-conductor switch means and said current sensing means being connected in closed series relation.
said ballast comprising: an a-c input circuit; a full wave bridge connected rectifier having a-c terminals connected to said a-c input circuit and having d-c output terminals; a filter capacitor connected across said d-c output terminals, said filter capacitor having a value less than about 30 microfarads; a transformer means having primary and secondary windings; a gas discharge lamp connected across the terminals of said secondary winding; a single semi-conductor switching means having a control electrode and first and second power terminals; a current sensing means; a control circuit connected to said control electrode and operable to turn said switching means on and off at a controlled duty cycle rate; a current level set circuit connected to said control circuit to set said duty cycle to a value related to a given value of current in said lamp; said current sensing means being connected to said control circuit and operable to adjust said duty cycle to a value which maintains the current through said current sensing means at a value related to that called for by said current level set circuit; said d-c output terminals, said primary winding, said single semi-conductor switch means and said current sensing means being connected in closed series relation.
20. The electronic ballast of claim 19, wherein said secondary winding has heater tap sections connected to the heaters of the filaments of said gas discharge lamp.
21. The electronic ballast of claim 19, which includes a second transformer having primary and secondary windings; said primary winding of said transformer and of said second trans-former connected in series in said closed series circuit; and inductor means connected in parallel with said series connected primary windings; and a second gas discharge lamp connected across the terminals of said secondary winding of said secondary transformer.
22. The electronic ballast of claim 21, wherein at least two series connected gas discharge lamps are connected across the terminals of each of said secondary windings.
23. The electronic ballast of claim 19, which further includes a dielectrically isolated coupling device for coupling said current level set circuit to said control circuit.
24. The electronic ballast of claim 19, wherein said single semi-conductor switching means comprises a transistor.
25. An electronic ballast for a gas discharge lamp;
said ballast comprising: a bridge connected rectifier having a-c terminals connected to said a-c input circuit and having d-c out-put terminals; a filter capacitor connected across said d-c out-put terminals, said filter capacitor having a value less than about 30 microfarads; an energy storage element having first and second terminals; a gas discharge lamp inductively coupled to said energy storage element; a single semi-conductor switching means having a control electrode and first and second power ter-minals; a current sensing means; a control circuit connected to said control electrode and operable to turn said switching means on and off at a controlled duty cycle rate; a current level set circuit connected to said control circuit to set said duty cycle to a value related to a given value of current in said lamp; said current sensing means being connected to said control circuit and operable to adjust said duty cycle to a value which maintains the current through said current sensing means at a value related to that called for by said current level set circuit; said d-c out-put terminals, said first and second terminals of said energy storage element, said single semi-conductor switch means and said current sensing means being connected in closed series relation.
said ballast comprising: a bridge connected rectifier having a-c terminals connected to said a-c input circuit and having d-c out-put terminals; a filter capacitor connected across said d-c out-put terminals, said filter capacitor having a value less than about 30 microfarads; an energy storage element having first and second terminals; a gas discharge lamp inductively coupled to said energy storage element; a single semi-conductor switching means having a control electrode and first and second power ter-minals; a current sensing means; a control circuit connected to said control electrode and operable to turn said switching means on and off at a controlled duty cycle rate; a current level set circuit connected to said control circuit to set said duty cycle to a value related to a given value of current in said lamp; said current sensing means being connected to said control circuit and operable to adjust said duty cycle to a value which maintains the current through said current sensing means at a value related to that called for by said current level set circuit; said d-c out-put terminals, said first and second terminals of said energy storage element, said single semi-conductor switch means and said current sensing means being connected in closed series relation.
26. The electronic ballast of claim 25, which further includes dielectrically isolated coupling device for coupling said current level set circuit to said control circuit.
27. The electronic ballast of claim 25, wherein said single semi-conductor switching means comprises a transistor.
28. The electronic ballast of claim 19, comprising at least two series connected gas discharge lamps connected across the terminals of each of said secondary windings.
29. The electronic ballast of claim 14, wherein said dielectrically isolated coupling device comprises an optoisolator component.
30. The electronic ballast of claim 17, wherein said dielectrically isolated coupling device comprises an optoisolator component.
31. The electronic ballast of claim 23, wherein said dielectrically isolated coupling device comprises an optoisolator component.
32. The electronic ballast of claim 26, wherein said dielectrically isolated coupling device comprises an optoisolator component.
33. An electronic ballast for a gas discharge lamp;
said electronic ballast being dimmable over a large range; said ballast comprising: an a-c input circuit; a rectifier having a-c terminals connected to said a-c input circuit and having d-c out-put terminals; a filter capacitor connected across said d-c out-put terminals; a transformer means having primary and secondary windings; a gas discharge lamp connected across the terminals of said secondary winding; a single semi-conductor switching means which comprises a high voltage bipolar transistor and a low voltage power MOSFET connected in cascode relation, said swit-ching means having a control electrode and first and second power terminals; a current sensing means; a control circuit connected to said control electrode and operable to turn said switching means on and off at a controlled duty cycle rate; a dimming level set circuit connected to said control circuit to set said duty cycle to a value related to a given degree of dimming of said lamp; said current sensing means being connected to said control circuit and operable to adjust said duty cycle to a value which maintains the current through said current sensing means at a value related to that called for by said dimming level set circuit; said d-c output terminals, said primary winding, said single semi-conductor switch means and said current sensing means being connected in closed series relation; said ballast further comprising current transformer means having a tap terminal means and end terminals; said end terminals connected to the base of said bipolar transistor and the drain of said MOSFET, respec-tively; said tap terminal connected to one of the emitter and collector of said bipolar transistor; the source of said MOSFET
and one of the emitter and collector of said bipolar transistor which is not connected to said tap terminal being connected in said series circuit; and a zener diode connected between said base of said bipolar transistor and said source of said MOSFET.
said electronic ballast being dimmable over a large range; said ballast comprising: an a-c input circuit; a rectifier having a-c terminals connected to said a-c input circuit and having d-c out-put terminals; a filter capacitor connected across said d-c out-put terminals; a transformer means having primary and secondary windings; a gas discharge lamp connected across the terminals of said secondary winding; a single semi-conductor switching means which comprises a high voltage bipolar transistor and a low voltage power MOSFET connected in cascode relation, said swit-ching means having a control electrode and first and second power terminals; a current sensing means; a control circuit connected to said control electrode and operable to turn said switching means on and off at a controlled duty cycle rate; a dimming level set circuit connected to said control circuit to set said duty cycle to a value related to a given degree of dimming of said lamp; said current sensing means being connected to said control circuit and operable to adjust said duty cycle to a value which maintains the current through said current sensing means at a value related to that called for by said dimming level set circuit; said d-c output terminals, said primary winding, said single semi-conductor switch means and said current sensing means being connected in closed series relation; said ballast further comprising current transformer means having a tap terminal means and end terminals; said end terminals connected to the base of said bipolar transistor and the drain of said MOSFET, respec-tively; said tap terminal connected to one of the emitter and collector of said bipolar transistor; the source of said MOSFET
and one of the emitter and collector of said bipolar transistor which is not connected to said tap terminal being connected in said series circuit; and a zener diode connected between said base of said bipolar transistor and said source of said MOSFET.
34. An electronic ballast for a gas discharge lamp;
said electronic ballast being dimmable over a large range; said ballast comprising an a-c input circuit; a bridge connected rec-tifier having a-c terminals connected to said a-c input circuit and having d-c output terminals; a filter capacitor connected across said d-c output terminals; an energy storage element hav-ing first and second terminals; a gas discharge lamp inductively coupled to said energy storage element; a single semi-conductor switching means which comprises a high voltage bipolar transistor and a low voltage power MOSFET connected in cascode relation, said switching means having a control electrode and first and second power terminals; a current sensing means; a control cir-cuit connected to said control electrode and operable to turn said switching means on and off at a controlled duty cycle rate;
a dimming level set circuit connected to said control circuit to set said duty cycle to a value related to a given degree of dim-ming of said lamp; said current sensing means being connected to said control circuit and operable to adjust said duty cycle to a value which maintains the current through said current sensing means at a value related to that called for by said dimming level set circuit; said d-c output terminals, said first and second terminals of said energy storage element, said single semi-con-ductor switch means and said current sensing means being con-nected in closed series relation, said ballast further including current transformer means having a tap terminal means and end terminals; said end terminals connected to the base of said bipo-lar transistor and the drain of said MOSFET, respectively; said tap terminal connected to the emitter of said bipolar transistor;
the source of said MOSFET and the collector of said bipolar tran-sistor connected in said series circuit; and a zener diode con-nected between said base of said bipolar transistor and said source of said MOSFET.
said electronic ballast being dimmable over a large range; said ballast comprising an a-c input circuit; a bridge connected rec-tifier having a-c terminals connected to said a-c input circuit and having d-c output terminals; a filter capacitor connected across said d-c output terminals; an energy storage element hav-ing first and second terminals; a gas discharge lamp inductively coupled to said energy storage element; a single semi-conductor switching means which comprises a high voltage bipolar transistor and a low voltage power MOSFET connected in cascode relation, said switching means having a control electrode and first and second power terminals; a current sensing means; a control cir-cuit connected to said control electrode and operable to turn said switching means on and off at a controlled duty cycle rate;
a dimming level set circuit connected to said control circuit to set said duty cycle to a value related to a given degree of dim-ming of said lamp; said current sensing means being connected to said control circuit and operable to adjust said duty cycle to a value which maintains the current through said current sensing means at a value related to that called for by said dimming level set circuit; said d-c output terminals, said first and second terminals of said energy storage element, said single semi-con-ductor switch means and said current sensing means being con-nected in closed series relation, said ballast further including current transformer means having a tap terminal means and end terminals; said end terminals connected to the base of said bipo-lar transistor and the drain of said MOSFET, respectively; said tap terminal connected to the emitter of said bipolar transistor;
the source of said MOSFET and the collector of said bipolar tran-sistor connected in said series circuit; and a zener diode con-nected between said base of said bipolar transistor and said source of said MOSFET.
35. An electronic ballast for a gas discharge lamp;
said ballast comprising: an a-c input circuit; a full wave bridge connected rectifier having a-c terminals connected to said a-c input circuit and having d-c output terminals; a filter capacitor connected across said d-c output terminals; a transformer means having primary and secondary windings; a gas discharge lamp con-nected across the terminals of said secondary winding; a single semi-conductor switching means which comprises a high voltage bipolar transistor and a low voltage power MOSFET connected in cascode relation, said switching means having a control electrode and first and second power terminals; a current sensing means; a control circuit connected to said control electrode and operable to turn said switching means on and off at a controlled duty cycle rate; a current level set circuit connected to said control circuit to set said duty cycle to a value related to a given value of current in said lamp; said current sensing means being connected to said control circuit and operable to adjust said duty cycle to a value which maintains the current through said current sensing means at a value related to that called for by said current level set circuit; said d-c output terminals, said primary winding, said single semi-conductor switch means and said current sensing means being connected in closed series relation;
said ballast further comprising current transformer means having a tap terminal means and end terminals; said end terminals con-nected to the base of said bipolar transistor and the drain of said MOSFET, respectively; said tap terminal connected to the emitter of said bipolar transistor; the source of said MOSFET and the collector of said bipolar transistor connected in said series circuit; and a zener diode connected between said base of said bipolar transistor and said source of said MOSFET.
said ballast comprising: an a-c input circuit; a full wave bridge connected rectifier having a-c terminals connected to said a-c input circuit and having d-c output terminals; a filter capacitor connected across said d-c output terminals; a transformer means having primary and secondary windings; a gas discharge lamp con-nected across the terminals of said secondary winding; a single semi-conductor switching means which comprises a high voltage bipolar transistor and a low voltage power MOSFET connected in cascode relation, said switching means having a control electrode and first and second power terminals; a current sensing means; a control circuit connected to said control electrode and operable to turn said switching means on and off at a controlled duty cycle rate; a current level set circuit connected to said control circuit to set said duty cycle to a value related to a given value of current in said lamp; said current sensing means being connected to said control circuit and operable to adjust said duty cycle to a value which maintains the current through said current sensing means at a value related to that called for by said current level set circuit; said d-c output terminals, said primary winding, said single semi-conductor switch means and said current sensing means being connected in closed series relation;
said ballast further comprising current transformer means having a tap terminal means and end terminals; said end terminals con-nected to the base of said bipolar transistor and the drain of said MOSFET, respectively; said tap terminal connected to the emitter of said bipolar transistor; the source of said MOSFET and the collector of said bipolar transistor connected in said series circuit; and a zener diode connected between said base of said bipolar transistor and said source of said MOSFET.
36. An electronic ballast for a gas discharge lamp;
said ballast comprising: a bridge connected rectifier having a-c terminals connected to said a-c input circuit and having d-c out-put terminals; a filter capacitor connected across said d-c out-put terminals; an energy storage element having first and second terminals; a gas discharge lamp inductively coupled to said energy storage element; a single semi-conductor switching means which comprises a high voltage bipolar transistor and a low volt-age power MOSFET connected in cascode relation, said switching means having a control electrode and first and second power ter-minals; a current sensing means; a control circuit connected to said control electrode and operable to turn said switching means on and off at a controlled duty cycle rate; a current level set circuit connected to said control circuit to set said duty cycle to a value related to a given value of current in said lamp; said current sensing means being connected to said control circuit and operable to adjust said duty cycle to a value which maintains the current through said current sensing means at a value related to that called for by said current level set circuit; said d-c out-put terminals, said first and second terminals of said energy storage element, said single semi-conductor switch means and said current sensing means being connected in closing series relation;
said ballast further comprising current transformer means having a tap terminal means and end terminals; said end terminals con-nected to the base of said bipolar transistor and the drain of said MOSFET, respectively; said tap terminal connected to the emitter of said bipolar transistor; the source of said MOSFET and the collector of said bipolar transistor connected in series cir-cuit; and a zener diode connected between said base of said bipo-lar transistor and said source of said MOSFET.
said ballast comprising: a bridge connected rectifier having a-c terminals connected to said a-c input circuit and having d-c out-put terminals; a filter capacitor connected across said d-c out-put terminals; an energy storage element having first and second terminals; a gas discharge lamp inductively coupled to said energy storage element; a single semi-conductor switching means which comprises a high voltage bipolar transistor and a low volt-age power MOSFET connected in cascode relation, said switching means having a control electrode and first and second power ter-minals; a current sensing means; a control circuit connected to said control electrode and operable to turn said switching means on and off at a controlled duty cycle rate; a current level set circuit connected to said control circuit to set said duty cycle to a value related to a given value of current in said lamp; said current sensing means being connected to said control circuit and operable to adjust said duty cycle to a value which maintains the current through said current sensing means at a value related to that called for by said current level set circuit; said d-c out-put terminals, said first and second terminals of said energy storage element, said single semi-conductor switch means and said current sensing means being connected in closing series relation;
said ballast further comprising current transformer means having a tap terminal means and end terminals; said end terminals con-nected to the base of said bipolar transistor and the drain of said MOSFET, respectively; said tap terminal connected to the emitter of said bipolar transistor; the source of said MOSFET and the collector of said bipolar transistor connected in series cir-cuit; and a zener diode connected between said base of said bipo-lar transistor and said source of said MOSFET.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/642,072 US4663570A (en) | 1984-08-17 | 1984-08-17 | High frequency gas discharge lamp dimming ballast |
| US642,072 | 1984-08-17 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1254610A true CA1254610A (en) | 1989-05-23 |
Family
ID=24575071
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000488850A Expired CA1254610A (en) | 1984-08-17 | 1985-08-16 | High frequency gas discharge lamp dimming ballast |
Country Status (6)
| Country | Link |
|---|---|
| US (1) | US4663570A (en) |
| KR (1) | KR920007773B1 (en) |
| CA (1) | CA1254610A (en) |
| DE (1) | DE3528549A1 (en) |
| FR (1) | FR2569327A1 (en) |
| GB (1) | GB2163309B (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5668446A (en) * | 1995-01-17 | 1997-09-16 | Negawatt Technologies Inc. | Energy management control system for fluorescent lighting |
Families Citing this family (62)
| Publication number | Priority date | Publication date | Assignee | Title |
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| US4894587A (en) * | 1984-08-17 | 1990-01-16 | Lutron Electronics Co., Inc. | High frequency gas discharge lamp dimming ballast |
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-
1984
- 1984-08-17 US US06/642,072 patent/US4663570A/en not_active Expired - Lifetime
-
1985
- 1985-08-02 GB GB08519442A patent/GB2163309B/en not_active Expired
- 1985-08-08 DE DE19853528549 patent/DE3528549A1/en not_active Ceased
- 1985-08-12 FR FR8512271A patent/FR2569327A1/en not_active Withdrawn
- 1985-08-16 CA CA000488850A patent/CA1254610A/en not_active Expired
- 1985-08-17 KR KR8505939A patent/KR920007773B1/en not_active IP Right Cessation
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5668446A (en) * | 1995-01-17 | 1997-09-16 | Negawatt Technologies Inc. | Energy management control system for fluorescent lighting |
| US5962989A (en) * | 1995-01-17 | 1999-10-05 | Negawatt Technologies Inc. | Energy management control system |
Also Published As
| Publication number | Publication date |
|---|---|
| GB8519442D0 (en) | 1985-09-11 |
| US4663570A (en) | 1987-05-05 |
| KR870002745A (en) | 1987-04-06 |
| KR920007773B1 (en) | 1992-09-17 |
| GB2163309A (en) | 1986-02-19 |
| DE3528549A1 (en) | 1986-02-27 |
| FR2569327A1 (en) | 1986-02-21 |
| GB2163309B (en) | 1988-03-16 |
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