EP0326114A1

EP0326114A1 - Drive device for a discharge lamp

Info

Publication number: EP0326114A1
Application number: EP89101287A
Authority: EP
Inventors: Satoshi Suzuki; Fujinori Kimura; Hiroshi Kubota
Original assignee: Tokyo Electric Co Ltd
Current assignee: Toshiba TEC Corp
Priority date: 1988-01-26
Filing date: 1989-01-25
Publication date: 1989-08-02
Also published as: JPH01189897A

Abstract

A drive device for a discharge lamp comprises a series inverter circuit (24) with first and second output terminals for producing an AC voltage between the first and second output terminals, a capacitor (33) connected between first ends of first and second filaments (32A, 32B) of the discharge lamp (32), and an inductor section for igniting the discharge lamp (32) in cooperation with the capacitor (33) and restricting a discharge current to be flow between the first and second filaments (32A, 32B) after the discharge lamp (32) has been ignited. In the drive device, the inductor section includes first and second inductors (37A, 37B) respectively connected between the first output terminal of the series inverter circuit (24) and a second end of the first filament (32A) and between the second output terminal of the series inverter circuit (24) and a second end of the second filament (32B).

Description

The present invention relates to a drive device for a discharge lamp, and more particularly to a drive device for a discharge lamp such as a fluorescent lamp.
A lamp drive device as shown in Fig. 1 has been used for driving a fluorescent lamp. In the drive circuit, an AC voltage from commercial power source 1 is converted into a DC voltage by rectifier circuit 2. The DC voltage is then applied to series inverter circuit 4 through a couple of power lines VDD and VSS. Series inverter circuit 4 is arranged in a half-bridge connection. Inverter circuit 4 is made up of a couple of transistors 5 and 6 and another couple of capacitors 7 and 8. These transistors 5 and 6 are connected in series between paired power lines VDD and VSS, and capacitors 7 and 8 are likewise connected between power lines VDD and VSS. Fluorescent lamp 12 is set between lamp sockets LS. Capacitor 13 is connected to the first ends of filaments 12A and 12B. The second end of filament 12A is connected by inductor coil 11 to the node of transistors 5 and 6. The second end of filament 12B is connected to the node of capacitors 7 and 8. In this case, capacitor 13 and inductor coil 11 form a resonance circuit which ignites fluorescent lamp 12.
In operation, when an alternate switching operation of transistors 5 and 6 starts, a resonance current flows through a series path of inductor coil 11, filament 12A, capacitor 13, and filament 12B, to warm up or preheat filaments 12A and 12B. In the warm-up mode, a voltage across capacitor 13 rises and exceeds a discharge start voltage at which a discharge begins between filaments 12A and 12B. Upon start of the discharge, the fluorescent lamp 12 starts to emit light.
The above lamp drive device, however, is defective in that replacement of a lamp during operation of the inverter circuit is dangerous. When, during the removal of lighting lamp 12 from socket pair LS, the discharge accidentally stops or when a new fluorescent lamp is set, the warm-up mode is set up. In this mode, a potential difference between the surface of fluorescent lamp 12 and ground becomes excessively high, increasing with potential VA at end P of filament 12A. In such a high voltage condition, if a user contacts the fluorescent lamp 12, he is placed in the path between the lamp surface and ground through which a leak current would flow, and may receive an electric shock.
Accordingly, an object of the present invention is to provide a drive device for a discharge lamp which eliminates the risk of electric shock to someone being very close to the lamp or replacing the lamp during operation of this device.
To achieve the above object, there is provided a drive device for a discharge lamp comprising:
a power source circuit with first and second output terminals for producing an AC voltage between the first and second output terminals;
a capacitor connected between the first ends of first and second filaments of the discharge lamp; and
first and second inductors respectively connected between the first output terminal of the power source circuit and the second end of the first filament, and between the second output terminal of the power source circuit and the second end of the second filament, the first and second inductors cooperating with the capacitor to form a resonance circuit which ignites the discharge lamp, and serving as elements to restrict a discharge current flowing between the first and second filaments after a discharge begins in the ignited discharge lamp.
In the discharge lamp drive device thus arranged, an inductance required for the resonance circuit and the current restricting elements is set as the sum of an inductances of the first inductor connected to the first filament and an inductance of the second inductor connected to the second filament. In this case, the voltage between the first and second output terminals of the power source circuit is divided in accordance with an inductance ratio of the first and second inductors. The voltage division suppresses the partial increase of a potential on the lamp surface, uniforms the lamp surface potential distribution, and sets its value at a lower voltage than that obtained when the inductor is coupled with one of the filaments.
This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a circuit diagram showing a conventional drive device for a fluorescent lamp;
Fig. 2 is a circuit diagram showing a drive device for a fluorescent lamp according to the first embodiment of the present invention;
Fig. 3 shows a circuit diagram of a measurement circuit used to measure a shock current flowing when a someone contacts with the surface of a fluorescent lamp coupled with the lamp drive circuit of Fig. 2;
Fig. 4 is a graph showing relationships of the shock currents and measurement points in the surface of a fluorescent lamp;
Fig. 5 is a circuit diagram showing a lamp drive device for two fluorescent lamps according to the second embodiment of the present invention;
Fig. 6 is a circuit diagram showing a drive device for a fluorescent lamp according to the third embodiment of the present invention, into which a series inverter of the full bridge type is provided;
Fig. 7 shows a circuit diagram of a measurement circuit used to measure a current leaking from an electronic stabilizer board on which lamp drive device of Fig. 2 is formed;
Fig. 8 is a graphical representation of a variation of a leak current from the board of Fig. 7 with respect to inductance ratio of first and second inductors; and
Fig. 9 is a circuit diagram showing a drive device for a fluorescent lamp according to the fourth embodiment of the present inventions.

There will now be described a drive device for a fluorescent lamp according to the first embodiment of the present invention with reference to Figs. 2 to 4.
Fig. 2 shows the drive device for a fluorescent lamp. The drive device is made up of DC power source circuit 22, trigger circuit 20, series inverter circuit 24, and ignition circuit 10. DC power source circuit 10 includes filter section 22A, rectifier section 22B and smoothing section 22C to convert an AC voltage from commercial AC power source 21 into a DC voltage (= 220 V). The DC voltage is applied through paired power lines VDD and VSS to trigger circuit 20 and series inverter circuit 24 serving as a high frequency oscillator for 25 KHz. Series inverter circuit 24 is includes NPN transistors 25 and 26, oscillation transformer T1, and commutating capacitors 27 and 28 (= 0.022 µF). Ignition circuit 10 is connected between the node of transistors 25 and 26 and the node of capacitors 27 and 28. The oscillation transformer T1 has exciting coil L1 (= 2 turns), and induction coils L2 and L3 (= 4 turns). Induction coil L3 is wound in such a direction as to place transistor 25 in a conductive state. Induction coil L2 is wound in such a direction as to place transistor 26 in the conductive state. Exciting coil L1 is coupled in series with ignition circuit 10. The voltages across induction coils L2 and L3 as induced when a current flows through exciting coil L1, are opposite in polarity to each other. Those induced voltages are applied across the base-emitter paths of the transistors 25 and 26, respectively. Ignition circuit 10 includes inductors 37A and 37B (= 2.8 mH), and a capacitor 33 (= 0.012 µF). Capacitor 33 is connected between the first ends of filaments 32A and 32B. Inductor 37A is connected between the second end of filament 32A and the node of transistors 25 and 26, and inductor 37B is connected between the second end of filament 32B and the node of capacitors 27 and 28. Inductors 37A and 37B, and the capacitor 33 form a resonant circuit which causes a discharge between the filaments 32A and 32B to ignite fluorescent lamp 32. After the discharge begins, inductors 37A and 37B serve as current restricting elements. Trigger circuit 20 includes resistor R4, capacitor C5 charged through resistor R4 at a fixed time constant, and diac TD which is made conductive when a charged voltage across the transistor C5 exceeds a predetermined voltage.
The operation of the drive circuit thus arranged will be described. An AC voltage is applied from the commercial power source 21 to DC power source circuit 22 capacitor C5 is charged by current applied through resistor R4. When the voltage across capacitor C5 exceeds a predetermined voltage, diac TD allows a discharge current from capacitor C5 to pass therethrough into the diac TD. In response to the discharge current, the transistor 26 is turned on, to cause a warm-up current to flow through a series current path formed of inductor 37A, filament 32A, capacitor 33, filament 32B, and inductor 37B. Consequently, a resonant voltage exceeding a discharge start voltage (= 300 V) of the lamp 32 is applied across capacitor 33, and a discharge begins between filaments 32A and 32B to light on fluorescent lamp 32. A lamp current flowing through lamp 32 also flows through exciting coil L1. The current of exciting coil L1 gradually decreases due to a magnetic saturation of transformer T1, and eventually the transformer is magnetized in the reverse direction. With the reversal of the magnetization, the voltage induced in induction coils L2 and L3 are also reversed in polarity. The induced voltage becomes large as the lamp current decreases. Subsequently, as the lamp current becomes zero, transistor 26 is instantaneously turned off by the voltage developed in induction coil L3, and transistor 25 is turned on by the voltage developed in induction coil L2. Then, a lamp current flows through a series pass formed of transistor 25, exciting coil L1, inductor 37A, fluorescent lamp 32, and inductor 37B in the direction DR′, so that lamp 32 is lit on. As in the previous case, the oscillation transformer T1 is saturated so that the discharge current gradually decreases and the polarity of the magnetization of transformer T1 is again reversed. At the instant that the discharge current becomes zero, the voltage is induced in induction coils L2 and L3, and renders transistor 25 nonconductive and transistor 26 conductive. Under this condition, a lamp current flows through a series path formed of inductor 37B, fluorescent lamp 32, inductor 37A, exciting coil L1, and transistor 26 in the direction DR2, so that the lighting of lamp 32 is maintained. Subsequently, the alternately reversed magnetization of oscillation transformer T1 alternately turns on and off the transistors 25 and 26 in a repeated manner, thereby maintaining the lighting of fluorescent lamp 32.
In the Fig. 2, diode D8 and resistor R5 are provided for discharging the charge of capacitor C5 through transistor 26 transistors Q1 and Q2, diodes D6 and D7, resistors R10 and R11, and resistors R8 and R9 are provided for stabilizing the voltage applied to lamp 32 when inductors 37A and 37B an d capacitor 33 resonate in the warm-up stage. Specifically, in the preheat or warm-up stage before fluorescent lamp 32 is lit on, a resonance by inductors 37A and 37B and the capacitor 33 causes an excessive current to flow the resistors R8 and R9 respectively coupled with transistors 25 and 26. The voltages across the resistors become high enough to render the transistors conductive. By the voltages, those transistors 25 and 26 are turned on. The result is to quicken the magnetic saturation of oscillation transformer T1. In a transient state that the transistors 25 and 26 are being turned off, it quickens the change of their conduction state, to restrict the resonance current.
A leak current between the surface of fluorescent lamp 32 and ground was measured by using a connection as shown in Fig. 3. That is, a tube of lamp 32 is wrapped with an aluminum foil 34, resistor 35 is inserted between the foil 34 and ground, and oscilloscope 36 is connected between a movable terminal of resistor 35 and ground.
The AC power source was a 3-phase power source of 200 V. The measurement was repeated for three different locations (1) to (3) of the foil wrapping on the lamp tube surface. A voltage developed across resistor 35 was measured by the oscilloscope 35 when lamp 32 is in a preheat or warm-up condition, while varying a ratio of the inductances "x" and "y" of inductors 37A and 37B. The obtained voltages were converted into corresponding peak values of a shock current. As shown in Fig. 4, variations of the peak values of the shock current were plotted against the foil wrapping locations (1) to (3), with parameters of inductance ratios of the inductors. In the graph of Fig. 4, curve A represents a variation of the peak value of the shock current when the inductance ratio, x : y = 0 : 1. Similarly, curve B was obtained for x : y = 1 : 0; curve C, for x : y = 2 : 1; curve D, for x : y = 3 : 2;, curve E, for x : y = 1 : 2; curve F, for x : y = 1 : 1; curve G, for x : y = 2 : 3.
The peak current variations of the conventional lamp drive device could be represented by curve A or B. These curves show that a maximum peak value of the shock current of 150 mA is observed at the tube surface location ① or ③ wrapped with the aluminum foil that is closer to the filament connecting to an inductor. The figure of 150 mA is very high as a shock current. In the curve C for x : y = 1 : 2, a maximum shock current is found at the location ①, and its value is 100 mA at most. In the curves D to F, the peak values are further reduced. In the case of curve G, a maximum peak value at the location ① is only 40 mA. As seen from the above curves, when two inductors are coupled with both ends of the discharge lamp, the shock current may remarkably be reduced. Such figures of the shock current little shock a user when he contact the lamp tube surface with his hand. Incidentally, it is known that an electrical shock produced when a human being contacts a discharge lamp is proportional to a peak value of the shock current.
A couple of resonance inductors 37A and 37B, which are not magnetically coupled with each other, are used in the above-mentioned embodiment. In the second embodiment each pair of inductors 37A and 37B magnetically are coupled with each other as shown in Fig. 5.
Additionally, a pair of capacitors 27 and 28 are used in the first embodiments. In the third embodiment, capacitors 27 and 28 are replaced by a pair of NPN transistors 38 and 39 so as to form series inverter circuit 24 of a full bridge type.
Fig. 7 shows another measuring circuit for measuring a leak current flowing between ground and electronic stabilizer board on which a drive circuit of each embodiment is formed. When the drive device of Fig. 2 is formed on electronic stabilizer board 42, a 3-phase AC power source of 200 V is connected to the drive circuit fluorescent lamp 43 is connected to the drive circuit. Resistor 44 of 1 kilo ohms is inserted between the stabilizer board 42 and ground, and ammeter 45 for measuring a leak current is connected across the resistor 44.
A leak current was measured by using the above measuring circuit, while varying an inductance ratio of the inductors (choke coils) of x : y. The results of the measurements were plotted as shown in Fig. 8. The graph of Fig. 8 clearly shows that a minimum leak current resides in the vicinity of the inductance ratio of 2 : 3.
Fig. 9 shows a drive device for a fluorescent lamp according to the fourth embodiment. Switching element SW is connected between first ends of filaments 32A and 32B of fluorescent lamp 32, second ends of filaments 32A and 32B are respectively connected through inductances 37A and 37B to power terminals between which an AC power source voltage is applied from commercial AC power source (12). In this case, it is also possible to reduce the electric shock.

Claims

1. A drive device for a discharge lamp characterized by comprising:
a power source circuit (22, 20, 24) with first and second output terminals for producing an AC voltage between said first and second output terminals;
capacitive means (33) connected between first ends of first and second filaments (32A, 32B) of said discharge lamp (32); and
inductive means for igniting said discharge lamp (32) in cooperation with said capacitive means (33) and restricting a discharge current which flows between said first and second filaments (32A, 32B) after the discharge lamp (32) has been ignited;
characterized in that said inductive means includes first and second inductors (37A, 37B) respectively connected between said first output terminal of said power source circuit (22, 20, 24) and a second end of said first filament (32A), and between said second output terminal of said power source circuit (22, 20, 24) and a second end of said second filament (32B).

2. A drive device according to claim 1, characterized in that said power source circuit includes oscillating means (22, 20, 24) for producing said AC voltage at a high frequency.

3. A drive device according to claim 2, characterized in that said oscillation means includes rectifying means (22) for rectifying an AC voltage supplied from a commercial AC power source (21), and inverter means (20, 24) for inverting a DC voltage supplied from said rectifying means.

4. A drive device according to claim 3, characterized in that said inverter means includes a series inverter circuit (24) of a full bridge type.

5. A drive device according to claim 3, characterized in that said inverter means includes a series inverter circuit (24) of a half bridge type.

6. A drive device according to claim 3, characterized in that said series inverter circuit includes a first and second switching transistors (25, 26) connected in series between a pair of power lines (VDD, VSS) for receiving said DC voltage, first and second capacitors (27, 28) connected in series between said power lines (VDD, VSS), and a control circuit (T1) for alternately turning on said first and second switching transistors (25, 26), the node of said first and second switching transistor (25, 26) serving as said first output terminal, and the node of said first and second capacitors (27, 28) serving as said second output terminal.

7. A drive device according to claim 1, characterized in that said the inductance ratio of said first and second inductors (37A, 37B) is set within a range from 0.5 to 2.

8. A drive device according to claim 1, characterized in that said first and second inductors (37A, 37B) are magnetically coupled to each other.

9. A drive device for a discharge lamp characterized by comprising:
first and second power terminals between which an AC voltage is supplied;
switching means (SW) connected between first ends of first and second filaments (32A, 32B) of said discharge lamp (32) for igniting said discharge lamp (32); and
inductive means for restricting a discharge current to be flow between said first and second filaments (32A, 32B) after the discharge lamp (32) has been ignited;
characterized in that said inductive means includes first and second inductors (37A, 37B) respectively connected between said first power terminal and a second end of said first filament (32A) and between said second power terminal and a second end of said second filament (32B).

10. A drive device according to claim 9, characterized in that said the inductance ratio of said first and second inductors (37A, 37B) is set within a range from 0.5 to 2.

11. A drive device according to claim 9, characterized in that said first and second inductors (37A, 37B) are magnetically coupled to each other.