|Publication number||USRE38780 E1|
|Application number||US 10/375,914|
|Publication date||Aug 23, 2005|
|Filing date||Feb 26, 2003|
|Priority date||Sep 1, 1999|
|Publication number||10375914, 375914, US RE38780 E1, US RE38780E1, US-E1-RE38780, USRE38780 E1, USRE38780E1|
|Inventors||Charles E. Hawkes, Michael M. Walters, Robert H. Isham|
|Original Assignee||Intersil Americas Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (11), Non-Patent Citations (2), Referenced by (22), Classifications (6), Legal Events (6)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application claims the benefit of U.S. Provisional Application No. 60/151,971, filed Sep. 1, 1999. This Reissue Application is a continuation of Reissue Application Ser. No. 10/045,169 which is a reissue of Application Ser. No. 09/591,360 (U.S. Pat. No. 6,181,120 ), which claims the benefit of Provisional Application Ser. No. 60/151,971.
The present invention relates to DC/DC converters.
As the complexity and clock speed of CPUs continue to rise, greater demands are placed on the power supplies (DC/DC converters) that supply the operating voltage to the CPUs. Typically, the operating voltage of CPUs is specified with a relatively tight tolerance to ensure proper operation of the CPU. The tight tolerances on CPU operating voltages are being further narrowed as CPU clock and CPU bus speeds increase, and CPU operating voltages decrease. The decrease in permissible tolerances on CPU operating voltages has resulted in a corresponding increase in the regulation specifications of power supplies that supply operating voltages to CPUs.
The current drawn by a CPU generally undergoes frequent variation and rapid changes of substantial magnitude. For example, the current a CPU draws from a power supply may change by as much as 10-75 Amps per microsecond. These frequently varying and rapidly changing demands for substantial amounts of current are referred to as a load transients. These extreme load transients cause a corresponding voltage transient on voltage output of the power supply, thereby making it very difficult for a power supply to comply with tight power supply regulation specification. Many power supplies incorporate very large capacitors to reduce the effect of these large and rapid load transients, and thereby lessen the resultant corresponding voltage transients on the output voltage of the power supply to an acceptable level. However, the use of large capacitors adds significantly to the cost, size and weight of the power supply.
In order to reduce the number and size of capacitors needed to lessen the effect of a given load transient on power supply output voltage, a technique known as “droop” is employed. Normally, power supplies are designed to have an output voltage that is essentially independent of the load current. However, in applications where a power supply will be required to comply with tight regulation specifications in a high-local-transient environment, there is an advantage in carefully controlling and/or adjusting the output impedance of the power supply to thereby cause the power supply output voltage to decrease by a predetermined amount in response to an increase in current demanded by or being supplied to the load.
In conventional current-mode DC/DC converters, the duty cycle of the DC/DC converter is modulated by a negative-feedback voltage loop to maintain the desired output voltage. The feedback voltage loop has a DC voltage gain which determines the amount of “droop” in the output impedance of the power supply. The DC voltage gain of the feedback loop is, therefore, designed to be relatively low in order to achieve a relatively small amount of droop and thereby maintain a substantial degree of voltage regulation to comply with the tight tolerances placed upon the operating voltage supplied to the CPU.
The low DC gain in the feedback loop, however, results in any variations or offsets in the voltages within the DC/DC converter being reflected in a corresponding error in the output voltage of the converter. The only known solution to this problem is to design precise circuitry using components having tight tolerances in order to achieve low-offset voltages and/or precise internal voltages within the DC/DC converter. The inclusion of such precise circuitry adds substantially to the cost and complexity of the converter.
Therefore, what is needed in the art is a converter that maintains voltage regulation in a high-load-transient environment.
Furthermore, what is needed in the art is a converter which does not depend upon large capacitors to maintain voltage regulation in a high-load transient environment, and is therefore less expensive to build, smaller in size and lighter in weight.
Moreover, what is needed in the art is a converter which achieves voltage regulation in a high-load transient environment without the use of precision circuitry, and is therefore less complex and less expensive to build.
The present invention provides a DC/DC converter having a controlled output impedance and which provides for a controlled droop in the output voltage in response to load transients.
The invention comprises, in one form thereof, a DC/DC converter having an output voltage and sourcing an output current to a load. The DC/DC converter includes an error amplifier with a reference input and a summing input. The reference input is electrically connected to a reference voltage. The summing input is electrically connected to the output voltage and the output current. The summing input is configured for adding together the output voltage and the output current. The error amplifier issues an error signal and adjusts the error signal dependent at least in part upon the output voltage and the output current. A comparator receives the error signal. The comparator has a ramp input electrically connected to a voltage ramp signal. The comparator issues an output signal that is based at least in part upon said error input. A power switch has an on condition and an off condition, and supplies dc current to the load when in the on condition. The power switch has a control input electrically connected to the comparator output signal. The power switch is responsive to the control input to change between the on condition and the off condition to thereby adjust the output current of the DC/DC converter.
An advantage of the present invention is that droop in the output voltage of the converter in response to a load transient is controlled and reduced.
Another advantage of the present invention is that the need for a plurality of large capacitors to maintain regulation of the output voltage in a high-load transient environment is eliminated, and therefore the present invention is less expensive to manufacture, is of a lighter weight and smaller in size than conventional DC/DC converters.
A further advantage of the present invention is that it is essentially immune to errors in internal reference and offset voltages.
The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become apparent and be better understood by reference to the following description of one embodiment of the invention in conjunction with the accompanying drawings, wherein:
Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner.
Referring now to the drawings, and particularly to
Referring now to
Referring now to
As will be described in more detail hereinafter, the duty cycle of DC/DC converter 10 is modulated by a negative-feedback voltage loop to maintain the desired output voltage VOUT across load 22. In a current-mode converter (as in FIG. 2), output voltage regulation is achieved in an indirect fashion by controlling a sensed current. The current through power switch 14 is sensed, and therefore controlled, by current sensor 24, and signal V1SENSE, which is proportional to the current sensed by current sensor 24, is issued. However, is it to be understood that either the current through indicator 20 or the current through diode 18 can be sensed instead.
To achieve output voltage regulation, output voltage VOUT is sensed and divided down by the voltage divider formed by R1 and R2 to produce the voltage VFB at node 26. Error Amp 28 amplifies the difference between VFB and the voltage reference VREF at node 30 and produces the error voltage VERROR at node 32. Thus, error amp 28 adjusts the VERROR voltage at node 30 as needed to achieve a power switch 14 duty cycle that forces VFB at node 26 to be equal to VREF. Subtraction circuit 35 subtracts V1SENSE from VERROR. Because the current sensed by current sensor 24 is subtracted from VERROR is the form of V1SENSE, error amp 28 also adjusts VERROR at node 32 in accordance with V1SENSE to produce the needed duty cycle. This results in an effective control, or programming, of the current sensed by current sensor 24. Depending on the gain of the signal conditioning block 34, the VERROR signal at node 32 can be proportional to the intra-cycle peaks of the sensed current (known as Peak Current Control) or the VERROR signal may be proportional to the average value of the sensed current (known as Average Current Control).
To implement either Peak Current or Average Current Control, it is necessary to add frequency compensation to the voltage feedback loop to achieve stability. Frequency compensation is accomplished by CCOMP and R1. CCOMP and R1 add a high frequency pole into the feedback loop that cancels a zero that is due to the Equivalent Series Resistance (ESR) of the output capacitor CL. Depending on the details of the circuit values, this compensating pole is sometimes not needed. The feedback resistor RFB is adjusted to control the DC gain of error amplifier 28, and thereby provide the desired amount of droop in the output voltage VOUT of converter 10. Since the voltage VERROR at node 32 is proportional to V1SENSE, which represents the current sensed by current sensor 24 and which is proportional to load current IOUT, a reduction in DC gain will cause the output voltage VOUT to vary with the load current IOUT. In this manner, a controlled droop in the output impedance of converter 10 is achieved. For example, the voltage V1SENSE may vary by 2V as the load current IOUT varies from 0 to 10 Amps. If the ratio of RFB to R1, is equal to 10 (ten), the voltage VOUT will decrease by 0.1V as the load current is increased from 0 to 10 Amps (hence, “Droop”).
The fundamental problem with the method of converter 10 in achieving and controlling droop resides in the low DC gain of the voltage feedback loop. The low gain is used to provide the drooping characteristic, but it also has an undesirable side-effect. As a result of this low DC gain, any variations in the VRAMP signal or DC offsets in current sensor 24 or comparator 16 will be reflected in a corresponding error in the voltage VOUT. For example, if the average value of the voltage VRAMP has tolerance of ±200 mV, and the ratio of RFB to R1 is equal to 20, an additional error term of ±10 mV on the voltage VOUT will result. The only known solution to this problem is to design precise circuitry in order to achieve low-offset voltages and/or a precise VRAMP voltage. The inclusion of such precise circuitry adds substantially to the cost and complexity of a DC/DC converter.
Referring now to
The most functional feature of DC/DC converter 100 is that current sensor 124 is electrically connected to the output voltage feedback loop. More particularly, V1SENSE is divided by the voltage divider formed by R1 and R2, and this divided portion forms part of VFB. However, it is to be understood that the current through inductor 120 or the current through diode 118 can be sensed and similarly connected to the output voltage feedback loop, rather than the current through power switch 114. V1SENSE is connected to the voltage feedback loop without first being frequency compensated by error amplifier 128, as in conventional DC/DC converter 10 of FIG. 2. The principle advantage of not performing frequency compensation upon signal V1SENSE prior to the connection thereof with the output voltage feedback signal is that the gain of error amp 128 is thereby permitted to be arbitrarily high at DC (note the absence of RF), thus providing DC/DC converter 100 excellent output voltage accuracy that is essentially immune to variations in the VRAMP voltage and offset voltages, etc.
To understand how DC/DC converter 100 creates the desired drooping output voltage characteristic, first consider the operation of DC/DC converter 100 under a no-load condition and IOUT=0. In this case, V1SENSE=0, and the output voltage VOUT of converter 100, under this no-load condition, is given by Vref (R1+R2)/R2. Note that R1 and R2 here are intentionally chosen so that the so-load output voltage of converter 100 is a predetermined amount greater than the desired target voltage. At full load, when IOUT=IMAX, V1SENSE will equal V1SENSE, MAX, and thus we have VOUT=[VREF(R1+R2)/R2]−V1SENSE,MAX. Thus, as the current through load 122 increases from zero to full load current, output voltage VOUT decreases, or droops, by V1SENSE,MAX Volts.
Note especially that the same frequency compensation provided by RCOMP and CCOMP is applied to both the VFB voltage signal and the V1SENSE current signal. In this way, average current mode control is implemented without the need for a separate signal conditioning block (Gc(s) in FIG. 2). This is another advantage of DC/DC converter 100. Average current mode control and accurate droop are achieved using a single amplifier. The frequency compensation in DC/DC converter 10 introduces a pole at very low frequency, which is set by the characteristics of error amp 128, and a zero which is set by RCOMP and CCOMP. For the voltage feedback loop, a high DC gain is provided, which makes the output voltage of DC/DC converter 100 essentially immune from errors in VRAMP and offset voltage errors. Likewise, in regards to current, the high DC gain and averaging characteristic of the frequency compensation provide excellent response to the average value of the sensed current. Because of the current-mode control, the two poles associated with the LC filter formed by inductor 120 and load capacitor 121 are split, with one pole moving to a relatively high frequency and the other pole moving to a relatively low frequency. The zero is placed before the crossover of the frequency compensation loop, which effectively cancels the effect of the low-frequency pole associated with the LC filter formed by inductor 120 and load capacitor 121. The high frequency gain of error amp 128 is determined by the ratio RCOMP/R1. This ratio is adjusted to provide suitable high frequency current gain (and the associated pole-spliting of the LC filter poles). The high-frequency pole associated with the LC filter formed by inductor 120 and load capacitor 121 is used to compensate for the zero associated with the ESR of load capacitor 121. In this manner, a response that is essentially a single-pole response having excellent phase margin is achieved.
Referring now to
While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, users, or adaptations of the present invention using the general principles disclosed herein. Further, this application is intended to cover such departures from the present disclosure as come within the known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.
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|U.S. Classification||323/282, 323/285|
|Cooperative Classification||H02M3/156, H02M2001/0019|
|Apr 29, 2008||CC||Certificate of correction|
|Jul 30, 2008||FPAY||Fee payment|
Year of fee payment: 8
|Nov 16, 2009||AS||Assignment|
Owner name: INTERSIL CORPORATION, FLORIDA
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|May 5, 2010||AS||Assignment|
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