Electric power conversion systems – Current conversion – With means to introduce or eliminate frequency components
Reexamination Certificate
2000-10-06
2001-11-20
Nguyen, Matthew (Department: 2838)
Electric power conversion systems
Current conversion
With means to introduce or eliminate frequency components
Reexamination Certificate
active
06320768
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to power supply pulse width modulation (PWM) control systems and methods, and more particularly to a power supply PWM method and system that uses peak current program mode (CPM) control for large duty ratios with a smooth transition to voltage mode control at small duty ratios down to zero duty ratio.
2. Description of the Prior Art
Current program mode (CPM) is a popular PWM control technique used in switched mode power supply technology. The duty ratio is determined when the switch current reaches a peak value that is requested by a control voltage. Often, the control voltage is a compensated version of the error between a power supply output voltage and a fixed reference. At normal operating levels, the switch current and the requested peak current are larger than the noise in the system, and they are sufficiently centered between comparator supply voltages and error-amplifier supply voltages in order to have smooth operation at large duty ratios and large currents. When a small duty ratio is required however, (i.e., below a few percent), the current sense signal becomes too small and too close to ground for the PWM comparator to successfully distinguish the greater of the current sense signal and the control voltage signal. This scenario is encountered in a PWM controlled power supply operating at light load conditions. The unfortunate result is that the duty ratio erratically jumps between zero and a minimum duty ratio. The result to the rest of the converter is that the output voltage will lose regulation and transformer isolated topologies may experience over-current conditions from saturation that is caused by the erratic behavior. Small duty ratios and high switching frequencies are also limited by the delay in the logic that is required to detect and latch the pulse width information each switch cycle.
In view of the foregoing,, a need exists for a power supply pulse width modulation (PWM) scheme that improves light load/low duty ratio performance in current program mode (CPM) controlled switched mode converters and that increases speed by eliminating the PWM latch and its associated delay.
SUMMARY OF THE INVENTION
The present invention is directed to a power supply PWM control technique that uses peak current program mode (CPM) control for large duty ratios with a smooth transition to voltage mode control at small duty ratios down to zero duty ratio and that improves the speed of modulation. A PWM comparator is implemented so that it operates in three different modes, depending on the current sense level. The PWM comparator performs CPM control (first mode) over most of the control range where the current sense levels are large. For small current sense levels, the PWM comparator performs voltage mode control (second mode), causing the duty ratio to be proportional to the control voltage. Voltage mode control is where the duty ratio is determined by a voltage or voltage error signal. The PWM comparator has a small operating range where the first and second modes overlap in order to facilitate a smooth transition between CPM and voltage mode control ranges. A third mode of operation occurs when the control voltage becomes low enough to drive the duty ratio to zero without any intermittent pulses.
In one aspect of the invention, a power supply PWM control technique is implemented using, an unbalanced comparator and relying on a cut-off threshold in order to determine the mode of modulation.
In another aspect of the invention, a power supply PWM control technique is implemented additionally using a differential amplifier in order to achieve balanced operation for high noise immunity at high speeds.
In yet another aspect of the invention, a power supply PWM technique is implemented using another differential amplifier whose purpose is to set the point where the mode of modulation changes by a reference instead of relying, on parasitic cut-off thresholds.
In still another aspect of the invention, a power supply PWM technique is implemented using a capacitor to store the state of the modulator in contrast to the PWM latch.
In still another aspect of the invention, a power supply PWM technique is implemented in which the PWM controller smoothly transitions from a first mode through a third mode and back so that the power supply output remains smoothly controlled.
In still another aspect of the invention, a power supply PWM technique is implemented in which the amount of delay is greatly reduced due to the size of the PWM state capacitor and its functional location.
REFERENCES:
patent: 4353113 (1982-10-01), Billings
patent: 5815381 (1998-09-01), Newlin
patent: 6011706 (2000-01-01), Adragna et al.
patent: 6188588 (2001-02-01), Hemena et al.
Madigan Michael T.
Pham Mau
Brady III Wade James
Holmbo Dwight N.
Nguyen Matthew
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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