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Buffer amplifier and LED improve PWM power controller's low-load operation

-September 15, 2005

Texas Instruments' UCC3895 offers a good base for building a high-efficiency, pulse-width-modulated, switched-mode power supply that suits either current- or voltage-mode control. Designed for driving a full-bridge power inverter using two sets of complementary outputs, Out A through D, the circuit controls power by phase-shifting outputs C and D with respect to A and B. The manufacturer's data sheet provides a detailed description (Reference 1). However, when lightly loaded and configured for current-mode control, the controller can produce asymmetric-width pulses on its lagging outputs, C and D, under start-up conditions. Reference 2 provides a complete description of the problem and a workaround.

Unfortunately, the workaround evokes other problems when you use the IC in other circuit implementations. Figure 1, from Reference 2, shows a partial schematic featuring the UCC3895 in a peak-current-mode-control circuit in which R1 serves as a pullup resistor, providing a dc offset for the voltage ramp. However, for a significant portion of the ramp waveform, diode D1 doesn't conduct and therefore narrows the power supply's dynamic range by cutting off a portion of the ramp voltage at IC1's Pin 3.

Figure 2 shows another approach that requires additional components but delivers the full magnitude of the voltage ramp to Pin 3 of IC1 and provides the approximately 1V-dc offset that Reference 1 requires. Transistors Q1 and Q2, resistors R1 and R2, and LED D3 form an emitter-follower amplifier for the ramp voltage available at IC1, Pin 7 across timing capacitor C1. This arrangement provides reliable current-mode operation over the full range from no-load to full-load output current by delivering a sawtooth drive with a dc offset to IC1's ramp input. Diode D3, a yellow LED, performs a 1.7V level translation without introducing any substantial signal loss. The component values not shown depend on the application.


References
  1. "UCC3895 BICMOS Advanced Phase Shift PWM Controller," Texas Instruments data sheet, http://focus.ti.com/docs/prod/folders/print/ucc3895.html.

  2. Mappus, S, "UCC3895 OUTC/OUTD Asymmetric Duty Cycle Operation," Texas Instruments Application Report, SLUA275, September 2002.




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