Current-mode PWM controller for single-ended topologies
The Texas Instruments UC3844DTRG4 is a current-mode PWM controller designed for single-ended power converter topologies including boost, buck, buck-boost, flyback, and forward converters. It integrates a transistor driver output, frequency control, and a 48% maximum duty-cycle clamp that prevents transformer saturation in flyback stages.
Switching frequency and duty-cycle limits
The oscillator runs up to 500 kHz, which sets the practical transformer size and snubber loss trade-off. At the upper end, core volume shrinks but switching losses in the MOSFET and rectifier rise; most designs land between 100 kHz and 250 kHz for a balanced flyback. The 48% duty-cycle ceiling is hard — the controller shuts off the output pulse before the on-time reaches half the period, guaranteeing a reset interval for the transformer core.
Supply rail and UVLO thresholds
Vcc range is 12 V to 28 V. The undervoltage lockout (UVLO) turn-on threshold is 16 V typical, turn-off 10 V typical — the 6 V hysteresis prevents chatter during startup on a slowly rising bias rail. Below 12 V the controller stays off; above 28 V the internal zener clamp limits the supply to protect the IC.