48% duty cycle — the flyback/forward ceiling
The UC3844AD8G4 is a current-mode PWM controller from Texas Instruments designed for step-up, step-down, step-up/step-down topologies including boost, buck, flyback, and forward converters. Its 48% maximum duty cycle is the hard limit that sets the transformer turns ratio and output voltage range in isolated flyback and forward designs — push past it and the controller skips pulses.
Switching up to 500 kHz — magnetics shrink, layout tightens
Switching frequency reaches up to 500 kHz, which lets you downsize the transformer and output filter capacitors. At these speeds the gate-drive loop and sense resistor layout become critical — a few nanohenries of stray inductance in the current-sense path can shift the cycle-by-cycle limit. The controller does not include clock synchronisation, so multiple converters on the same board need external beat-frequency management.
The undervoltage lockout thresholds are set inside that window — the controller stays off until Vcc rises above the start threshold and shuts down cleanly when it drops. Output configuration is positive, isolation-capable, with a single transistor-driver output.