Push-pull PWM controller — 49% duty cycle, no internal FETs
The UCC28083DR is a current-mode push-pull PWM controller from Texas Instruments, designed for isolated offline and DC-DC power supplies. It operates with a maximum duty cycle of 49%, which is the practical ceiling for push-pull topologies to avoid transformer flux imbalance. Because it has no internal power switches, the controller drives external MOSFETs through a gate-drive transformer or a dedicated driver stage — the output stage is a pulse transformer interface, not a totem-pole.
50 kHz to 1 MHz switching — sizing the magnetics
The switching frequency is programmable from 50 kHz to 1 MHz via an external resistor on the RT pin. At 50 kHz the transformer core volume is larger but core losses are lower — typical for cost-sensitive designs with standard ferrites. At 1 MHz the transformer shrinks but the gate-drive losses and snubber dissipation increase; the practical upper end is usually 500–700 kHz in a push-pull offline supply. The 12.5 V start-up threshold and 8.3 V to 15 V supply range mean the controller powers up from an auxiliary winding once the converter is running. The undervoltage lockout hysteresis (typically 1 V) determines the minimum hold-up capacitor on Vcc — a 10 µF ceramic is usually sufficient for a 50 W design.