1 MHz PWM controller for isolated offline supplies
The Texas Instruments UCC3809D-1 is a current-mode PWM controller for isolated offline power converters. It supports boost, buck, flyback, and forward topologies, switching at 1 MHz. The 70% maximum duty cycle limits the on-time per cycle, which sets the transformer turns ratio and output voltage range in flyback and forward designs. The 8V to 19V supply range and 15V startup threshold match common bias rails from a bootstrap winding or auxiliary supply. No internal power switch is integrated — the controller drives an external MOSFET through the gate-drive output.
The 8-SOIC surface-mount package keeps board area small.
Control features and design flexibility
The UCC3809D-1 includes frequency control and soft-start as control features. Frequency control allows the oscillator to be synchronised or adjusted externally, useful for spread-spectrum EMI reduction or multi-phase interleaving. Soft-start limits inrush during startup, which is critical for designs powering large output capacitors or loads that draw heavy surge current. The part is an isolated controller — the feedback path crosses the isolation barrier via an optocoupler or transformer.
For BOM freeze or dual-sourcing reviews, the active status means no imminent LTB risk, though the 1 MHz switching frequency and 70% duty cycle should be cross-checked against any candidate alternate for pin compatibility and control-loop compensation.