Current-mode PWM controller for single-ended topologies
The Texas Instruments UC3845AD8 is a current-mode PWM controller designed for single-ended power converters. It supports boost, buck, flyback, and forward topologies, with a single transistor-driver output. The 48% maximum duty cycle is tailored for flyback and forward converters where transformer reset requires the off-time to exceed the on-time. Switching frequency reaches up to 500 kHz, enabling compact magnetics. The wide supply range from 7.6 V to 30 V allows direct connection to rectified DC bus voltages without an intermediate regulator.
48% duty cycle — why it matters for flyback designs
The 48% duty cycle ceiling is a deliberate limit for single-ended topologies. In a flyback converter, the transformer core must reset during the off-time; exceeding 50% duty cycle risks saturation and primary-side switch stress. This part ensures the off-time is always longer than the on-time, simplifying transformer design and improving reliability. The controller lacks clock synchronization and synchronous rectification capability, so it is best suited for non-interleaved, single-phase designs where external sync is not required.