Current-mode PWM controller for isolated and non-isolated supplies
The UC3842D8TR is a fixed-frequency current-mode PWM controller from Texas Instruments, designed for step-up, step-down, step-up/step-down topologies including boost, buck, buck-boost, flyback, and forward converters. It drives a single transistor output with a maximum duty cycle of 97%, which is critical for maintaining regulation in high step-ratio flyback designs where the off-time must be short. Switching frequency reaches up to 500 kHz, allowing the use of smaller magnetics and reducing the overall power-stage footprint — a direct trade-off against switching losses in the MOSFET and snubber network.
The controller operates from a 12 V to 28 V supply (Vcc/Vdd), which sets the bias rail headroom. The internal undervoltage lockout (UVLO) thresholds are tied to this range — a supply below 12 V may not reliably start the oscillator. Output configuration is positive and isolation-capable, meaning the controller can drive a primary-side MOSFET in an isolated flyback while the secondary-side feedback loop crosses the isolation barrier via an optocoupler.
Package format and sourcing posture
Housed in an 8-SOIC (0.154", 3.90 mm width) surface-mount package, supplied on Tape & Reel (TR) or Cut Tape (CT). ROHS3 compliant.