Current-mode PWM controller for flyback and forward converters
The UC3842ADW is a current-mode PWM controller from Texas Instruments, designed for single-ended topologies including boost, buck, flyback, and forward converters. It delivers a single transistor-driver output with a maximum duty cycle of 96% and a switching frequency that can be set up to 500 kHz.
500 kHz switching — transformer size vs. loss trade-off
Switching at up to 500 kHz lets you shrink the magnetic components — a flyback transformer at 100 kHz might be three times the core volume of one running at 500 kHz. The trade-off is higher switching losses in the MOSFET and snubber dissipation; the UC3842ADW's current-mode architecture gives cycle-by-cycle current limiting that helps keep the switch within its SOA during transients. The 96% maximum duty cycle is useful in boost and flyback stages where the input voltage is well below the output — for example, a 12 V input to a 48 V output flyback can run near full duty without pulse-skipping, which keeps the output ripple predictable.
The undervoltage lockout (UVLO) threshold is typically 8.4 V on the UC3842 family, so the controller stays off until the bias supply is stable — no partial startup glitches.
16-SOIC package — fits a standard footprint
Housed in a 16-SOIC with 7.50 mm body width, the UC3842ADW uses the same footprint as the industry-standard UC3842 SOIC-16 layout. The wide body gives decent creepage between the high-voltage sense pins and the low-voltage control pins — important in flyback designs where the primary-side ground reference is noisy.
The UC3842ADW can be specified into new BOMs without a near-term end-of-life risk.