Current-mode PWM controller for isolated and non-isolated topologies
The Texas Instruments UC2844D8 is a current-mode PWM controller in an 8-SOIC package, designed for step-up, step-down, and step-up/step-down converter topologies including boost, buck, flyback, and forward converters. It switches at up to 500 kHz and delivers a single transistor-driver output with a maximum duty cycle of 48%, making it suited for medium-frequency isolated power supplies where duty-cycle clamping prevents transformer saturation. The controller operates from a 12 V to 28 V supply rail and includes frequency control as a programmable feature, giving the designer direct adjustment of the oscillator timing via an external RC network.
Switching frequency and duty-cycle limits affect transformer design
The 500 kHz switching ceiling sets the practical transformer or inductor size — at this frequency, a flyback transformer for a 5 W isolated supply fits in an EP7 or EE8 core, but the 48% max duty cycle limits the input-to-output voltage ratio in a buck or flyback stage. Because the controller lacks clock synchronization and synchronous rectification, the design relies on an external freewheeling diode or Schottky for the output rectifier; the switching frequency must be chosen to keep the diode's reverse-recovery losses within the thermal budget. The output configuration is positive and isolation-capable, meaning the controller can drive a primary-side FET while the secondary-side regulation loop is closed through an optocoupler or transformer winding — typical for offline flyback supplies.