A zero-current-switching PWM flyback converter with a simple auxiliary switch

A simple and effective approach of turning an isolated hard-switched converter design into a soft-switched one is presented. By adding an auxiliary winding, switch and small capacitor to the conventional pulsewidth modulation (PWM) isolated flyback converter, all switches and diodes are softly turned on and off. No extra active or passive voltage clamp circuit is needed to suppress voltage stress on the switching devices that were usually found in classical converters. A zero-current-switching (ZCS) PWM flyback converter topology with multiple outputs is analyzed and examined. The proposal inherently utilizes the leakage inductance of the "flyback" transformer to achieve ZCS of all switching devices. A complete steady-state DC analysis and the operating principle are described. The performance of an 80 W experimental converter prototype with dual-voltage outputs is included.