Soft Switching and Optimal Resonance Conditions of APWM HB Flyback Converter for High Efficiency under High Output Current

Abstract

The soft switching and optimal resonance conditions of the asymmetrical pulse-width modulated (APWM) half-bridge (HB) flyback converter for high efficiency under high output current are proposed. This converter can operate under high efficiency by using soft switching techniques such as the zero-voltage switching (ZVS) of primary power switches and the zero-current switching (ZCS) of output rectifiers. Under high output current condition, however, the converter satisfied with the ZCS of output rectifiers does not always have higher efficiency than no ZCS cases. The ZCS condition grows the rms value of the output rectifier current to finish its resonance operation transferring power from the primary side to the secondary. This high rms current increases the conduction loss of the transformer's secondary wire and output rectifier, especially the synchronous rectifier (SR). Therefore, the resonance should be considered by several factors such as the conduction loss of the output rectifier and its reverse recovery loss. Experimental results using a 300 W APWM HB converter are obtained to verify the suggested efficiency-optimal conditions by demonstrating the efficiency data of the converter under load variations.