Analysis, design and control of a resonant forward-flyback converter

A resonant forward-flyback converter is proposed in this paper, in which the high efficiency resonant forward part delivers most of the power and the flyback part achieves tight regulation and magnetic resetting. Operating characteristics are analyzed in detail. As the key design parameters, this paper studies the influence of transformer's turns-ratios on component stress and power loss. A minimum duty cycle control strategy (Dmin-fs) is proposed to guarantee ZCS of forward diode and valley switching. An experimental prototype of 400V input 48V output is built to verify the analysis. The prototype achieves a high efficiency of 96 % at 120W output.