Feed-Forward Compensator of Operating Frequency for APWM HB Flyback Converter

A feed-forward frequency compensator for an asymmetrical pulsewidth modulated (APWM) half-bridge (HB) flyback converter is proposed to increase its power conversion efficiency. Under a variable input voltage, the APWM HB flyback converter cannot be satisfied with the zero-current switching (ZCS) condition of output rectifiers because the variation of the turn-ON duration of main switches, which regulate the output voltage, induces their improper turn-OFF duration. This means that the resonance of a primary current during the turn-OFF of the switches is out of the ZCS operation of the output rectifiers. To avoid this improper operation, the resonant network should be designed with a marginal turn-OFF duration that guarantees the ZCS. However, this makes the converter operate with poor efficiency because of increasing conduction losses. The proposed feed-forward frequency compensator can properly change the converter´s operating frequency to reduce the conduction losses according to the variation of the input voltage satisfying the ZCS condition of the output rectifiers. Operation principles, steady-state analysis, and soft-switching conditions are analyzed to derive the frequency compensation algorithm of the APWM HB flyback converter. Experimental results using a 240-W prototype converter show that the proposed compensation algorithm can improve the power conversion efficiency of the APWM HB flyback converter under the input-voltage variation.