A Scheme of a Resonant Forward-Flyback Converter With Suppressed Frequency Variation

This letter proposes a simple and low-cost scheme to soft switch an isolated converter. The new circuit merges resonant forward converters and flyback converters by using shared power transformers and primary switches. Specifically, the resonant forward mode operation fulfills soft-switching for the primary switches, and the flyback mode operation resets the transformers by directly delivering the energy to the output. This also allows us a lower ratio between the leakage inductance and the magnetizing inductance. Thus, the main attraction is that simpler structure is utilized to regulate the output voltage for a resonant converter with a constant switching frequency, and higher power efficiency is achievable by taking the full use of transformer magnetizing energy. Also, an input-series-output-parallel connection is utilized to further reduce the voltage stress on the primary side. In addition to detailed theoretical analysis, experimental results verify the feasibility and effectiveness of the proposed concept.