A Forward Converter with Separated Resonant Tanks and Mutually Affected Resonance

This paper proposes a novel LLC-type resonant forward structure that achieves wide range operation with high efficiency. The proposed converter employs an Input-Series-Output-Parallel (ISOP) configuration by connecting two resonant tanks. This configuration allows the adjustment of the equivalent resonant state of the converter based on the phase difference between the two series-connected resonant tanks. The performance for step-down conversion is enhanced by the series connection on the input side, addressing the requirement for wide operation. To achieve wider voltage regulation, the structure adopts a mixed control method, combining adjustable turning-off time control and phase-shifted control. This approach eliminates the requirement for frequency modulation and offers improved efficiency, compared to conventional pulse frequency modulation (PFM). Furthermore, the LLC-type operation enables soft switching for main switches and secondary diodes, contributing to overall efficiency enhancement. In additional to detailed theoretical analysis, a 150W laboratory prototype validates the feasibility and effectiveness of the proposed concept.