Bi-Directional High-Conversion-Ratio CLLC Resonant Converter with a New Synchronous Rectification Scheme for Low Conduction Loss

This paper proposes a new synchronous rectification (SR) scheme for frequency-modulated bi-directional CLLC resonant converters. By introducing a small-phase-shift between the two full-bridges and utilizing the switching transient, the proposed scheme effectively avoids the magnetizing current being provided from the low voltage side. This leads to considerable reduction in the conduction losses and current stress of the switches in a high-step-down design. Furthermore, the output voltage characteristic is analyzed using a novel mathematical model, which, comparing to the fundamental harmonic approximation method, can not only acquire a more accurate result of steady-state characteristic, but also calculate an accurate set of initial values for the passive components, which is crucial to determine the proper dead-time and phase-shift. Based on the new model, the boundaries of dead-time and phase-shift are discussed in detail. Finally, a prototype is built, and experiments are conducted to verify the principles of the proposed scheme.