An Ultrawide Output Range ${LLC}$ Resonant Converter Based on Adjustable Turns Ratio Transformer and Reconfigurable Bridge

This article proposes an ultrawide output range LLC resonant converter based on adjustable turns ratio transformer and reconfigurable bridge. Due to the reconfiguration of the transformer by two four-quadrant switches, the transformer exhibits three different effective turns ratios, which correspond to three output ranges. Moreover, in case of lower output voltage, the primary side is reconfigured as a half-bridge to further extend the output voltage range. The design of resonant tank parameters is less constrained by the voltage gain requirement. The switching frequency range in each mode can be squeezed close to the resonant frequency, which contributes to an improved soft switch performance. The operational principles, design considerations, and mode transition are analyzed. A 4 A rated prototype that converts 390 V input to a 60-450 V output is designed and tested to validate the concept. A smooth mode transition is achieved without transient issues. The design prototype demonstrates 97.50$\%$ peak efficiency and good overall efficiency performance over this ultrawide output range.