Soft switching, frequency control, and bidirectional power flow of an isolated quasi-switched-capacitor dc/dc converter for automotive applications

This paper presents the soft switching, frequency control, and bidirectional power flow of an isolated quasi-switched-capacitor (QSC) dc/dc converter which was proposed for automotive applications, connecting the high-voltage (HV) dc bus and the low-voltage (LV) dc bus. The converter employs a QSC dc/ac front-stage circuit with a 3:1 voltage step-down ratio, and a synchronous-rectifier, current-doubler post-stage circuit. In this paper: 1) a more accurate equivalent circuit of the frontstage QSC dc/ac circuit is presented, with the transformer leakage inductance integrated. The circuit is analyzed and its output dc-offset current is calculated; 2) soft switching is analyzed, and a wide ZVS range for the HV-side switches is achieved; 3) a frequency control strategy is proposed to solve the resonance problem caused by the deadband control. The operation principles are described and the output power is derived; 4) a bidirectional power flow is implemented; 5) guidelines are given to estimate key circuit parameters, including the capacitance of the switched capacitors, the transformer dc-bias flux density, and the average currents of the post-stage inductors. Experiment results from a 1 kW prototype based on SiC MOSFETs on the HV-side are provided to verify the feasibility and superior performance of the converter.