Efficiency Analysis of 11 kW Bidirectional CLLC Converter with Symmetric and Asymmetric Resonant Frequency Design Considering Single-phase/Three-phase Compatible On-board Charger

This paper compares the efficiency of bidirectional CLLC converters under symmetric and asymmetric resonant frequency designs. Typically, CLLC converters have identical primary and secondary resonant frequencies. However, when the input and output voltage ranges or voltage gain requirements differ during charging and discharging, the operating frequency may increase excessively, thereby reducing system efficiency. To address this, a method is proposed to design a secondary resonant frequency that differs from the primary. Equivalent circuits and voltage gain equations were derived using fundamental harmonic approximation (FHA) for charging and discharging operations, and an 11 [kW] CLLC converter was designed. Loss analysis was conducted for one symmetric and four asymmetric designs, and their efficiencies were compared. Experimental validation revealed that, while a minor efficiency reduction of up to 0.38% was observed in some charging conditions, the discharging operation demonstrated improved efficiency across all conditions, with a maximum of 0.81% and an average of 0.48%.