Optimal design of line level control resonant converters in plug-in hybrid electric vehicle battery chargers

The series-parallel resonant converter (also called line level control (LLC) resonant converter) is one of the most suitable topologies for dc-dc power supply. This study introduces the LLC resonant converter into the on-board battery chargers for the plug-in hybrid electric vehicle (PHEV) applications. Different from the previous literature which has focused on the wide operation range and hold-up time requirement in the LLC design, this study is mainly focused on battery load characteristics and its impact on the charger design. First, to guarantee high efficiency in the light-load condition in the constant voltage charging stage, the optimum LLC switching frequency range is derived. Second, considering the constant current charging function in the battery charger, the impact of peak load current on the LLC converter is discussed. The boundary between zero-voltage switching (ZVS) and zero-current switching (ZCS) in the constant current charging application is analysed. The trade-off among the minimum load voltage, maximum charge current and resonant capacitor is studied in detail. Finally, the optimal design method for the LLC resonant converter used in the PHEV battery charger is proposed. The proposed methods are validated through experiments on a 400 V/6 kW PHEV charger system with 97% efficiency.