A Flexible Resonant Converter Based Battery Charger with Power Relays

Battery charger applications require constant current (CC) charging stage and constant voltage (CV) charging stage. From the output voltage profile, the converter is required to work in a wide gain range. Due to wide switching frequency operation range, the performances for traditional LLC converter are degraded. The circulating current and conduction loss for traditional LLC converter are increased to satisfy the voltage gain requirement. To improve the converter performance, a flexible resonant converter topology is proposed. With the aid of power relays, the proposed converter is operating as traditional LLC converter with resonant frequency operation during CV charge stage. During CC charging stage, the LCL-T resonant tank is connected in the secondary side to provide constant output current. Thanks to the high accuracy characteristics of two resonant tanks, almost fixed frequency operation is achieved to satisfy the charging profile. Comparisons have been made with traditional LLC converter to demonstrate the advantages. Experimental results are provided to validate the proposed converter.