Full-Bridge LLC Resonant Converter with Series-Parallel Connected Transformers for Electric Vehicle On-Board Charger

A full-bridge LLC resonant converter with series-parallel connected transformers for an onboard battery charger of electric vehicles is proposed, which can realize zero voltage switching turn-on of power switches and zero current switching turn-off of rectifier diodes. In this converter, two same small transformers are employed instead of the single transformer in the traditional LLC resonant converter. The primary windings of these two transformers are series-connected to obtain equal primary current, while the secondary windings are parallel-connected to be provided with the same secondary voltage, so the power can be automatically balanced. Series-connection can reduce the turns of primary windings. Parallel-connection can reduce the current stress of the secondary windings and the conduction loss of rectifier diodes. Compared with the traditional LLC resonant converter with single transformer under same power level, the smaller low-profile cores can be used to reduce the transformers loss and improve heat dissipation. In this paper, the operating principle, steady state analysis, and design of the proposed converter are described, simulation and experimental prototype of the proposed LLC converter is established to verify the effectiveness of the proposed converter.