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Improved Magnetic Design for 12.5 kW CLLC Resonant dc-dc On-board Charger

Three-phase Bi-directional Converter
Fig. 1. Three-phase Bi-directional Converter
High power and high-density battery charging converters are gaining popularity in recent years thanks to the achieved advantage of reduced size and high efficiency. In particular, the three-phase dc/dc converter has the added benefit of reduced current stress on switching devices. However, few applications take advantage of integrating magnetics components. The implementation of wide-bandgap devices provides the opportunity for increasing switching frequency. The higher frequency makes printed circuit board (PCB) transformers a possibility. In this work, an improved magnetic design is introduced for the 12.5 kW conventional LLC resonant converter. The improved magnetic structure has lower loss both for the transformer winding and the soft ferret core. Shielding technique is implemented to enhance common mode (CM) noise rejection performance and to improve reliability additional design consideration is committed to thermal management for the transformer structure. By improving the magnetic structure, the PCB based transformer reaches a higher power capacity without additional thermal constraint, enabling the converter to achieve a higher power level and higher power density. The three-phase bi-directional dc/dc converter, shown in Fig. 2, is designed to operate with a 550-800 V variable DC bus. The bi-directional operation of the converter is achieved with the help of a resonant component on both the primary and secondary side of the converter. With magnetic integration, the converter is able to operate with six core legs. The primary and secondary side mirror ensure identical gain characteristics for both forward and reverse operation.
Hardware Prototype
Fig. 2. Hardware Prototype for 12.5kW Bi-directional CLLC converter