High-Frequency DC-transformer Design for High Power Density Phase-Modular On-Board Chargers

This work presents the design, implementation, and experimental verification of a high-power-density, bidirectional DC-transformer (DCX) intended for a phase-modular onboard charger (OBC) supporting both grid-to-vehicle (G2V) and vehicle-to-everything (V2X) operation. Three identical isolated DCX modules, each based on 650 V GaN devices and operated at 670 kHz, are paralleled to achieve 11 kW over a wide battery voltage range (250-450 V). A planar high-frequency transformer is developed using PCB windings and flux-canceling integration, reducing core volume and core losses by approximately 12% and 19% respectively. To ensure proper current sharing among the parallel DCXs, a three-phase current-equalizing transformer (CET) is introduced, providing high differential-mode impedance to guarantee balanced operation. Experimental results confirm zero-voltage switching (ZVS), high conversion efficiency, and adequate thermal behavior across operating points. The proposed design demonstrates the feasibility of high-frequency, modular DCX architectures for next-generation OBC systems.