T07.5 - Three-Winding HF Transformer Design for a Multiport System Integrating Battery Storage or Renewables with the Grid for EV Charging

This paper presents the optimized design of a high-frequency three-winding transformer for a triple active bridge dc–dc converter used in an unfolding-based grid-tied multiport electric vehicle (EV) charging system. The appropriate spacing between the transformer windings is determined using the H-field distribution, which relates the winding placement to the resulting leakage inductance. The accuracy of this estimation is further improved by applying the Rogowski correction factor, and the resulting spacing is implemented in hardware to achieve the desired leakage inductance. To ensure compatibility with a wide range of EV battery voltages from 200 to 800 V, an LCL resonant tank is employed, and the transformer winding on the EV side is designed to minimize the reactive-to-active power ratio of the tank over this entire voltage range. The transformer design is first validated using ANSYS MAXWELL, then fabricated and tested with a 5 kW, 100 kHz multiport converter prototype. Measured leakage inductance closely matches analytical predictions, and the experimentally obtained coupling and inductance matrices show strong agreement with finite element analysis results.