Design of 500 kHz, 18 kW Low Leakage Inductance Intraleaved Litz Wire Transformer for Bi-directional Resonant DC-DC Converter

This paper focuses on the design of a 18 kW, 500 kHz, 8:4 planar E core litz wire transformer for a bi-directional CLLC resonant dc-dc converter. In order to reduce gain variance around the resonant frequency, the transformer design focuses on minimizing leakage inductance. Multiple winding topologies are surveyed and the selected P-S1-P-S2 winding topology achieves primary leakage inductance of 747 nH and secondary leakage inductance of 120 nH. In order to improve current sharing among two parallel windings, an "intra-leaved" winding structure is proposed which achieves a current distribution of 42 A(Peak) for S1 and 38 A(Peak) for S2 while maintaining the low leakage inductance benefits of the P-S1-P-S2 structure. The transformer winding is potted in thermal epoxy to improve winding heat spreading and peak winding temperature is 126 degrees C at 18 kW load. An easy-to-use ac core loss measurement technique is demonstrated which minimizes measurement error due to transformer ac resistance. The designed transformer achieves 99.6% efficiency at 18 kW load and a power density of 1720 W/in(3).