Analysis of a Capacitor-Free Self-Resonant Coil for High Power Wireless EV Charging

This paper investigates a capacitor-free, self-resonant wireless power transfer (WPT) coil for electric vehicle (EV) charging in the SAE J2954 band (nominal 85 kHz). The proposed four-terminal, two-layer Litz-wire coil leverages its intrinsic interlayer capacitance to form a distributed LC network, eliminating external compensation capacitors and the associated cost, interconnect complexity, and reliability risks. A symmetric equivalent lumped-element model is developed to capture the coil’s resonant behavior. Finite-element analysis (FEA) quantifies the frequency-dependent resistance and examines the effects of ferrite backing, turn-to-turn pitch, and turn count. Inductance is primarily set by the number of turns, pitch, and ferrite, with a trade-off between higher inductance and increased AC losses. By adjusting these mechanical and material parameters, together with the distributed capacitance, the coil can be tuned to application requirements, demonstrating a flexible and robust design approach for high-performance EV WPT.