A High Frequency Wireless Power Transfer System for Electric Vehicle Charging Using Multi-layer Non-uniform Self-resonant Coil

Wireless EV (Electric Vehicle) charging is an emerging technology with rapid development in the past decade. Compared to wired EV chargers, wireless power transfer (WPT) enables safe and unobtrusive charging for EVs.

This work proposes high frequency wireless charging using a self-resonant (SR) coil at several megahertz. A multi-layer self-resonant coil structure is proposed, allowing high quality factor coils to be fabricated from layers of inexpensive copper foil and dielectric film. Additionally, the self-resonant coil utilizes its interlayer capacitance for resonance, eliminating the external compensation capacitor and shrinking the overall volume of passive component to increase the power density. Comparing to other self-resonant coils in the literature, it exhibits the characteristics of achieving high quality factor and high inductance simultaneously.

Prototype coils with 200 mm radius are fabricated and tested, achieving quality factor over 450 at 3 MHz. The fabricated air-core coil structure is low-cost and lightweight, with 200 mm radius, 3 mm thickness and only 2 oz copper traces.

The power stages, including GaN (Gallium Nitride) transistor based inverter and SiC (Silicon Carbide) diode based rectifier, are designed with emphasis on reduction of PCB (Printed Circuit Board) layout parasitics. Experimental tests show 95.2% dc-dc efficiency with 6.6 kW power transferred across a 100 mm coil-to-coil distance. The power density is 52.5 kW/m², without need for any external compensation components. This work validates the concept of high frequency compact WPT system for EV.

Practical shielding design is proposed for the WPT system with self-resonant coils, considering the high frequency parallel resonance effect. Complete coil pads are fabricated and assembled, incorporating the ferrite cores, PTFE (Polytetrafluoroethylene) spacer, and aluminum plate. The system is validated with shielded SR coils, achieving 92.3% DC-DC efficiency and 7.1 kW/dm³ volumetric power density. This work demonstrates the first 6.6-kW WPT system using compact self-resonant coils with practical shielding implementation.

The concept of proposed multi-layer self-resonant coil is extended to other possible structures. Different multi-layer self-resonant coil structures are compared and analyzed, giving design guidelines for their capabilities at different system operating frequencies.