Compensate Capacitor Optimization for Kilowatt-Level Magnetically Resonant Wireless Charging System

Magnetically resonant wireless power transfer (WPT) technique excels in delivering power over a relatively long distance, and WPT systems for bio-implants have been successfully developed. However, the preconception of resonant-operating results in a fact that most previous works pay little attention to the optimization of the compensate capacitors; these design methods cannot meet the newly arisen challenges when designing a high-power wireless charging system (WCS) for electric vehicles (EVs). This paper presents a design method featuring compensate capacitor optimizing for better design of practical kilowatt-level WCS. Comparison with conventional frequency tuning method is made based on equivalent circuit model analysis. Compensating characteristics of a typical WCS are studied to find out how the compensate capacitors affect the systemic performance. Furthermore, considering peculiar constraints and requirements of EV-oriented WCS, we present detailed optimizing procedure and adjustment criterion. Finally, correctness and effectiveness of the proposed optimizing method are verified by a 3.3-kW wireless charging prototype. A transfer efficiency value of 92% over 21 cm and 88.5% over 36 cm is achieved; voltages of the transmit and receive coils are simultaneously minimized and balanced for security.