Challenges and Solutions to Passive Rectification in Multi-MHz Frequency Capacitive Wireless Power Transfer Systems for Electric Vehicle Charging

This paper presents challenges and solutions to implementing a passive rectifier in a high-frequency capacitive wireless power transfer (WPT) system suitable for electric vehicle (EV) charging. For this study, a prototype 13.56-MHz 12-cm air-gap capacitive WPT system utilizing 22-cm diameter coupling plates is designed, built and tested. The system is first tested in dc-to-high-frequency ac mode with a resistive load. In this mode the prototype system transfers 895 W at an efficiency of 93%. The prototype is then evaluated in dc-to-dc mode with a full-bridge passive rectifier. In this mode the power transfer and efficiency are severely reduced. It is found that this degradation is partly due to detuning of the system's resonant frequency, caused by the parasitic output capacitance of the rectifier diodes. To retune the system, its operating frequency is changed to 13.28 MHz, allowing the performance to be partially recovered, but only up to 533 W at an efficiency of 69%. The remainder of the degradation is found to be due to the rectifier's capacitance unfavorably changing the gain and reactive compensation of the L-section matching network that connects the coupling plates to the rectifier. This paper introduces the use of an appropriate-valued inductor in parallel with the rectifier to nullify the effects of its parasitic capacitance. Incorporating this inductor, the system is able to operate at the original 13.56-MHz frequency and transmits 774 W of power at an efficiency of 81%.