Theoretical Limits and Optimal Operating Frequencies of Capacitive Wireless Charging Systems

This paper presents a framework to determine the theoretical limits of maximum power that can be delivered by capacitive wireless power transfer (WPT) systems suitable for electric vehicle (EV) charging and their optimal operating frequencies. The limits on system performance imposed by physical constraints such as air breakdown, application-specific constraints such as allowable fringing field levels and constraints imposed by the semiconductor devices such as device thermal limits are studied. The proposed framework is used to predict and compare the maximum power transfer capability of different capacitive WPT systems designed to charge EVs operating at different frequencies and also find the optimal operating frequency as a tradeoff between power, efficiency, and physical size of magnetics. The analytical framework is validated using a 13.56-MHz 12-cm air-gap prototype capacitive WPT system that transfers 1 kW power with a dc-dc efficiency of 86%.