This paper presents an optimally soft-switched modular capacitive wireless power transfer (CWPT) system for electric vehicle (EV) charging. Multi-MHz CWPT systems must operate with zero-voltage switching (ZVS) of their inverter and rectifier transistors to avoid excessive losses and thermal instability. Two different techniques to achieve ZVS, one with an additional series inductor and the other with an additional shunt inductor, are considered. For both these techniques, the required additional inductance and dead time between switching transitions are determined optimally so as to achieve ZVS of the inverter transistors with minimized reactive power in all the modules. The efficiency implications of both the approaches are formulated, and it is shown that while efficiency remains nearly constant with varying number of modules, the shunt inductor approach consistently provides a higher efficiency. A 6.78-MHz, 2-kW, 15-cm air-gap two-module CWPT prototype is designed using the shunt inductor approach, and the desired soft switching is shown to be achieved.
