High-performance large air-gap capacitive wireless power transfer system for electric vehicle charging

This paper introduces a high-performance large air-gap capacitive wireless power transfer (WPT) module as part of a multi-modular capacitive WPT system for electric vehicle charging. This WPT module utilizes two pairs of metal plates separated by an air-gap as the capacitive coupler, incorporates L-section matching networks to provide gain and reactive compensation, and is driven by a GaN-based inverter operating at 6.78 MHz. The system achieves high efficiency and simplicity by eliminating the need for high-voltage capacitors, and instead utilizes the parasitic capacitances formed between the coupling plates and the vehicle chassis and roadway as part of the matching networks. This paper also presents a comprehensive design methodology for the capacitive WPT system that guarantees high performance by ensuring zero-voltage switching of the inverter transistors, and by selecting matching network component values to maximize efficiency under practical constraints on inductor quality factor and self-resonant frequency. Two prototype 6.78-MHz 12-cm air-gap capacitive WPT systems have been designed, built and tested. The first prototype with 625 cm
²
coupling plate area transfers up to 193 W of power and achieves an efficiency greater than 90%, with a power transfer density of 3 kW/m
²
. The second prototype with 300 cm
²
coupling plate area transfers up to 557 W of power and achieves an efficiency of 82%, with a power transfer density of 18.5 kW/m
²
, which exceeds the state-of-the-art for capacitive WPT systems by more than a factor of four.