Design and Analysis of CPT System with Wide-Range ZVS and Constant Current Charging Operation Using 6.78 MHz Class-E Power Amplifier

High frequency is beneficial to reduce the volume and interplate voltage for capacitive power transfer (CPT) systems. The Class-E power amplifier (PA) is a suitable high-frequency inverter because of its fewer components and higher reliability. However, the output current and zero-voltage switching (ZVS) of the Class-E PA-based CPT system is affected by load variations. In this article, a 6.78 MHz Class-E PA-based CPT system with wide-range ZVS and quasi-constant current (CC) output is proposed. The rectifier, capacitive coupler, and Class-E PA are modeled. Based on these models, the influence of the rectifier's diode junction capacitances on its input impedance is analyzed. The relationship between the Class-E PA's equivalent load and the secondary-side impedance is given. A general ZVS boundary of Class-E PA with complex impedance load is studied, and the influence of diodes on the CC output of the system is researched. Finally, the parametric design and diodes selection methods aiming at wide-range ZVS and quasi-CC output for the system are proposed. A 144 W prototype is built and experimental results show the proposed system maintains ZVS operation and the fluctuation of output current is 6.1% when the load changes between 10 and 135~Ω. The peak efficiency achieves 90.5%.