High-Frequency and High-Power Capacitive Power Transfer Technology
Authors:   , Wang Yao , Lu Fei , Miu Karen Nan 
Published: October 2023

Capacitive power transfer (CPT) technology offers a promising wireless power transfer approach using the coupling capacitance of metal plates, eliminating the need for metal cables. Emerging within the last two decades, CPT has demonstrated significant advantages, such as cost-effectiveness, lightweight design, absence of eddy-current losses, and excellent misalignment tolerance. However, compared to the well-established inductive power transfer (IPT) technology, CPT technology still faces challenges in insufficient theoretical research, underdeveloped system design methodology, and limited power transfer capability. This dissertation aims to advance the fundamental theoretical research of CPT and provide a comprehensive design methodology for achieving high-frequency, high-power capacitive power transfer. The main work of this dissertation mainly includes four aspects. First, this dissertation compares CPT and IPT systems in terms of coupling structures, power transfer mechanisms, compensation circuits, and system efficiency. A unified two-port parameter-based modeling and analysis methodology is developed for both CPT and IPT systems, achieving theoretical uniformity and enhancing the fundamental theoretical research of CPT technology. Furthermore, the duality between IPT and CPT circuits is also investigated, leveraging the mature IPT technology to uncover CPT opportunities, not yet reported in existing literature. Second, a universal methodology for constructing CPT systems with desirable characteristics of zero-phase angle and load-independent output is proposed. T/[pi]/M-type high-order resonant networks are proposed and employed on dual sides of four basic SS/SP/PS/PP CPT compensations, and then, 324 feasible CPT topologies are derived, encompassing all the existing CPT circuits and predicting new possibilities. Taking zero-reactive power circulation, compatibility with voltage-source inverter (VSI) and voltage-source rectifier (VSR), and system simplicity into account, 7 high-performance CPT circuits are recommended for various power levels. Particularly, the MSP-PP-MPS CPT circuit is chosen for high-power applications due to its advantageous zero-reactive power circulation properties Third, the limitations of a multi-MHz multi-kW silicon carbide (SiC) full-bridge inverter are investigated, and an inverter design methodology is proposed to improve switching performance, contributing to a high-frequency high-power CPT system design. The zero-voltage switching (ZVS) mode of the full-bridge inverter is analyzed, and the issue of oscillations in drain-source current and voltage of the inverter under ZVS operation is revealed, which emerges as a critical limitation for the inverter's functionality in multi-MHz multi-kW conditions. Inverter optimization is conducted in terms of switching device selection, gate driving circuit design, zero-voltage switching realization, and inverter layout. The implemented inverters can work from 1 MHz at 6.59 kW to 5 MHz at 1.09 kW, which creates new power and frequency records of a SiC inverter. Lastly, a high-power MSP-PP-MPS CPT system is developed based on the implemented multi-MHz multi-kW inverter. This CPT system achieves a record-breaking power transfer milestone, delivering 4.34 kW power at 3 MHz over a distance of 120 mm with a dc-dc efficiency of 94.14%. The comparison with existing state-of-art CPT works shows the advantages of the implemented CPT system in power level, minimized reactive power circulation, and suppressed voltage stress. To sum up, this dissertation explicates the theoretical uniformity between CPT and IPT technologies and demonstrates the methodology to analyze, design, and implement high-frequency and high-power CPT systems. A record-breaking 3 MHz 4.34 kW capacitive power transfer over a distance of 120 mm is achieved with an efficiency of over 94%. In future work, gallium nitride (GaN) based converters will be leveraged to further improve the power and efficiency performance of the CPT system.