Study on Impedance Matching Techniques for Capacitive Power Transfer

Abstract

While inductive power transfer (IPT) technology has become the most popular method for transferring electricity wirelessly, studies have shown that capacitive power transfer (CPT) can be a good alternative. CPT has unique features such as transferring power across a metal barrier, simple coupling structure, low standing losses, and EMI emission. Despite many compensation topologies have been introduced to improve the performance of CPT systems, there is a lack of general understanding and characterization of impedance matching networks for designing CPT systems. This thesis presents a fundamental study of impedance-matching methods that can be used for improving the CPT performance regarding power transfer capability and efficiency. First, single inductor L, double-sided LC, and double-LCLC impedance-matching networks are analyzed and characterized. The transfer functions of these typical topologies are developed, and the frequency responses of fixed-parameter systems, load-variant systems, and coupling-variant systems are evaluated and validated by simulation and experimental results. It is found that while the single inductor compensation method is the simplest, it is not suitable for a loosely coupled CPT system due to the high tuning sensitivities; whereas double LC and LCLC topologies better cope with circuit parameter variations, especially under poor coupling conditions. An improved single inductor L impedance-matching technique is introduced using a dc-dc converter on the pickup side for load impedance transformation. Numerical analysis and experimental results have demonstrated that the dc-dc converter operating in buck mode can also effectively increase the system bandwidth and reduce the circuit sensitivity. Furthermore, a detuning method for the single inductor compensation option is proposed by introducing a new index named detuning factor (kr). The maximum power transfer and corresponding power efficiency of the system are analyzed and practically measured. The results have shown that a CPT system with a higher detuning factor (kr) will lead to lower maximum power output compared to the fully tuned condition; however, the corresponding power transfer efficiency is increased. This finding is useful to trade off the maximum power transfer and efficiency in designing a practical CPT system. Finally, a closed-loop CPT system is proposed by combining the detuning method and dc-dc converter to track the maximum power output with high efficiency. A controller is designed based on the perturb and observe algorithm to dynamically achieve impedance matching. Experimental results have shown the maximum transfer of 10 W is maintained across an effective coupling capacitance of 1nF against load variations from 5 Ω to 500 Ω. And a power efficiency of 70% is achieved across the full load variation range.