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Abstract:Conditions for load-independent output voltage or current in two-coil wireless power transfer (WPT) systems have been studied. However, analysis of load-independent output current in three-coil WPT system is still lacking in previous studies. This paper investigates the output current characteristics of a three-coil WPT system against load variations, and determines the operating frequency to achieve a constant output current. First, a three-coil WPT system is modeled by circuit theory, and the analytical expression of the root-mean-square of the output current is derived. By substituting the coupling coefficients, the quality factor, and the resonant frequency of each coil, we propose a method of calculating the frequency for load-independent output current in a three-coil WPT system, which indicates that there are two frequencies that can achieve load-independent output current. Experiments are conducted to validate these analytical results.

Multiple-receiver wireless power transfer (MRWPT) systems have revolutionary potential for use in applications that require transmitting power to multiple devices simultaneously. In most MRWPT systems, impedance matching is adopted to provide maximum efficiency. However, for most MRWPT systems, achieving target power levels and maximal efficiency is difficult because the target output power and maximum efficiency conditions are mostly not satisfied. This study establishes a target power control (TPC) strategy to balance providing target transfer powers and operating under high efficiency. This study is divided into the following points: First, this study derives the optimal mutual inductance to verify that it’s difficult for two-receiver wireless power transfer (WPT) system to achieve both maximum efficiency and power distribution simultaneously; Second, this study illustrates that for impedance matching method the mutual inductances play a more important role than equivalent impedances in increasing the system efficiency, and hence system should give priority in improving the mutual inductance as large as possible; Third, this study proposes a simplified system model which helps to derive the analytic solutions of equivalent impedances; Fourth, this study developed a 100-kHz two-receiver WPT system and establishes a TPC strategy for enabling the system to achieve target output power levels with high efficiency; At last, the proposed system is proved to achieve an efficiency level of more than 90 % and satisfies the target output power levels requirements.