A methodology to achieve the maximum transfer efficiency for magnetic coupling wireless power transfer systems

Transfer efficiency is one of the most significant indexes of magnetic coupling wireless power transfer (MC-WPT) systems, which is severely affected by frequency splitting. In this paper, a novel method is proposed for MC-WPT systems to achieve the maximum transfer efficiency by introducing and controlling the circuit detuning factors while maintaining the operating frequency at constant. Firstly, the circuit model of tri-coil MC-WPT systems is established along with the derivation of some key concepts, including the transfer efficiency and circuit detuning factors. Then the analytical expressions of the optimal circuit detuning factors, maximum transfer efficiency and a comprehensive discriminant which could quickly and accurately determine the critical value for every system parameters are derived. Further, the physical interpretation of the maximum transfer efficiency is firstly elucidated from the perspective of energy flow: (1) when the system is in non-splitting region, the energy flow path of the maximum transfer efficiency is transmitter–relay–receiver; (2) when system is in splitting region, the energy flow path of the maximum transfer efficiency is directly from the transmitter to the receiver. Finally, simulation models and practical prototypes are designed to verify the theoretical results. The simulation and experimental results show that the proposed system can maintain an almost constant maximum transfer efficiency over a wide range of transfer distance. The derived discriminant is very useful for guiding the system design in the future, and the energy flow analysis method will be beneficial for maximizing the efficiency for multi-coil MC-WPT systems.