Nonresonant Compensation Optimization for Efficiency Improvement of Wireless Power Transfer System With Relay Coil

Relay coils can effectively increase the transmission distance of wireless power transfer (WPT) systems. However, the poor transmission efficiency hinders the application of this structure. Based on the modeling of series-compensated WPT systems with relay coils, this study analyzes the drawbacks of the conventional compensation method and reveals that system efficiency may not be optimal when resonance is achieved in each coil. Then, a nonresonant compensation method for efficiency improvement and better output performance is proposed. With this method, the compensation parameters of the relay and receiving coils can be optimized to minimize losses in the coupling coils and power devices, while the compensation parameter of the transmitting coil is determined by the zero-voltage switching demand of the high-frequency inverter. Furthermore, the applied performance of the proposed method is investigated by finite element analysis considering misalignment tolerance. Finally, experimental validation is performed on a 1-kW WPT prototype. The results show that, compared with the conventional system, the dc-dc efficiency can be improved by 3% and 2.5%, under alignment and 200-mm misalignment, respectively.