1.School of Electrical Engineering, Southeast University, Nanjing 210096, China;2.Jiangsu Provincial Key Laboratory of Smart Grid Technology and Equipment, Nanjing 210096, China

Lithium-ion batteries are commonly used as loads in current wireless power supply applications. The wireless battery charger should provide an initial constant current (CC) and a subsequent constant voltage (CV) output required by the battery. The additional converter at the secondary side, the hybrid topology, or the high-order network can realize CC-to-CV transition, which have problems of complicated control and poor reliability. To solve this problem, this paper proposes a three-coil wireless battery charging system for self-adapting to the battery charging curve. By adding an auxiliary coil and a passive rectifier bridge on the primary side, the system can automatically realize CC-to-CV transition without any additional control. The auxiliary coil is connected in parallel with the input source through a specific compensation network, and the output voltage threshold is not limited by the magnetic coupling mechanism parameter. Thus the self-adapting to the battery charging curve is realized. Meanwhile, the proposed three-coil wireless battery charger operates at the same frequency in both charging modes, which ensures the soft switching of the inverter and the near zero reactive power circulation of the converter, to reduce the device stress and improve the transmission efficiency. Moreover, the proposed charger has the function of automatic open-circuit protection. In this paper, the design principles and operation principle of the charger are systematically described, and an experimental prototype with 72 V/2.5 A output is built to verify the effectiveness of the proposed charger.