Analysis and Design of Three-Coil Structure WPT System With Constant Output Current and Voltage for Battery Charging Application

Compared with the traditional plug-in charging system, the wireless charging system for battery charging has broad application prospects due to its significant advantages, such as security, convenience, and aesthetics. In practical applications, in order to prolong the battery lifecycles, it is preferred to charge the battery with constant current (CC) and constant voltage (CV) modes. However, since the battery equivalent resistance varies greatly during charging, it is not easy to design a complete charging system owning CC and CV output characteristics. Besides, the equivalent input impedance of the system is preferably resistive to improve efficiency and enhance power transfer capability; therefore, achieving the zero phase angle (ZPA) operation is extremely important. Hence, a novel three-coil structure WPT system is proposed in this paper to solve the above issues. A comprehensive theoretical analysis for the three-coil system to realize the CC and CV charging modes with perfect ZPA operation at two fixed operating frequencies is presented. Furthermore, due to the parasitic losses of passive components and the instability of the dc input voltage, it is unrealistic to achieve accurate and stable CC and CV outputs through the inherent properties of the circuit itself. Consequently, a simple closed-loop controller is introduced into the system to enable the desired CC and CV charging characteristics with zero voltage switching (ZVS) operation by slightly adjusting the operating frequency. Finally, a confirmatory experimental prototype with 4.6-A charging current and 56-V charging voltage is fabricated to confirm the feasibility and validity of the proposed method. The experimental results agree well with the theoretical analysis.