Optimization of Time-Weighted Average Efficiency for Reconfigurable IPT Battery Charging System

Constant current (CC) and constant voltage (CV) combined charging is considered as the most popular and efficient method for charging batteries. There are a lot of works done to achieve CC/CV characteristics by utilizing reconfigurable circuits for easy control but without the consideration of efficiency optimization in the CV mode. In this paper, the optimization of mutual-inductance proportion between transmitter/receiver mutual inductance and transmitter/intermediate mutual inductance of the three-coil circuit is investigated to maximize the time-weighted average efficiency (TWAE). With two switches, a section of receiver coil of the series-series (SS) circuit, which corresponds to the optimized mutual inductance proportion, can be converted into an intermediate coil of the three-coil circuit. Therefore, the system can be transferred from the CC mode to CV mode, and with the optimized mutual-inductance proportion, the efficiency in the CV mode can be improved. An experimental prototype is built to validate the feasibility of the proposed approach. In the CC (CV) mode, the impedance range is 30-108 Ω (108-500 Ω), and the maximum fluctuation for current (voltage) is 3.37% (4.55%). Besides, the TWAE is 94.5% (maximum 95.3% and minimum 94.49%) for the three-coil circuit, while that of SS circuit with control is 91.54% (maximum 95.20% and minimum 85.31%).