Analysis and design of load-independent output current or output voltage of a three-coil wireless power transfer system

The design of operating frequencies and compensation topologies for load-independent output current or output voltage in two-coil wireless power transfer (WPT) system has been widely studied. The three-coil WPT system has been proven to be more efficient and shows a better performance compared to the two-coil WPT system with an increasing transmission distance. However, the research on load-independent output voltage or output current of the three-coil WPT system is still at pilot stage. This paper systematically investigates the load-independent output voltage and output current characteristics of a three-coil WPT system to achieve a constant output voltage (CV) and constant output current (CC) with near zero reactive power, which can simplify the control. The CV and CC characteristics are achieved by well-designed passive parameters of the receiver side compensation. Additionally, a hybrid receiver side compensation with two additional switches for the three-coil WPT system is developed to charge the electric vehicle battery by combining the CC and CV characteristics. Finally, a scaled-down experimental setup with 4-A charging current and 96-V cell voltage is constructed to verify the proposed approach. The experimental results agree well with the theoretical analysis.