Study on efficiency maximization design principles for Wireless Power Transfer system using magnetic resonant coupling

Wireless Power Transfer (WPT) brings convenience and safety in many applications and serves as a research hot spot in recently years. Magnetic resonant coupling is widely implemented in WPT applications such as mobile devices and electric vehicles where large distance, large power amount and high efficiency are the three key requirements in real application. However, there are always trade-offs between these requirements even in theory. Former literatures failed to illustrate the complete relationships between these requirements. Based on the phasor analysis, this paper illustrates the operating principle of the whole circuit, attains the equivalent circuit models of the system and derives the physical essence of frequency characteristics. Then Maximum Efficiency Conditions (MEC), which achieves the maximum efficiency without sacrificing the requirements for power transfer distance and power amount, is summarized. It should be noted that the highest efficiency is achieved at the natural frequency of the receiver, instead of any split frequencies of the coupled resonances. Following the MEC, a WPT prototype was designed, which was composed of a full bridge inverter, a LC resonant transmitter, a LC resonant receiver and a full bridge rectifier. The resonant frequency of the transmitter was designed to be slightly lower than the inverter operation frequency-446 kHz to make an inductive load for the inverter so that all the MOSFETs operated in Zero-Voltage-Switching (ZVS) condition. For the experimental results, 300 W output power was obtained over a distance of 22 cm with 84% overall efficiency.