Efficiency Analysis of Magnetic Resonance Wireless Power Transfer With Intermediate Resonant Coil

This letter presents an efficiency analysis of a magnetic resonance wireless power transfer (WPT) system with an intermediate resonant coil. A helical coil and a spiral coil with an additional capacitor are considered as resonant coils for the WPT system. The intermediate resonant coil is set up coaxially and perpendicular to both the Tx and Rx resonant coils in order to observe the efficiency change according to the directions. The power efficiency is calculated using the temporal coupled mode theory (CMT). Impedance matching conditions are also shown by using the CMT. Analysis results show that using an intermediate coil properly improves efficiency and extends the distance between the transmitter and receiver. Both calculated and measured efficiencies are in good agreement. It is also shown that the intermediate resonant system has a good efficiency and is superior to nonintermediate systems.