Wireless resonant-type power transfer links with relay elements: Harmonic balance design

In a wireless power transmission system the oscillator power is transmitted through resonant coils over variable distances. The simplest systems are realized with direct connection between the oscillator and the resonant coil. To improve the system performance one or more relay elements can be used. In order to take into account the resonant frequency variations, which also tune oscillation frequency, a broadband harmonic balance (HB) design of the system is carried out with the oscillator load including the resonant coupling and the rectifier. The oscillator efficiency, the RF-to-DC system efficiency and the DC power are then directly optimized for any coupling distance of interest. A Royer-type oscillator is designed and prototyped using the proposed technique. The oscillator exhibits 40 W and 75% conversion efficiency while the WPT system efficiency, DC-to-DC, is better than 60% for coupling distances of the order of 10 cm.