a reasonable cost. The research team from MIT published a
paper in Science [3], in which 60 W power is transferred at
a 2-m distance with the so called strongly coupled magnetic
resonance theory. The result surprised the academia and the
WPT quickly became a hot research area. A lot of interesting
works were accomplished with different kinds of innovative
circuit, as well as the system analysis and control [4]–[9]. The
power transfer path can even be guided using the domino-form
repeaters [10], [11]. In order to transfer power more efficiently
and further, the resonant frequency is usually selected at
MHz level, and air-core coils are adopted.