I. Introduction
With the development of modern industry, energy and environmental problems become increasingly prominent. A demand for development and popularization of electric vehicles is more urgent. In terms of convenience, a noncontact wireless charging technology is more applicable, which is divided into three categories, namely, electro-magnetic induction, microwave, and electromagnetic resonance [1]. Based on the principle of electromagnetic induction, power energy can be transported and received at a certain distance without the need for a ferromagnetic or ferrite core transformer [2]– [4]. The operating frequency of the coils is closely relevant to its ac resistance in the wireless power transmission system, and its power transmission efficiency is also closely relevant to the quality factor of the coils. In such a high frequency, the ac resistance of the coil should be reduced as much as possible for increasing the quality factor and thereby improving the power transfer efficiency [5]–[6].