Frequency, Impedance Characteristics and HF Converters of Two-Coil and Four-Coil Wireless Power Transfer

Magnetically coupled wireless power transfer (WPT) can be roughly divided into two kinds of inductively coupled and strongly coupled magnetic resonances. In this paper, the frequency characteristics, impedance characteristics and high frequency (HF) converters of two-coil and four-coil magnetically coupled WPT are outlined between inductively coupled power transfer (ICPT) and resonant coupling WPT. First, the frequency bifurcation in ICPT and frequency splitting in resonant coupling WPT are analyzed, and the natural similarities are explained, which have not been pointed out before. Second, the differences of the conventional two-coil (often used in ICPT) and four-coil WPT are analyzed by their input impedance characteristics. The results show that the two-coil WPT is appropriate for short-range and the four-coil WPT suitable for mid-range applications. In the end, as one of the most important parts of magnetically coupled WPT systems, HF converters used in WPT systems are summarized and a new soft-switched converter is put forward, which has 70% efficiency though it is not precisely designed.