Circuit Modeling of a Wireless Power Transfer System Containing Ferrite Shields Using an Extended Impedance Expansion Method

The impedance expansion method (IEM) is a circuit modeling technique based on the method of moments (MoM). This study extended the conventional IEM to be applied to wireless power transfer (WPT) systems containing dielectric/magnetic (DM) bodies such as ferrites. The advantages of the extended IEM are that it can consider arbitrarily shaped DM bodies, self-resonant coils, and radiation loss, and that the equivalent electromagnetic currents on DM bodies are not treated as unknowns in the resulting circuit model. Numerical experiments revealed that the extended IEM is applicable up to the frequency at which the dimensions of the DM bodies are approximately 0.3 times the wavelength of electromagnetic waves inside the medium. An equivalent circuit model of a WPT system containing ferrite shields was obtained using the extended IEM and validated using the full-wave MoM and the finite-difference time-domain (FDTD) method.