Spurious electromagnetic emissions from a Magnetic Field Wireless Power Transfer system

Magnetic Field Wireless Power Transfer (MF-WPT) is appropriate for high-power applications such as the charging of electric vehicle batteries. Most systems operate essentially as resonant converters with the couplers (primary and secondary windings) and matching networks absorbed into the tank circuit of a resonant converter topology. As in a well-designed resonant converter, the currents in the couplers are quasi-sinusoidal. The extraneous electromagnetic field of such a system viewed in the frequency domain naturally includes a strong contribution at the fundamental frequency. However, it also necessarily includes components due to rectifier harmonics and short-time-scale ringing both in the inverter and rectifier circuits. We examine the relationship between the primary and secondary coupler currents and the extraneous electromagnetic field. It is seen that the electromagnetic field at some distance from the system has a spectral character markedly different from that of the primary and secondary currents. This is due to the fact that the induction and radiation fields of magnetic multipoles (predominantly dipole and quadrupole) depend not only on the associated magnetic moments but also on frequency. While the static fields depend directly on moment which, in turn, depends on current magnitude, the quasi-static induction and the radiation fields depend on the product of moment and a non-zero power of frequency.