The High Frequency Magnetic Field Emissions Produced by a Wireless Power Transfer System Employing DD Polarized Couplers

The electromagnetic compatibility (EMC) of magnetic field wireless power transfer (WPT) systems for vehicular applications is currently under consideration. We examine the high-frequency magnetic field emissions of one particular coupler (primary/secondary winding assembly) design, the DD topology, which is described in the SAE J2954 standard. Operation of this coupler at the fundamental frequency has previously been analyzed. The analysis here concerns frequencies well above the 85 kHz fundamental. Emissions in this range are typically due to harmonics produced by the H-bridge and the rectifier as well as short-time-scale ringing which in turn is due to parasitic capacitance and inductance, and are present at some level in all switched-mode WPT systems. At the fundamental frequency the extraneous field of the DD coupler is largely that of a horizontal magnetic dipole with an admixture of a high-order multiple as has been shown. Because of the strongly dispersive nature of the power Mn-Zn ferite employed in the coupler, it is not obvious how it will behave at high frequencies. It is shown that the ferrite material, despite being overwhelmingly lossy in the dispersion region, has a profound effect on the extraneous field in this frequency range. It is also shown that a parasitic horizontal dipole moment orthogonal to the principal moment rotates the extraneous field pattern some 17 degrees. It is argued that this is due to the spiral winding of the coupler and this behavior is consistent with earlier analyses of other spiral couplers.