Mechanism and Optimization Method of Leakage Magnetic Field Reduction Using Active Shielding in Wireless Power Transfer

Leakage magnetic field is one of the problems of wireless power transfer, which is now used in a variety of situations. Leakage magnetic fields are feared to have adverse effects on the human body and electronic devices and must be for reducing leakage magnetic fields. Passive shielding confines leakage magnetic fields with metal or magnetic materials. Active shielding counteracts leakage magnetic fields by using magnetic fields generated by shielding coils. In this paper, it is shown that the leakage magnetic field can be reduced by shifting the phase of the current flowing in the shielding coil from that of the transfer coil. And it is also shown that the optimum phase difference differs depending on the resonance method and the location of the shielding coil. Phase difference between transfer coil and shieling coil causes leakage magnetic field reduction. The influence of the phase difference between the transfer coil and the receiver coil, and between the shielding coil and the receiver coil is negligible. The phase difference is not necessarily the optimum one to be in the opposite phase. The case of a 600 V input to the transfer coil and a shielding coil installed next to the transfer coil or around the receiver coil is considered. In all cases of S-S, Double-LCC, and LCC-S, leakage magnetic fields can be reduced to about 1 mA/m by adjusting the phase difference between the transfer coil and the shielding coil. The reduction of leakage magnetic field by the phase difference and its optimum value are shown using the theoretical equation and electromagnetic field analysis using the method of moments.