Reducing unwanted emissions of a wireless power transfer coil by using a parasitic loop element

This paper reports on a coil structure that reduces magnetic fields formed in the surrounding area by providing a parasitic loop element with added parallel resonance circuit around the inner exited coil. It was shown that magnetic field strength can be reduced by approximately 30dB by superposition of the magnetic fields formed by currents flowing in the inner coil and the parasitic loop element. It was also shown that, by matching the resonance frequency of the parallel resonance circuit with the power transfer frequency, it is possible to suppress degradation of power transfer efficiency due to installation of the parasitic loop element.