Analysis and Modeling of the Common-Mode Conducted EMI From a Wireless Power Transfer System for Mobile Applications

Wireless power transfer (WPT) is a safe and convenient technology for battery charging thanks to the elimination of electrical contact. Despite the great advantages, the electromagnetic interference (EMI) noise generated by a WPT system coulddegrade the performance of nearby electrical circuits. In particular, common-mode (CM) EMI noise generated by the WPT system will conduct along the cable and radiate in high frequencies, thus becoming one of the concerns. In this article, the noise source and current path of the conducted CM EMI noisefrom a Qi-compliant WPT system for mobile applications are analyzed. The transfer function relating the CM EMI noise to the noise source voltage is derived analytically in the frequency domain. The noise source voltage is measured in the time domain and then converted to the frequency domain via fast Fourier transform. The conducted CM EMI spectrum is predicted by multiplying the spectrum of noise source voltage by the transfer function. The prediction matches well with measurement. The analysis and modeling explain the mechanism of the CM EMI noise and provide guidelines to reduce the CM EMI noise.