A Transformer-Isolated Common-Mode Active EMI Filter Without Additional Components on Power Lines

This paper proposes a new structure of transformer-isolated active EMI filter (AEF) without additional components on power lines for the reduction of common-mode (CM) conducted emissions (CE). The noise attenuation performance and loop gain are rigorously analyzed, and the design guides for the proposed AEF are developed. The AEF is designed in a compact package for the application to a switching mode power supply. The performance, stability, and leakage currents of the AEF are experimentally validated by various measurements. The CM EMI filter, including the AEF, achieves the additional CE reduction by 10 to 20 dB at a frequency range lower than 1 MHz. Also, the noise peaks due to the resonances at several MHz are reduced by 7 dB. The weak points of the conventional CM EMI filter at both low and high frequencies are effectively improved by employing the proposed AEF.