Optimization of a Passive Common Mode EMI Filter by Adding an Active Feedback Loop

In this paper, a hybrid common mode EMI filter is presented. The EMI filter used consists of a passive EMI filter with an added active feedback loop. The passive components are not only used for the noise attenuation but also for measuring the noise and inserting the feedback signal. The noise is measured at the common mode choke on an additional winding and is fed back inversely amplified via the Y capacitors. This technique reduces the unwanted noise current by inserting a counter current. With the topology used, the size of the common mode choke can be reduced significantly in comparison to a pure passive EMI filter. The calculation of the proposed hybrid filter design as well as the implementation and noise measurements are shown in this paper. The hybrid EMI filter is compared with a passive EMI filter to display its advantages.