An Active EMI Filter for Robust Integration in Medium and High-Power AC/DC Converters

The filters of power converter systems typically have to attenuate electromagnetic interference (EMI) to a very large degree. This makes them bulky and heavy. Active EMI filters (AEF) can reduce the filter size and weight. Feedforward (FF) AEF topologies are best suited for good stability at varying grid impedances. Until now, FF topologies were discussed only for low-power systems, where the parasitic behavior of the circuit elements can be controlled easily. This contribution introduces an FF-AEF design that can be used in medium and high power three-phase ac-dc converter systems with varying grid impedances without the need to redesign the AEF. To improve the performance, a second-order high-pass filter is added and a current source for injecting the compensation current is optimized for compensating the parasitic inductance of the injection capacitor. The improved AEF is tested and analyzed in a simple common mode test circuit and a complex bidirectional 16 kW ac-dc converter system. An attenuation of 30 dB between 60 kHz and 400 kHz and a maximum attenuation of more than 42 dB at 150 kHz has been achieved.