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Improving High-Frequency Performance of an Input Common Mode EMI Filter Using an Impedance-Mismatching Filter

Fig. 1. Motor drive system with an EMI Filter
This paper investigates the impedance interaction between the electromagnetic interference (EMI) filter and the noise propagation path, and its influences on the filter's design. It proves that the impedance resonance in the propagation path decreases the filter's high-frequency in-circuit attenuation. This paper proposes a method to improve the filter's high-frequency perfor-mance using an impedance-mismatching filter. The impedance-mismatching filter damps the resonance in the common mode (CM) noise propagation path and eliminates the high-frequency noise spike. By applying this method in the filter design, the CM inductor of the EMI filter can be significantly reduced, since overdesign of the EMI filter - caused by its high-frequency perfor-mance degradation - is avoided, and the filter can potentially achieve high power density. This paper also proposes a design procedure for this impedance-mismatching filter, as well as an improved EMI filter design method taking impedance mismatching into account.

Fig. 2. Noise spectrum with the improved filter and the impedance mismatching filter
Fig. 3. Comparison of the CM inductances
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