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# EMI Filter Design Considering In-Circuit Impedance Mismatching

It is evident that the noise attenuation comes from three terms: Z

_{22}/Z

_{LISN}, Z

_{S}/Z

_{11}and T

_{21}. Other terms like Z

_{S}and Z

_{LISN}are fixed by the design and the standards used for EMI compliance. For the differential mode (DM) filter the impedance Z

_{S}is decided by the large input dc-link capacitor of the converter and when ZLISN equals 100%u2126 (standard LISN). Thus, for a simple LC filter and a very small Z

_{S}, the inductor (L) of the filter should be facing the converter so that Z

_{11}can be easily made greater than Z

_{S}, which results in Z

_{S}/Z

_{11}becoming very small. On the other hand, if the capacitor (C) of the LC filter is facing the LISN, this will make Z

_{22}/ Z

_{LISN}small as well. In this way the attenuation will be decided by T

_{21}only. In order to make the attenuation larger, T

_{21}should be as small as possible, which can be easily achieved when the LC filter with L is facing the converter Z

_{11}= ZL Z

_{C}and Z

_{21}= Z

_{C}.

A similar understanding is created in the case of the common-mode (CM) filter. Here contrary to the DM - LC filter, the C should be facing the converter as the CM capacitance of the converter is usually very small thus causing Z

_{S}to be very large. Moreover, in CM the LISN impedance is only 25 ω so the L should face the LISN. Such a choice will provide the optimum impedance mismatching in the case of CM as it will make both Z

_{S}/Z

_{11}and Z

_{22}/Z

_{LISN}very large, which causes the attenuation to be large as well.

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