# Common mode filters effectiveness

As we know, there are many types of common mode filters that can be implemented. One of this type is a single element topology where this element can be a series impedance element (usually R or L or common mode choke ) and a shut impedance element (usually a capacitor). My question is regarding when these two types have a high effectivness . According to electromagnetic compatibility engineering book by Henry Ott, the series impedance element must be larger than the sum of the impedance of the load and the source to be effective and for the shunt impedance element, it must have an impedance that is lower than the parallel combination of the source and load impedances. So why these conditions will make each filter element effective? • Post a picture of what you quote. – Andy aka Mar 17 '19 at 14:16
• @Andyaka Done . – John adams Mar 17 '19 at 14:36
• It's probably best to fire up your favorite simulator, and see for yourself why it's true. If you want an intuitive understanding, think about voltage dividers and current dividers. – The Photon Mar 17 '19 at 14:52

In the case of three series impedance's what will make the most difference to the total impedance? The largest impedance.

So if we have three series impedance's

$$\ Z_{total}=Z_{source}+Z_{filt}+Z_{load}\$$

if the source and the load are significantly smaller than filt, then the filter will be biggest contributor to current

In the parallel case we get this:

$$\ Z_{total}=Z_{source}+\frac{Z_{filt}Z_{load}}{Z_{filt}+Z_{load}}\$$

and using a capacitor for the filter is sufficient to change the overall filtering.

IF either of those don't work, then a multi-element filter will need to be used, and the calculating the impedance will be complex.