Common-Mode Filter Design

Question: I was searching the web and came across your web page for the outline of a Common-Mode Filter Course that you offer. I have a number of questions regarding common-mode filters that I am hoping that you could answer for me.

1. Should I use a ferrite choke or a shunt capacitor for a common-mode filter?

2. If I use a ferrite and a capacitor, should the capacitor face the cable or the circuit?

3. Why do ferrites only work over a narrow frequency range that seems to change in every application?

4. What are the advantages of multi-element filters?

Answer: The effectiveness of a filter depends upon the source and load impedances between which it is working. For a common-mode filter the source impedance is usually the PCB ground impedance (which is small, but increases with frequency), and the load impedance is the impedance of the cable acting as an antenna (which, except in the vicinity of cable resonance is large).

Filter attenuation occurs as a result of impedance mismatches. For a series impedance filter element, such as a ferrite, to be effective it must have an impedance larger than the sum of the source and load impedances. For a shunt filter element to be effective it must have an impedance less than the parallel combination of the source and load impedances.

1. Therefore there are three possible cases, (1) both source and load impedance are small, in which case a series ferrite will be effective, (2) both the source and load impedances are large, in which case a shunt capacitor will be effective, and (3) one of the impedances, source or load, is small and the other is large (and it does not matter which is which), in which case no single element filter will be effective, and a multi-element filter must be used.

2. When a ferrite and a capacitor are used together to form an L-filter, the ferrite should be placed on the circuit side and the capacitor on the cable side. This is because the capacitor cannot work against the low source impedance of the circuit ground. If the ferrite is placed on the circuit side of the capacitor the ferrite effectively raises the source impedance and now the capacitor can be effective. If the ferrite is placed on the cable side of the capacitor it increases the already large cable impedance and has very little effect on the filter’s attenuation.

3. The ferrite will only be effective when the load (cable) impedance is low, and this occurs only in he vicinity of the resonance of the cable. Hence when the cable length changes so does its resonant frequency, and the ferrite will be effective over a different frequency range--in the vicinity of the new resonant frequency. The effectiveness of a ferrite will will also change if the given source and/or load impedances chance. For example, if the cable is connected to the PCB through a transformer instead of being directly connected. In this case the common-mode source impedance will be high (the interwinding capacitance of the transformer) instead of low (the PCB ground impedance).

4. With respect to multi-element filters, the more elements a filter has, the less its attenuation is dependent on the terminating impedances. The majority of the mismatch can be made to occur between the elements of the filter and is less dependent of the actual source and load impedances. A multi-element filter can produce significantly more attenuation, over a broader frequency range, than a single element filter can. There are, therefore, basically two reasons to use a multi-element filter, (1) when one of the given impedances is high and the other is low (in which case no single element filter will be effective as discussed in 1 above), or (2) when more attenuation than possible with a single-stage filter is needed.

Henry Ott Consultants

Common-Mode Filter Design