Noise suppression filtering in electronic circuits

Interference suppression filters, more commonly known as electromagnetic compatibility (EMC) filters, are one way of reducing electromagnetic interference and complying with EMC standards. Their purpose is to act on conducted emissions, i.e. to reduce the disturbances that propagate along cables (or via connecting wires) and interfere with the smooth operation of an electrical or electronic system.

There are two modes of noise propagation in electronic equipment: common-mode noise and differential-mode noise. It is therefore essential to determine which type of noise needs to be attenuated in order to choose the right filter structure.

A filter is characterized by its attenuation, its filtering performance. The aim is to quantify this attenuation, either by measurement or theoretically, to determine whether or not the filter is suitable for the application.

In addition, there are many different filter structures, and each one is adapted to specific problems. It is therefore important to be able to model the complete filter to be able to predict whether it will be sufficient to attenuate disturbances. It is therefore necessary to model each of the passive components (capacitors, inductors and common-mode cores) that make up the filter.

Certain parameters can have a strong influence on the final attenuation of a filter and must be taken into account. The impedances seen by the filter can considerably affect performance, as can the filter's own resonances or temperature. What's more, the close proximity of components in the same filter makes it essential to take into account coupling, which has a strong influence on final attenuation.

The first section deals with the propagation of noise in equipment, and more specifically with the two modes of propagation: the common mode and the differential mode. The main property of a filter, its attenuation, is then discussed, moving from theory to practice. The third section deals with typical filter structures and their use. This is followed by the modeling of the passive components making up these filters, taking into account their parasitic elements. Finally, the parameters influencing filter performance, such as the impedances seen by the filter, natural resonances, inter-component coupling and temperature, are presented.

At the end of the article, readers will find a table of the symbols used.