EMC filters in practice — common mode, differential mode, X and Y.

A mains filter is not an off-the-shelf part; it has to match the disturbance. Anyone who understands common mode and differential mode picks the right components.

Discuss interference suppression

Common mode vs. differential mode

Conducted disturbances occur in two forms: differential mode (between the conductors) and common mode (all conductors against ground/earth). Both require different filter elements.

The building blocks

X capacitors sit between the conductors (L–N) and attenuate differential-mode disturbances.
Y capacitors sit from a conductor to ground/earth and attenuate common-mode disturbances.
Common-mode choke: for common-mode currents it acts as a high impedance (the fluxes add up), while for the useful signal in differential mode the fluxes cancel out — the useful signal is barely attenuated.

Sequence: first measure, then filter

It makes sense to first measure the modal nature of the disturbance (common or differential mode) and to build the filter accordingly. For high attenuation, two-stage filters (two chokes, additional X capacitors) are used.

The filter belongs at the boundary

A filter only works if it sits at the enclosure/interface transition and is contacted cleanly and at low impedance to ground. Poor grounding or long supply leads render even the best filter ineffective — as is also the case with shielding.

Frequently asked questions

Why doesn't my standard mains filter work?

Often because the disturbance is predominantly common mode while the filter was designed for differential mode — or because the ground connection is too high in impedance. First measure the modal nature, then filter in a targeted way.

Filtering or shielding?

Both: filters tackle conducted disturbances, shielding tackles radiated ones. More on this in Shielding & filtering and EMC measurement methods.

Sources: Absolute EMC — How to design an EMI filter, Würth Elektronik — Mains Filter.