How EMC is achieved in 199 words!

15 Apr 2020

Author: Keith Armstrong

How EMC1 is achieved, in 199 words!

EM propagation2 creates all the voltages and currents we measure in our circuits and cables, as well as creating all the unwanted voltage and current noises that can cause EMI3 – and depends entirely on impedances4

Currents – both wanted Differential Mode (DM) and unwanted stray Common-Mode (CM) – always prefer to flow in the loops with the lowest overall impedances.

So – putting it as simply as possible – all we have to do for EMC is to make sure that the paths we want our DM and CM currents to flow in, have sufficiently lower loop impedances5 than the paths we don’t want them to flow in!

The less EMI we want (whether emissions or immunity): the lower should be the wanted loop impedances with respect to the impedances of the unwanted current loops6.

We achieve this by carefully segregating the wanted DM and CM paths from the unwanted path – a structural issue in three dimensions that I call ‘EM Zoning’ 7.

If practical issues mean we can't get the ratio between wanted/unwanted path impedances low enough, we then add EM mitigation 8 between the wanted and unwanted DM and CM paths.

This is how EMC is achieved!

1 EMC = Electromagnetic Compatibility
2 EM = Electromagnetic
3 EMI = Electromagnetic Interference, caused by electromagnetic disturbances
4 AC impedance: the complex impedance taking into account the resistances, inductances and capacitances of electrically small 5 current paths, and the wave impedances of electrically large current paths 5.
5 Electrically small (large) means being physically shorter (larger) than a tenth of the wavelength, at the frequency being considered.
6 Note that AC and RF currents will happily travel more through the air or other insulating media when those paths have lower overall loop impedances than the paths that use the conductors our schematics or simulations assume they will take.
7 ‘EM zoning’ aims to reduce the amount of coupling between the wanted current paths and the unwanted paths by:
i) decreasing the loop impedances in the wanted zones (which usually means involves making them physically smaller); and,
ii) increasing the loop impedances associated with the unwanted zones (which usually involves locating them physically further away).
8 ‘EM mitigation’: a variety of techniques including: galvanic isolation; filtering; shielding; transient suppression, etc., that help achieve good EM Zoning 7.

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