Mounting

Lead inductance and stray capacitance can degrade filter performance markedly at high frequency, since they have the same effect as the parasitics which cause self resonance in single components. Two common faults in filter applications are:

creating excessive inductance in the ground connection, for instance with a length of (green-and-yellow) wire, and

to wire the input and output leads in the same loom or close to each other, or route PCB traces past each other.

A poor ground offers a common impedance which rises with frequency and couples HF interference straight through via the filter’s own ground terminal. Common input-output wiring does the same thing through stray capacitance or mutual inductance. The cures are to mount the filter so that its ground node is directly coupled to the lowest inductance ground of the equipment, preferably the chassis, and to keep the I/O leads separate, preferably screened from each other. The best solution is to position the filter so that it straddles the equipment shielding, where this exists.

Layout of internal components

As well as mounting to ensure a good connection to the ground reference and minimum cross coupling from one side of the filter to the other, you should be careful with the actual positioning of filter components, particularly large inductors.

Magnetic coupling

One potential issue which is specific to cored inductors is the hazard of leakage flux: two inductors in close proximity can suffer magnetic interaction, so that the flux from one inductor induces a voltage in the windings of the other. This can be a problem with small, tightly-packed switchmode power supplies, where the switching inductor or transformer is unavoidably close to the EMC filter's common mode choke. For this reason toroidal cores are popular, since they have low leakage flux. Pot core and E-core constructions are usually acceptable as well.

Electric coupling

Magnetic interaction is complemented by electric interaction. Any part of a circuit that carries a high dV/dt and has a large surface area is strongly capacitively coupled to its surroundings. The electric field leakage can be just as significant as magnetic field leakage, again with switchmode transformers being a particular threat. The problem is though more easily contained with local E-field shielding of the affected component(s).

Capacitors, generally being smaller, are more forgiving of placement constraints. But in general, maximum separation distance within and around filters is to be preferred.

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