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Why it is recommended to not to add copper planes and to avoid routing of signals under the common mode chokes?

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A common mode choke acts as a low impedance path for normal (wanted) differential currents flowing to and from a load. For these currents, the net magnetic field in the ferrite-core is largely zero. Or, put another way, the ampere-turns for the forward load current is cancelled by the same ampere turns flowing back from the load.

This means that copper planes and tracks local to the common-mode choke are not subject to fringing fields from the ferrite-core when purely differential currents are flowing.

This is not true for common-mode (normally unwanted) currents. The current in one coil is in the same direction as the current in the other and, this can lead to a substantial fringing field hence, undesirable emfs are induced in copper tracks and undesirable eddy current are induced in copper planes.

The same argument applies to inductors because there is no ampere-turn cancellation due to an inductor having only one winding. In some cases, the situation may be worse due to gaps in the ferrite-core producing a fringing H field that can melt copper. A lot of inductors use core-gaps to control the inductance value against temperature changes and this is why the same recommendation is given for inductors.


Additional information

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Good article by Coilcraft on CM chokes

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