Why it is recommended to not to add copper planes and to avoid routing of signals under the common mode chokes?
Electronic – PCB Layout – Common Mode Choke
common-mode-chokepcbpcb-design
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Chokes are used to suppress noise, i.e. to prevent noise and other EMI both from entering and going out of some piece of equipment.
What is usually referred to as common mode noise or as differential mode noise are simply two modes in which noise can be coupled conductively (i.e. through wires) into the piece of equipment.
This document explains in more detail the issue.
Essentially common mode (CM) noise is an unwanted signal which couples into both line conductors in the same direction, whereas differential noise is coupled into a single conductor. For simplicity I'm talking about noise coupled into mains line here, where common mode chokes are frequently used, but the same problem arises whenever some wire comes out of an apparatus, for example the leads of a multimeter, an oscilloscope probe or even the USB cable connecting an external HD to the PC.
Common mode chokes usually have two separate windings which are each put in series with each line conductor. These two windings are wound on the same ferromagnetic core in a way that exploits the different path that power line current and CMN currents take in the circuit. Therefore the choke presents very little impedance to the power line current, whereas common mode noise currents see each winding as much higher impedances, and this attenuates the amplitude of the noise.
Another interesting document is this application note about line filters in switching power supplies.
One thing to consider is that you will never achieve 100% elimination of conducted and radiated noise. The regulators know that too and take that into account when the standards are written. So it is necessary to just attenuate the conducted or radiated noise down to an acceptable level.
One way to picture things for selection of noise filter chokes is to consider the choke (or filter bead) impedance as part of a impedance divider. The other part of the impedance becomes either the source impedance of inputs or load impedance of outputs as referenced to the GND system. This can even be dealt with on an intuitive basis so that if, for example a power supply, as an input has a source impedance that is very low then the filter choke probably does not need to be one with an extremely high impedance at the frequencies of interest.
Similarly for an output if the signal terminates into higher impedance load then the high frequency impedance of the filter choke/bead needs to be much higher.
It also seems to be that high current filters are typically lower impedance chokes than the ones selected for low current signal lines.
Last comment I can make is that the proper thing to do in a product design is to think of how you want to architect your EMI suppression and the design support for that into the product using the best layout practices that you can apply. The key thing here is to select proper types of components and have footprints for them available in the design. Then when you build up first boards apply experience to populate the filter bead, inductor and capacitor sites with components with the best guess values. This permits you flexibility to swap out component values at the test lab in case problems are seen in particular parts of the product at certain frequencies.
Best Answer
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
Good article by Coilcraft on CM chokes