In order to introduce the effect of common-mode choke quickly and clearly, the following discussion can be considered :" the magnetic fields on both sides of the common-mode choke core cancel each other out, so there is no magnetic flux to saturate the core." Although this intuitive account of the common mode choke is concrete, it is not.
Refer to the following discussion around maxwell's equations
* assuming that the current density J generates magnetic field H, it can be concluded that another current nearby will not cancel out or block the magnetic field or the resulting electric field.
* the same adjacent current can cause a change in the magnetic field path.
* in special cases of circular common-mode inductors, the differential mode current density in each lead can be assumed to be equal and in the opposite direction. Therefore, the resulting magnetic field must be 0 on the periphery of the ring core, and not 0 on the outside!
The core ACTS as if it were split in half between the coil winding gaps. Each winding generates a magnetic field in half of the loop, meaning that the magnetic field through the air must form a self-closing loop.
The common mode choke can play a certain role because the co-mode reactor was several orders of magnitude larger than the khan dm, because the co-mode current is usually small, and the smaller dm can be obtained by keeping L/D at a lower value.
In order to obtain the common-mode inductance, and at the same time to minimize the differential inductance, it is best to use a magnetic core with a large cross-section area to make a multi-turn coil. It is not necessary to use the larger magnetic core of spiral tube, which can be incorporated into an effective differential mode inductor in the common mode choke. Because the differential mode flux is far away from the core (ring structure), it may produce extremely strong radiation. Especially when the filter is mounted on the PCB board, the radiation can be coupled to the power line to enhance the transmission emission. When magnetic materials are brought into the field (for example, the ring core is placed in an iron case), the differential mode permeability may increase significantly, resulting in saturation of the core due to the differential mode current.