Common mode Choke inductor’s Measurement Diagnosis 3

author: MagTop
common mode choke,filter inductors
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6. Differential mode and common mode flux in common mode choke For a quick and superficial introduction to the role of a common-mode choke, consider the following statement: "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 argument concretized the intuitive account of the action of a common-mode choke, it was not in essence.

7. Review of leakage
The common mode choke,filter inductors is useful because μcm is several orders of magnitude larger than μdm, because the common mode current is usually small and smaller μdm can be obtained by keeping L/D lower.
In order to obtain the common mode inductance and minimize the differential mode inductance, it is better to make a multi-turn coil with a large cross-sectional magnetic core. By using a larger coil core, which is not necessary, an effective differential mode inductor can be incorporated into a common mode choke. Because the differential mode flux is away from the magnetic core (annular structure), it can produce extremely strong radiation. Especially when the filter is installed on the PCB board, this radiation can be coupled to the power line, so that the conduction emission is enhanced. When a magnetic material is brought to the field (for example, a ring core is placed in an iron shell), the differential mode permeability may be significantly increased, resulting in saturation of the core due to the differential mode current.

8. Radiation-free common mode choke structure In order to achieve effective filter design, the radiation problem caused by flux leaving the core must be solved. This can be done by confining the differential mode flux to a magnetic structure (pot core), or by providing a path of high permeability for the differential mode flux (E core).

9. Kettle core structure If the common - mode choke has a kettle-shaped iron core structure, then two winding shafts are required. Two sets of coils in a kettle-shaped iron core pane and their resulting flux paths. The differential mode flux path with the same structure is also shown.

10. E-core structure
There is also a common mode choke, which is easier to wind than a toric-core coil but emits more radiation than a kettle-core coil. The common mode flux of an E-core coil connects both sets of coils on the outer lead. In order to obtain high permeability, there should be no air gap in the external lead. The differential mode flux, on the other hand, connects the outer lead to the center lead. The permeability in the differential mode path can be obtained by separating the center leads from each other, which are the main areas of radiation generation.