Measurement and Diagnosis of Common Mode Inductors 3

Assuming that the current density J generates a magnetic field H, it can be concluded that another current nearby will not cancel or prevent the magnetic field or the resulting electric field.

The same adjacent current can cause a change in the magnetic field path.

In the special case of ring common mode inductors, the differential current density in each lead can be assumed to be equal and opposite in direction. So the total sum of the magnetic field generated by this must be 0 around the annular magnetic core, but not 0 outside it!

The function of a magnetic core is similar to the effect it exhibits when it splits into two halves at the gap of a coil winding. Each winding generates a magnetic field within half of the annular coil, indicating that the magnetic field passing through the air will inevitably form a self enclosed circuit. Figure 3 is a schematic diagram of the annular magnetic core and differential current magnetic circuit.diode

Power cord enhances conduction and emission. When magnetic materials are brought into the field (for example, a ring-shaped magnetic core placed in an iron shell), the differential mode permeability may significantly increase, leading to saturation of the magnetic core due to differential mode current.

Non radiative common mode choke structure

In order to achieve effective filter design, the radiation problem caused by magnetic flux leaving the magnetic core must be addressed. The methods include restricting the differential magnetic flux within a magnetic structure object (pot shaped iron core), or providing a high permeability path for the differential magnetic flux (E-shaped iron core).

Pot shaped iron core structure

If the common mode choke adopts a pot shaped iron core structure, then two winding shafts are required. Figure 4 illustrates two sets of coils and the magnetic flux paths generated within the pot shaped iron core pane. It also indicates the differential mode magnetic flux path under the same structural conditions.diode

Figure 4: Magnetic Circuit in Common Mode Pot shaped Core Inductance

Pay attention to the first group, all magnetic flux is inside the iron core. It is precisely because of this structure that the length of the air gap between the outer surface of the iron core and the vertical partition at its center determines the magnitude of the pure magnetic resistance. After using a gasket with a magnetic permeability greater than 10, the pure magnetic resistance can be controlled by changing the size of the inner and outer radii of the gasket (whose value is equal to the length of the air gap). The differential inductance and common mode choke of the pot shaped iron core can be calculated according to the following formula:diode