Measurement and Diagnosis of Common-Mode Inductors 3

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.

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.  diode

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.

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:

The specific dimensions are shown in Figure 5.

Figure 5 Specific dimensions for calculating differential inductance of pot shaped iron core

Reducing the magnetic resistance on the differential mode path will increase the differential mode inductance. The most significant advantage of using this common mode choke is that the pot shaped iron core has inherent "self shielding" characteristics.

E-shaped iron core structure

In addition, there is another type of common mode choke coil that is easier to wind than a toroidal core coil, but has more intense radiation than a pot shaped core coil. The E-shaped core coil is shown in Figure 6. The figure indicates that the common mode magnetic flux connects both sets of coils on the external lead together. In order to achieve high magnetic permeability, there should be no air gap on the external lead. On the other hand, differential mode flux connects the external lead and the central lead. The magnetic permeability in the differential mode path can be obtained by isolating the central leads from each other, which are the main areas where radiation is generated. diode