NIPPON CHEMI-CON CORPORATION

The magnetic coupling characteristic of common mode choke coils is that even though the primary and secondary sides of the coil are electrically insulated, the magnetic flux generated in the core flows through both the primary and secondary coils. This is a phenomenon in which voltages are mutually induced according to changes in magnetic flux.

1. Primary and secondary winding ratio

With a common mode coil, it is necessary to balance the two windings with the same winding ratio on the primary and secondary sides (𝐿1=𝐿2) and also with the same polarity (winding direction).

2. Degree of coupling between primary and secondary sides (coupling coefficient k)

Coupling coefficient k represents the degree of magnetic coupling between the primary and secondary coils. Lower coupling leads to lower common-mode inductance.

Coupling coefficient k is a number between 0 and 1, with an ideal value of 1. Generally, in a toroidal coil, the value of k is infinitely close to 1, and k=0.9995000 to 0.9998000.

3. Mutual inductance M

Mutual inductance M is a virtual inductance used to express the induced voltage generated in the other coil by the magnetic flux generated in the core by both the primary and secondary coils.

Mutual inductance M acts to suppress common mode noise. Mutual inductance M is calculated from the above coupling coefficient k, primary coil (L1) and secondary coil (L2).

If L1=L2=L here, the following formula is obtained. When k is close to 1, M is almost the same as L.

4. Leakage inductance

Leakage inductance is the inductance generated by leakage flux that is not magnetically coupled between the primary and secondary coils.

【Magnetic flux leakage】
Magnetic flux leakage is the magnetic flux generated inside the coil that slightly leaks to the outside of the core. An increase in magnetic flux leakage lowers the common mode inductance.

Leakage inductance (Lℓeak) is also zero when completely magnetically coupled. Leakage inductance works to suppress normal mode noise. It can be measured with an impedance analyzer, etc.

Generally, when the coupling coefficient k is very close to 1, the mutual inductance M and the leakage inductance Lℓeak have the following relationship.

This indicates that M becomes the dominant inductance for common mode noise. For normal mode noise, Lℓeak becomes the dominant inductance since M is zero.