# Common-mode inductor

Working principle of common mode inductor
Common mode inductance In a circuit, when a differential mode signal passes through a common mode inductor, the magnetic flux produced by the differential mode signal in the core is equal in magnitude and opposite in direction. The net flux is zero, so the common-mode inductance to the differential mode signal is very small; When the common-mode EMI signal passes through the common-mode inductor, the magnetic flux generated by the two coils strengthens each other, increasing the inductive reactance of the two coils to the common-mode current, so that the common-mode current is suppressed more, and the purpose of attenuating the common-mode current is achieved. The common-mode inductance can transform EMI energy into heat and filter it out. (Red differential mode signal, blue common mode signal)
Common mode inductance magnetic leakage
There is no ideal common-mode inductor. In operation, of course, there is magnetic leakage. Leakage inductance test method: N2 short-circuit, measure the inductance at both ends of N1. You get a smaller sense, the leakage sense of this common-mode inductor. To:
N1 total flux: φ 1 total
N1 flux leakage: φ 1 leakage
N1 main flux: φ 1 main flux
N2 total flux: φ 2 total
N2 flux leakage: φ 2 leakage
N2 main flux: φ 2 main flux
The relationship is as follows:
(1) The total flux of N1 is equal to the main flux of N1 winding + leakage flux of N1 winding:
φ 1 Total = φ 1 Main + φ 1 leakage
(2) The main flux of winding N1 is equal to the total flux of winding N2:
φ 1 principal = N2 total
(3) The total N2 flux is equal to the main N2 flux + the leakage flux of N2 winding:
φ 2 total = φ 2 Main + φ 2 leakage
(4) The net flux φ of N1 is equal to the total flux of N1 -N2 winding to offset the flux of N1 winding (N2 main flux):
φ = φ 1 Total - φ 2 Main = φ 1 leakage + φ 2 leakage
According to this, the inductance is equal in value to the flux generated, so when N2 is short-circuited, the measured inductance (leakage inductance) corresponds to φ 1 leakage + φ 2 leakage, not just the leakage inductance generated by N1 winding. So the leakage inductance of common-mode inductance is N1 leakage inductance +N2 leakage inductance.
Magnetic saturation of common-mode inductor
1. Saturation principle of common mode inductor
The flux generated by N1 and N2 of the ideal common-mode inductors cancel each other and will not saturate. However, when the common mode inductor works in the circuit, there is leakage flux in the magnetic core due to the existence of leakage inductance. When the magnetic flux leakage is large, the core is saturated when the B obtained by dividing the cross-sectional area exceeds the core Bs.
2. Magnetic core flux leakage
(1) The main magnetic flux of the magnetic core is the magnetic flux generated by the coil through the magnetic core to form a closed loop;
(1) Magnetic flux leakage is the magnetic flux generated by the coil, which consists of a closed loop of the magnetic core surrounded by (at least) the coil and air, so the magnetic flux leakage passes through the magnetic core.
(3) The main flux generated by the common mode inductors N1 and N2 (which form a closed circuit through the magnetic core) is equal in magnitude and opposite in direction, and cancels each other. The magnetic flux in the magnetic core is only flux leakage. When the flux leakage passes large, the magnetic core will be saturated.
The inductance of common mode inductance in a circuit
1. Common mode inductance
The common mode inductance of the common mode inductance is the parallel connection of the two coils. The common mode inductance is regarded as a whole, and the two lines can be regarded as one line.
2. Differential mode inductance
The differential mode inductance of the common-mode inductance is the leakage inductance of the common-mode inductance. Most of the magnetic flux generated by N1 and N2 cancel each other, and a small part of the circuit is formed by space to form flux leakage and generate the differential mode inductance.