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Characteristics of common mode inductance and differential mode inductance

Characteristics of common mode inductance and differential mode inductance

2021-12-08

Before understanding the difference between common mode inductance and differential mode inductance, we must first understand what is common mode current and differential mode current.


Differential mode current: on a pair of differential signal lines, a pair of signals with the same size and opposite direction are generally the working current in the circuit, and the signal line is the current flowing between the signal line and the signal ground.


Common mode current: the current of a pair of signals (or noise) with the same amplitude and direction on the differential signal line. In the circuit, ground noise usually propagates in the form of common mode current, so it is also called common mode noise.


In addition to eliminating the common mode noise from the source, there are many methods to suppress the common mode noise, but the common suppression method is to filter the common mode noise through the common mode inductance, that is, to keep the common mode noise outside the target circuit. That is, online series common mode inductance device. The principle is to increase the impedance of the common mode loop so that the common mode current is consumed and blocked (reflected) by the choke, so as to suppress the common mode noise on the line.


Principle of common mode choke and inductor

If magnetic materials are used around a pair of magnetic rings in the same direction, when the AC flux is out of date, the magnetic flux will be generated in the coil due to electromagnetic induction. Because the magnetic flux generated by the differential mode signal is the same, opposite and offset each other, the differential mode impedance generated by the magnetic ring is very small; However, because the magnetic flux generated by the common mode signal has the same size and direction and overlaps with each other, the common mode impedance generated by the magnetic ring is very large. This characteristic makes the common mode choke have little influence on the differential mode signal and have a good filtering effect on the common mode noise.


The differential mode current passes through the common mode coil, the direction of the magnetic line of force is opposite, and the induced magnetic field is weakened. The direction of the magnetic force line can be seen from the figure below. The solid arrow indicates the current direction and the dotted line indicates the magnetic field direction.


When the common mode current passes through the common mode coil, the direction of the magnetic lines of force is consistent and the induced magnetic field is enhanced. As can be seen from the figure below, the direction of the magnetic force line is as follows: the solid arrow indicates the direction of the current, and the dotted line indicates the direction of the magnetic field.


It is well known that the inductance or self inductance coefficient of a common mode coil represents the ability to generate a magnetic field. For the common mode coil or common mode choke, when the common mode current flows through the coil, due to the same direction of magnetic flux and superposition of magnetic flux, the principle is mutual inductance. In the figure below, the magnetic flux generated by the red coil passes through the blue coil, and the magnetic flux generated by the blue coil passes through the red coil to generate mutual inductance.


In terms of inductance, the inductance is multiplied, and the flux linkage represents the total flux. In terms of common mode inductance, when the magnetic flux is twice the original magnetic flux, the number of turns remains unchanged and the current remains unchanged, which means that the inductance is doubled and the equivalent permeability is doubled.


Why is the equivalent magnetic conductivity doubled? According to the following inductance formula, since the number of turns n is constant, the cross-sectional area of the magnetic circuit and magnetic core is determined by the physical size of the magnetic core, so it is constant, but the permeability u is doubled, so more magnetic flux can be generated.


Therefore, when the common mode current passes through the common mode inductor, it works in the mutual inductance mode. In the mutual inductance mode, the equivalent inductor is consumed more, so the impedance of the common mode inductor will be doubled, which has a good filtering effect on the common mode signal, that is, blocking the common mode signal with large impedance and preventing it from passing through the common mode inductor, that is, not transmitting the signal to the next layer of the circuit, Such as inductive reactance ZL generated by inductance.


To distinguish between common mode choke and differential mode inductance, it is most important to have several windings, common mode choke of two windings and differential mode inductance of one winding.


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