# Detailed explanation of common mode inductors with images and texts

2023-11-15Archive

Differential current. In a pair of differential signal lines, a pair of signals of same size and opposite direction usually represents operating current in circuit. For signal lines, this is signal current. line and signal ground line current flowing between them.

Common mode current: on a pair of differential signal lines, a pair of signals (or noise) with the same magnitude and direction. In chain

Typically, ground noise is usually transmitted as common mode current, which is why it is also called common mode noise.

There are many ways to suppress common mode noise. In addition to reducing common mode noise from source, usually most common method of suppression is to use common mode inductors to filter common mode noise, i.e., to block common mode noise outside target circuit. That is, common-mode choke devices are connected to line in series. The purpose of this is to increase common-mode impedance so that common-mode current is drawn and blocked (reflected) by inductor, thereby suppressing common-mode noise on line.

The principle of a common mode choke or inductor

If a pair of coils are wound in same direction on a magnetic ring made of a certain magnetic material, when an alternating current passes through coil, a magnetic flux will be generated due to electromagnetic induction. For differential mode signals, magnetic flux generated is of same magnitude and opposite direction, and they cancel each other out, so differential mode impedance generated by magnetic ring is very small; while for common-mode signals, magnetic flux generated is of same magnitude and direction, and they overlap. The magnetic ring creates a large common-mode resistance. Due to this feature, common mode inductor has less effect on differential signal and provides good filtering of common mode noise.

(1) The differential current passes through common mode coil, direction of magnetic field line is opposite, and induced magnetic field is weakened. This can be seen from direction of magnetic field line in figure. at bottom, solid arrow indicates direction of current, and dotted line indicates direction of magnetic field

(2) Common mode current flows through a common mode coil, direction of magnetic field lines is same, and induced magnetic field increases. This can be seen from direction of magnetic field lines in figure below - solid arrow indicates direction of current, and dotted line indicates direction of magnetic field

The inductance of a common mode coil is also known as self-inductance factor. We know that inductance is ability to characterize ability to generate a magnetic field. In case of leakage inductance, magnetic flux is superimposed, The principle of mutual inductance. The magnetic field lines generated by red coil in figure below pass through blue coil, and magnetic field lines generated by blue coil also pass through red coil, inducing each other.

In terms of inductance, inductance is also doubled, and flux linkage is total magnetic flux. For common mode inductors, when magnetic flux is twice original flux, number of turns does not change and current does not change. , this means that inductance has doubled, and hence the equivalent magnetic permeability has doubled.

Why does equivalent magnetic permeability double? From following inductance formula, since number of turns N does not change, magnetic circuit and cross-sectional area of ​​the magnetic circuit are determined by physical dimensions of magnetic core, so there is no change. The only thing is that magnetic permeability u doubles, so more magnetic flux can be generated

So, when a common-mode current flows, common-mode inductance works in mutual inductance mode. Under effect of mutual inductance, equivalent inductance increases by cost, so common-mode inductance will be doubled, so common-mode signal has a good filtering effect, that is, common-mode signal is blocked by a large impedance, and it is not allowed to pass through common-mode inductor, that is, signal cannot be transmitted to next stage circuits. The following is inductance ZL created by inductor.

To understand inductance of common mode inductors in common mode, main thing is to understand mutual inductance, all magnetic components, regardless of their name, as long as you understand changing shape of magnetic field and see essence of change in magnetic field through phenomenon, it will be easy understand and we should always understand magnetic field lines which are intuitive shapes for us to understand magnetic field Just imagine that regardless of magnetic field concepts or phenomena such as end of same name or end of same name or mutual inductance, we are all draw lines of magnetic force to understand them - master previous explanations of "Magnet Winding Method".