Before figuring out the difference between common mode inductance and differential mode inductance, we must first understand what common mode current and differential mode current are. Differential mode current: On a pair of differential signal lines, a pair of signals with the same magnitude and opposite direction is 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 on a differential signal line for a pair of signals (or noise) with the same magnitude and direction. In circuits, ground noise usually propagates in the form of common-mode currents, so it is also called common-mode noise.
In addition to eliminating common-mode noise from the source, there are many ways to suppress common-mode noise, but the commonly used suppression method is to filter common-mode noise through common-mode inductors, that is, keep common-mode noise outside the target circuit. That is, the in-line 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 coil, thereby suppressing the common mode noise on the line.
Principles of Common Mode Chokes
If magnetic material is used around a pair of magnetic rings in the same direction, when alternating current is passed through, a magnetic flux will be generated in the coil due to electromagnetic induction. Since the magnetic fluxes generated by the differential mode signal are of the same size and opposite directions, they cancel each other out, so the differential mode impedance generated by the magnetic ring is very small; however, because the magnetic fluxes generated by the common mode signal are of the same size and direction and superimposed on each other, the common mode generated by the magnetic ring Impedance is very large. This characteristic makes the common mode choke coil have little influence on the differential mode signal, and has a good filtering effect on the common mode noise.
The differential mode current passes through the common mode coil, the direction of the magnetic field lines is opposite, and the induced magnetic field is weakened. The direction of the magnetic field lines can be seen from the figure below. The solid arrows indicate the direction of the current, and the dashed lines indicate the direction of the magnetic field.
When the common mode current passes through the common mode coil, the direction of the magnetic force lines is the same, and the induced magnetic field is enhanced. As can be seen from the figure below, the directions of the magnetic field lines are as follows: the solid arrows indicate the direction of the current, and the dashed lines indicate the direction of the magnetic field.
It is well known that the inductance or self-inductance of a common mode coil represents the ability to generate a magnetic field. For common mode coils or common mode choke coils, when the common mode current flows through the coil, since the magnetic fluxes are in the same direction, the magnetic fluxes are superimposed, and 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, resulting in mutual inductance. As far as inductance is concerned, the inductance is multiplied, and the flux linkage represents the total magnetic flux. In terms of common mode inductance, when the magnetic flux is twice the original magnetic flux, the number of turns is the same and the current is the same, which means that the inductance is doubled, and the equivalent permeability is also doubled.
Why is the equivalent magnetic permeability doubled? According to the following inductance formula, since the number of turns n is unchanged, the cross-sectional area of the magnetic circuit and the magnetic core is determined by the physical size of the magnetic core, so it is unchanged, but the magnetic permeability u doubles, So more magnetic flux can be generated.
Therefore, when the common mode current passes through the common mode inductance, it works in the mutual inductance mode. In the mutual inductance mode, the equivalent inductance is consumed more, so the impedance of the common mode inductance will be multiplied, which has a great effect on the common mode signal. A good filtering effect means blocking the common mode signal with large impedance and preventing it from passing through the common mode inductor, that is, not passing the signal to the next layer of the circuit, such as the inductive reactance ZL generated by the inductor.
To distinguish between common mode chokes and differential mode inductances, the most important thing is to have several windings, two winding common mode chokes and one winding differential mode inductance.