Common Mode Current: A pair of signals (or noise) with the same magnitude and direction on a pair of differential signal lines. In the circuit.Generally, the ground noise is generally transmitted in the form of common mode current, so it is also called common mode noise.
There are many ways to suppress common-mode noise. In addition to reducing common-mode noise from the source, the most commonly used method for suppressing common-mode noise is to use common-mode inductors to filter out common-mode noise, that is, to block common-mode noise from the target circuit. . That is, a common mode choke device is connected in series in the line. The purpose of this is to increase the impedance of the common-mode loop so that the common-mode current is dissipated and blocked (reflected) by the choke, thereby suppressing common-mode noise in the line.
Principles of Common Mode Chokes or Inductors
If a pair of coils in the same direction are wound on a magnetic ring made of a certain magnetic material, when an alternating current passes through, a magnetic flux is generated in the coils due to electromagnetic induction. For differential mode signals, the generated magnetic fluxes are of the same magnitude and opposite in direction, and they cancel each other out, so the differential mode impedance generated by the magnetic ring is very small; while for common mode signals, the magnitude and direction of the generated magnetic fluxes are the same, and the two are superimposed on each other. The magnetic ring has a large common mode impedance. This feature makes the common mode inductor less influence on the differential mode signal and has good filtering performance for the common mode noise.
(1) 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. It can be seen from the direction of the magnetic field lines in the following figure – the solid arrow indicates the direction of the current, and the dotted line indicates the direction of the magnetic field
(2) The common mode current passes through the common mode coil, the direction of the magnetic field lines is the same, and the induced magnetic field is strengthened. It can be seen from the direction of the magnetic field lines in the following figure – the solid arrow indicates the direction of the current, and the dotted line indicates the direction of the magnetic field.
The inductance of the common mode coil is also known as the self-inductance coefficient. We know that the inductance is the ability to generate a magnetic field. For the common mode coil or common mode inductance, when the common mode current flows through the coil, since the direction of the magnetic field lines is the same, the leakage inductance is not considered. In the case of , the magnetic flux is superimposed, and the principle is mutual inductance. The magnetic field lines generated by the red coil in the figure below pass through the blue coil, and the magnetic field lines generated by the blue coil also pass through the red coil and induce each other.
From the perspective of inductance, the inductance is also doubled, and the flux linkage represents the total magnetic flux. For common mode inductors, when the magnetic flux is twice the original, the number of turns does not change, and the current does not change, then it means that As the inductance is increased by 2 times, it means that the equivalent magnetic permeability is doubled.
Why is the equivalent magnetic permeability doubled? From the following inductance formula, since the number of turns N does not change, the magnetic circuit and the cross-sectional area of the magnetic core are determined by the physical size of the magnetic core, so it does not change, the only thing is the magnetic permeability. u is doubled, so more magnetic flux can be generated
Therefore, when the common mode current passes through, the common mode inductance works in the mutual inductance mode. Under the action of the mutual inductance, the equivalent inductance is increased by the cost, so the common mode inductance will be doubled, so it has a good effect on the common mode signal. The filtering effect is to block the common mode signal with a large impedance and prevent it from passing through the common mode inductor, that is, to prevent the signal from being transmitted to the next stage of the circuit. The following is the inductive reactance ZL generated by the inductor.
To understand the inductance of common mode inductors in common mode mode, the main clue is to understand mutual inductance, all magnetic components, no matter what the name is, as long as you grasp the change form of the magnetic field and see the nature of the magnetic field change through the phenomenon, it will be easy to understand, and then We must always grasp the magnetic field line, which is the intuitive form of our understanding of the magnetic field. Imagine that no matter the concept of the same name or the different name or mutual inductance or the magnetic field phenomenon, we always draw the magnetic field line to know them – master the “magnetic rod” explained earlier. Winding method”.
Post time: Mar-16-2022
