Introduction to the characteristics of common mode inductors and differential mode inductors -

Before figuring out the difference between common-mode inductors and differential-mode inductors, we must first figure out 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 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 common suppression method is to filter out common-mode noise through common-mode inductors, that is, to keep common-mode noise out of the target circuit. That is, an 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 coil, thereby suppressing the common-mode noise on the line. Principle of Common Mode Choke Coil and Inductor If a magnetic material is used around a pair of magnetic rings in the same direction, when alternating current passes through, magnetic flux will be generated in the coil due to electromagnetic induction. Since the magnetic flux generated by the differential mode signal has the same magnitude and opposite directions and cancels each other, the differential mode impedance generated by the magnetic ring is very small; but because the magnetic flux generated by the common mode signal has the same magnitude and direction and superimposes each other, the common mode generated by the magnetic ring The impedance is very high. This feature 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 force lines is opposite, and the induced magnetic field is weakened. The direction of the magnetic force lines can be seen from the figure below, the solid arrow indicates the direction of the current, and the dotted line indicates 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 consistent, and the induced magnetic field is enhanced. As can be seen from the figure below, the direction of the magnetic force lines 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 of a common mode coil indicates 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 flux direction is the same, the magnetic flux is 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 to generate mutual inductance. In terms of inductance, the inductance is multiplied, and the flux linkage represents the total flux. As far as the common mode inductor is concerned, 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 magnetic permeability is also doubled. Why is the equivalent magnetic permeability doubled? According to the following inductance formula, since the number of turns n is constant, 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 constant, but the magnetic 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, and in the mutual-inductance mode, the equivalent inductance is consumed more, so the impedance of the common-mode inductor will be multiplied, thus greatly affecting the common-mode signal A good filtering effect is to block the common-mode signal with large impedance and prevent it from passing through the common-mode inductor, that is, not to pass this 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 inductors, the most important thing is to have several windings, two windings for common mode chokes and one winding for differential mode inductors. the