Common Mode Inductors and Differential Mode Inductors Characteristics Introduction

Before figuring out the difference between common mode inductors and differential mode inductors, it is important to first figure out what common mode current and differential mode current are.

　　Differential mode current: A pair of signals of the same magnitude and opposite direction on a pair of differential signal lines 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 of a pair of signals (or noise) of the same magnitude and direction on a pair of differential signal lines. In circuits, ground noise usually propagates in the form of common-mode currents, hence the term common-mode noise.

　　In addition to eliminating common mode noise from the source, there are many ways to suppress common mode noise, but a commonly used suppression method is to filter out common mode noise by means of a common mode inductor, i.e., to keep the common mode noise outside the target circuit. That is, in-line series common mode inductor 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, thus suppressing the common-mode noise on the line.

　　Principle of Common Mode Chokes and Inductors

　　If a magnetic material is used around a pair of rings that are orientated in the same direction, a magnetic flux will be generated in the coil due to electromagnetic induction when an alternating current is passed through. Since the flux generated by the differential mode signal is of the same size and in the opposite direction and cancels each other out, the differential mode impedance generated by the magnetic rings is very small; however, since the flux generated by the common-mode signal is of the same size and in the same direction and is superimposed on each other, the common-mode impedance generated by the magnetic rings is very large. This characteristic makes the common-mode choke have little effect on the differential-mode signal, and has a good filtering effect on the common-mode noise.

　　Differential mode current passes through the common mode coil, the magnetic lines of force are in opposite directions and the induced magnetic field is weakened. The direction of the magnetic lines of force can be seen from the figure below, with the solid arrows indicating the direction of the current and the dashed lines indicating the direction of the magnetic field.

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

　　It is well known that the inductance or self-inductance coefficient of a common mode coil indicates the ability to generate a magnetic field. For common mode coils or common mode chokes, when the common mode current flows through the coil, the magnetic flux is superimposed due to the same direction of magnetic flux, which is based on the principle of 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, creating mutual inductance.

　　In the case of inductance, the inductances are multiplied and the magnetic chain represents the total magnetic flux. In the case of common-mode inductors, when the magnetic flux is twice the original flux, the number of turns remains the same and the current is unchanged, which means that the inductance is increased by a factor of two, and the equivalent permeability is also increased by a factor of two.

　　Why is the equivalent permeability doubled? According to the inductance equation below, the cross-sectional area of the magnetic circuit and core is constant as the number of turns n remains the same and is determined by the physical dimensions of the core, so it is constant, but the permeability u is doubled so that more magnetic flux can be produced.

　　Therefore, when a common-mode current passes through a common-mode inductor, it operates in the mutual inductance mode, where the equivalent inductance is consumed more, so the impedance of the common-mode inductor increases exponentially, which gives a good filtering effect on common-mode signals, i.e., it blocks large impedance common-mode signals and prevents them from passing through the common-mode inductor, i.e., it does not pass this signal on to the next layer of the circuit, e.g., inductive impedance ZL, which is generated by the inductor.

　　To distinguish between a common mode choke and a differential mode inductor, the most important thing is that there are several windings, a common mode choke with two windings and a differential mode inductor with one winding.

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