How much do you know about the working principle of common mode inductors?

What is a common mode inductor? What is its working principle? This article will give you a brief introduction to the principle and use of common mode inductors. The common mode inductor is a common mode interference suppression device with a ferrite core. It consists of two coils of the same size and the same number of turns symmetrically wound on the same ferrite ring core to form a four-terminal The device should exhibit a large inductance for common-mode signals and have a suppressive effect, while it should have a small leakage inductance for differential-mode signals and have almost no effect.

The principle is that when common mode current flows through the magnetic ring, the magnetic fluxes in the magnetic ring superimpose each other, thus having a considerable inductance, which inhibits the common mode current. When the two coils flow through differential mode current, the magnetic flux in the magnetic ring The currents cancel each other out and there is almost no inductance, so the differential mode current can pass without attenuation. Therefore, common mode inductors can effectively suppress common mode interference signals in balanced lines without affecting the normal transmission of differential mode signals on the line.

1. Common mode inductors should meet the following requirements during production:

1) The wires wound on the coil core should be insulated from each other to ensure that no breakdown short circuit occurs between the turns of the coil under the action of instantaneous overvoltage.

2) When a large instantaneous current flows through the coil, the magnetic core should not be saturated.

3) The magnetic core in the coil should be insulated from the coil to prevent breakdown between the two under the action of instantaneous overvoltage.

4) The coil should be wound in a single layer as much as possible. This can reduce the parasitic capacitance of the coil and enhance the coil's ability to withstand instantaneous overvoltage.

Under normal circumstances, pay attention to selecting the frequency band required for filtering. The larger the common mode impedance, the better. Therefore, we need to look at the device data when selecting a common mode inductor, mainly based on the impedance frequency curve. In addition, when selecting, pay attention to the impact of differential mode impedance on the signal, focusing mainly on differential mode impedance, and pay special attention to high-speed ports.

2. How does a common mode inductor suppress interference noise?

As we all know, the frequency range of common-mode noise generated by switching power supplies is 10 kHz to 50 MHz or even higher. In order to effectively attenuate or suppress these noises, the common-mode inductor is required to have high enough inductive reactance within this frequency range. How does a common mode inductor suppress interference noise? First, the two sets of coils of the common mode inductor are wound on the magnetic ring, with the same number of turns and the same direction, except that one set of coils is wound on the left side and the other A set of coils is wound on the right side. Common mode inductors use high magnetic permeability manganese-zinc ferrite or amorphous materials to improve common mode inductor performance.

Secondly, the normal AC current flowing through the common mode inductor is analyzed. The 220 V alternating current is a differential mode current. The direction in which it flows through the common mode inductors L3 and L4 is as shown in the figure below. The directions of the magnetic fields generated by the currents in the two inductors are opposite and offset. At this time, the normal signal current is mainly affected by the resistance of the inductor. (This effect is very small), and a small amount of damping (inductance) caused by leakage inductance. In addition, the frequency of 220 V AC is only 50 Hz, and the inductance of the common mode inductor is not large, so the common mode inductor is suitable for normal 220 The AC inductance is very small and does not affect the 220 V AC power supply to the entire machine.

Finally, the common mode current flowing through the common mode inductor is analyzed. When the common mode current flows through the common mode inductor, the current flows in the same direction in the common mode inductor, and magnetic fields in the same direction are generated in the common mode inductors L3 and L4. At this time, the common mode inductor L3 increases and the inductance of L4, that is, the inductive reactance of L3 and L4 to the common mode current is increased, causing the common mode current to be more suppressed, achieving the purpose of attenuating the common mode current, and suppressing the common mode interference noise. effect.

3. What are the functions of common mode noise and common mode inductance?

Common mode noise is also called asymmetric noise or line-to-ground noise. This noise exists at the input ends of electrical equipment using AC power (transmission lines and neutral lines), and the phases of the two to ground remain in the same phase. Common mode noise has current flowing in the same direction on both power lines and back through the ground wire.

Common mode noise can be suppressed by placing an inductor in series with each power line in the EMI filter and using a Y capacitor to connect between the two power lines and ground. Common mode inductors are also called common mode chokes and are often used in computer switching power supplies to filter common mode electromagnetic interference signals. In the board design, the common mode inductor also plays the role of EMI filtering, which is used to suppress the outward radiation and emission of electromagnetic waves generated by high-speed signal lines.

As switching power supplies are increasingly used in industrial and household appliances, mutual interference between appliances has become an increasingly serious problem, and people are increasingly concerned about the electromagnetic environment. There are many types of electromagnetic interference, among which common-mode interference below 30MHz is a very important type. They mainly propagate through conduction, causing great harm to the safe and normal operation of the instrument and must be controlled. A common-mode filter is usually added to the input end to reduce external common-mode interference from entering the instrument through the power line, and to prevent common-mode interference generated by the instrument from entering the power grid. The core of the common mode filter is a common mode inductor with a soft magnetic core, and its performance determines the level of the filter.

Common mode noise and common mode inductor Common mode noise is mainly generated when various switching devices are turned on and off. It can be decomposed into different harmonic forms and has a relatively wide spectrum range. For interference signals below 30MHz, they are generally propagated through conduction. The common mode inductor consists of a soft magnetic core and two sets of coils wound in the same direction. For differential mode signals, since the magnetic fields generated by the two sets of coils are in opposite directions, they cancel each other out. The core is not magnetized and has no suppression effect on the signal.

For common-mode signals, since the magnetic fields generated by the two sets of coils do not cancel each other, but superimpose each other, the iron core is magnetized. Due to the high magnetic permeability of the iron core material, the iron core will produce a large inductance, and the impedance of the coil will inhibit the passage of common mode signals. The above is an analysis of the working principle of common mode inductors. I hope it can help you.