Since most of the problems faced by EMC are common-mode interference, common-mode inductors are also one of our commonly used powerful components! Here is a brief introduction to the principle and use of common mode inductors. Common mode Inductor is a common mode interference suppression device with ferrite core. It consists of two coils of the same size and the same number of turns symmetrically wound on the same ferrite toroidal core to form a four-terminal The device has a suppressive effect on the large inductance of the common-mode signal, while the small leakage inductance for the differential-mode signal is almost ineffective.
The principle is that the magnetic flux in the magnetic ring superimposes each other when the common mode current flows, which has a considerable inductance, which can inhibit the common mode current. When the two coils flow through the differential mode current, the magnetic flux in the magnetic ring Passes cancel each other out, there is almost no inductance, so the differential mode current can pass without attenuation. Therefore, the common mode inductance can effectively suppress the common mode interference signal in the balanced line, and has no effect on the differential mode signal normally transmitted by the line.
The common mode inductor should meet the following requirements when making:
1) The wires wound on the core of the coil must be insulated from each other to ensure that no short-Circuit breakdown occurs between the turns of the coil under the action of instantaneous overvoltage.
2) When the instantaneous large 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 transient 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 ability of the coil to impart transient overvoltage.
Under normal circumstances, pay attention to selecting the frequency band to be filtered at the same time. The larger the common-mode impedance, the better. Therefore, we need to look at the device data when selecting the common-mode inductor, mainly based on the impedance frequency curve. In addition, pay attention to the influence of differential mode impedance on the signal when choosing, mainly pay attention to differential mode impedance, and pay special attention to high-speed ports.
It is well known that the frequency range of common-mode noise generated by switching power supplies is 10 kHz～50 MHz or even higher. In order to effectively attenuate or suppress these noises, common-mode inductors are required to have a high enough inductance in this frequency range. How does the common mode inductor suppress the 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 in the same direction, except that one set of coils is wound on the left side and the other set of coils is wound on the right side. Common-mode inductors use high-permeability manganese-zinc ferrite or amorphous materials to improve the performance of common-mode inductors.
Secondly, the normal AC current flows through the common mode inductor analysis. 220 V alternating current is a differential mode current, it flows in the direction of common mode inductors L3 and L4 as shown in the figure below. The magnetic fields generated by the currents in the two inductors are in opposite directions and offset. At this time, the normal signal current is mainly affected by the inductor resistance. The impact (this impact is very small), and a small amount of damping (inductance) caused by leakage inductance, plus the frequency of 220 V alternating current is only 50 Hz, the inductance of common mode inductors is not large, so common mode inductors are more The AC inductance is very small and does not affect the power supply of the 220 V AC to the whole machine.
Orientation of common mode inductors L3 and L4
Finally, the common mode current flows through the common mode inductor for analysis. When the common mode current flows through the common mode inductor, the common mode current flows in the same direction in the common mode inductor. The common mode inductors L3 and L4 generate magnetic fields in the same direction, which increases the common mode inductor L3. The inductance of L4 and L4 increases the inductance of L3 and L4 to the common mode current, so that the common mode current is more suppressed, achieving the purpose of attenuating the common mode current and suppressing common mode interference noise. effect.
What is the role 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 end (transmission line and neutral line) of electrical equipment that uses AC power, and the phases of the two to the ground remain the same.
The current of common mode noise flows in the same direction on the two power lines and returns through the ground wire.
Common mode noise can be suppressed by placing an inductor in series with each transmission line in the electromagnetic interference filter, and connecting the two transmission lines with the ground using Y Capacitors.
Common-mode inductors are also called common-mode chokes, which are often used in computer switching power supplies to filter common-mode electromagnetic interference signals. In the board design, the common mode inductance also plays the role of EMI filtering, which is used to suppress the electromagnetic wave generated by the high-speed signal line from radiating to the outside.
With more and more applications of switching power supplies in industry and household appliances, the mutual interference between electrical appliances has become an increasingly serious problem, and the electromagnetic environment has become more and more concerned by people. There are many types of electromagnetic interference, among which common mode interference below 30MHz is a very important category. They are mainly transmitted by conduction and cause 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 to reduce external common mode interference from entering the instrument through the power line, while preventing common mode interference generated by the instrument from entering the power grid. The core of a 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 inductance Common-mode noise is mainly generated by various switching devices during turn-on and turn-off. It can be decomposed into different harmonic forms and has a relatively wide spectrum range. For interference signals below 30MHz, it is generally transmitted through conduction. The common mode inductor is composed 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, and the iron core is not magnetized, which has no effect on the signal.
For common-mode signals, since the magnetic fields generated by the two sets of coils are not offset, but superimposed on each other, the iron core is magnetized. Due to the high permeability of the iron core material, the iron core will generate a large inductance, and the impedance of the coil suppresses the passage of common mode signals.