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Common Mode Inductance

by:Catech      2023-05-04

Common mode Choke, also known as common mode choke, is often used in switching power supplies of computers to filter common mode electromagnetic interference signals. In the board design, the common mode inductor also plays the role of EMI filter, which is used to suppress the electromagnetic wave generated by the high-speed signal line from radiating outward.

The filter circuit of the common mode inductor, La and Lb are the common mode inductor coils. The two coils are wound on the same iron core with the same number of turns and phases (wound in opposite directions). In this way, when the normal current in the circuit flows through the common-mode inductor, the current generates a reverse magnetic field in the inductance coil wound in the same phase to cancel each other out. At this time, the normal signal current is mainly affected by the coil resistance (and a small amount due to leakage) Damping caused by inductance); when there is a common-mode current flowing through the coil, due to the isotropy of the common-mode current, a magnetic field in the same direction will be generated in the coil to increase the inductive reactance of the coil, making the coil appear as high impedance, resulting in Strong damping effect, in order to attenuate the common mode current and achieve the purpose of filtering.

In fact, if one end of this filter circuit is connected to the interference source and the other end is connected to the interfered device, then La and C1, Lb and C2 form two sets of low-pass filters, which can control the common-mode EMI signal on the line to a very low level. level. The circuit can not only suppress the incoming external EMI signal, but also attenuate the EMI signal generated when the line itself works, and can effectively reduce the EMI interference intensity.

A small common mode inductor produced in China, using high frequency noise suppression countermeasures, common mode choke coil structure, no signal attenuation, small size, easy to use, good balance, easy to use, high quality and other advantages. Widely used in double-balanced tuning devices, multi-frequency transformers, impedance transformers, balanced and unbalanced conversion transformers...etc.

There is also a common mode filter inductor/EMI filter inductor using a ferrite core, two wires wound in parallel, good noise suppression countermeasures, high common mode noise suppression and low differential mode noise signal suppression, low differential mode noise signal suppression interference It is difficult to deform in high-speed signals, and has the advantages of small size, good balance, convenient use, and high quality. It is widely used to suppress EMI noise of electronic equipment, USB lines of personal computers and peripheral equipment, DVC, IEEE1394 lines of STB, liquid crystal display panels, low-voltage differential signals, etc.

meet the following requirements

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

(2) When the coil flows through the instantaneous large current, 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, which can reduce the parasitic capacitance of the coil and enhance the coil's ability to withstand instantaneous overvoltage.

Usually, pay attention to the selection of 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 a common-mode inductor, mainly based on the impedance-frequency curve. In addition, when choosing, pay attention to the influence of differential mode impedance on the signal, mainly focus on differential mode impedance, and pay special attention to high-speed ports.

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