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The difference between mode inductors and differential mode inductors content sharing

by:Catech      2023-04-26

Common mode inductors and differential mode inductors:

The following focuses on two types of inductors used for anti-electromagnetic interference: common-mode inductors and differential-mode inductors.

The disturbance electromagnetic field generates a differential mode current between the line and the line, causing interference on the load, which is differential mode interference; the disturbance electromagnetic field generates a common mode current between the line and the ground, and the common mode current generates a differential mode voltage on the load, causing interference. This is common mode ground loop interference.

The filter inductance that suppresses common-mode interference is called common-mode inductance. The filter inductance that suppresses differential mode interference is called differential mode inductance.

Common mode inductors are two sets of coils wound on the same core with the same number of turns, equal wire diameters, and opposite winding directions. A differential mode inductor is a coil wound on an iron core.

The characteristic of the common mode inductor is that since the winding directions of the two sets of coils on the same core are opposite, the core is not afraid of saturation. The most common core material used in the market is high conductivity ferrite material.

The characteristics of differential mode inductors: it is used in high current occasions. Because a coil is wound on a core, when the current flowing into the coil increases, the core in the coil will be saturated, so the most used core material on the market is metal powder core material. Especially iron powder core material (due to cheap price).

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 now adopts 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.

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