The frequency range of common-mode noise generated by switching power supply is 10 kHz ~ 50 MHz or even higher. In order to effectively attenuate these noises, the common-mode **inductor** is required to provide high enough inductive reactance in this frequency range.

The structure of common mode inductors should be understood before explaining the principle of common mode inductors.

The two coils of a common-mode inductor are wound around a magnetic ring in the same direction with the same number of turns, except that one coil is wound on the left and the other on the right.High permeability manganese zinc ferrite or amorphous material is used to improve the performance of common mode inductors.

(1) analysis of normal ac current flowing through common mode inductor.220 V alternating current is the differential mode current which flows through the common mode inductor.

(2) analysis of common mode current flowing through common mode inductor.When the common mode current flows through the common mode inductor, because the common mode current is in the same direction in the common mode inductor, the common mode current is more restrained, thus achieving the purpose of attenuating the common mode current and playing the role of suppressing the common mode interference noise.

The main functions of common mode inductance in circuit are filter, oscillation, delay and trap.The common mode inductance is composed of ferrite and magnetic ring.The function of common mode inductor is directly related to the function of raw material.

In order to make common mode harassment more useful to filter out, common mode electrical sense has a large enough amount of inductance.Therefore, the high permeability of ferrite material is the basic requirement of common mode inductance.On the other hand, the frequency characteristic of ferrite material is also an important factor in determining the function of the device.

Because the common mode disturbance has a wide spectrum, the ferrite resistance to common mode disturbance only has a large control value in a certain frequency band.Therefore, in order to filter out the common-mode disturbance of a certain band, the frequency characteristics of the core should make the impedance of the device have a large mismatch between the band and the circuit behind it, so as to make a large enough disturbance.

In the essential common-mode inductance, inductive reactance causes a reflection of common-mode disturbance, and impedance is due to the absorbed part such as ferrite dispersion.Both parts were successful in suppressing common-mode harassment.Therefore, the total impedance of common-mode ferrite inductors represents the power of the device to suppress common-mode disturbance.Most ferrite suppliers of common mode inductors use the relationship between impedance and frequency to express the frequency characteristics of products.

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Post time: Aug-30-2019