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The important role of ferrite core in EMI filter design

The important role of ferrite core in EMI filter design

2021-11-20

The important role of ferrite core in EMI filter design, electromagnetic interference suppression, ferrite magnetic ring, magnetic beads, etc. are favored by designers due to its convenient use and low price. Its main advantages are as follows:

silicon steel core

1) It is very convenient to use, just put it directly on the cable that needs to be filtered.


2) It does not need to be grounded like other filtering methods, so there are no special requirements for structural design and circuit board design.


3) When used as a common mode choke, it will not cause signal distortion, which is very valuable for wires that transmit high-frequency signals.


The biggest difference between electromagnetic interference suppression ferrite and ordinary ferrite is that it has a large loss. The inductor made with this kind of ferrite as a magnetic core has a characteristic closer to resistance. It is a resistance whose resistance value increases as the frequency increases. When a high-frequency signal passes through the ferrite, the electromagnetic energy is dissipated in the form of heat.


To give full play to the performance of ferrite, the following precautions are very important:


A) The effect of the ferrite bead (magnetic bead) is related to the circuit impedance: the lower the impedance of the circuit, the better the filtering effect of the ferrite bead (magnetic bead). Therefore, in the general ferrite material product manual, the insertion loss of the ferrite material is not given, but the impedance of the ferrite material is given. The greater the impedance of the ferrite material, the better the filtering effect. .


B) The influence of current: When a large current flows through the ferrite wire, the low-frequency insertion loss of the filter will become smaller, and the high-frequency insertion loss will not change much. To avoid this situation, when using the power cord, you can pass the power cord and the power return line through the ferrite at the same time.


C) Selection of ferrite materials: According to the frequency of interference suppression, choose ferrite materials with different permeability. The higher the magnetic permeability of the ferrite material, the higher the impedance at low frequencies and the lower the impedance at high frequencies. Sp3 safety and electromagnetic compatibility network


D) Determination of the size of the ferrite magnetic ring: the greater the difference between the inner and outer diameters of the magnetic ring, the longer the axial direction and the greater the impedance. But the inner diameter must be tightly wrapped. Therefore, in order to obtain large attenuation, use a larger magnetic ring as far as possible under the premise that the inner diameter of the ferrite magnetic ring is tightly wrapped with the wire.


E) The number of turns of the common mode choke: Increasing the number of turns passing through the magnetic ring can increase the low-frequency impedance, but due to the increase of the parasitic capacitance, the high-frequency impedance will decrease. It is a common mistake to blindly increase the number of turns to increase the attenuation. When the interference band to be suppressed is wider, different numbers of turns can be wound on the two magnetic rings.


F) The number of ferrite magnetic rings on the cable: increasing the number of ferrite magnetic rings on the cable can increase the low-frequency impedance, but the high-frequency impedance will decrease. This is because of the increase in parasitic capacitance.


G) The installation position of the ferrite magnetic ring: generally as close as possible to the interference source. For the cable on the shielded case, the magnetic ring should be as close as possible to the inlet and outlet of the case cable.


H) The effect of using together with capacitive filter connector is better: Since the effect of ferrite magnetic ring depends on the impedance of the circuit, the lower the impedance of the circuit, the more obvious the effect of the magnetic ring. Therefore, when capacitive filter connectors are installed at both ends of the original cable, the impedance is very low, and the effect of the magnetic ring is more obvious.

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