The important role of ferrite ring in the design of EMI filter

Electromagnetic Interference Suppression Ferrite magnetic rings, magnetic beads, etc. are favored by designers because of their ease of use and low cost. Its main advantages are as follows:

1) It is very convenient to use and can be directly put on the cable that needs filtering.

2) Unlike other filtering methods that require grounding, there are no special requirements for structural design or circuit board design.

3) When used as a common-mode choke coil, 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, using this ferrite as magnetic core inductors, its characteristics are closer to the resistance. It is a resistor whose resistance value increases with increasing frequency. When the high-frequency signal passes through the ferrite, electromagnetic energy is dissipated as 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 (bead). Therefore, in the ferrite material's product manual, ferrite material insertion loss is not given, but ferrite material resistance is given. The greater the ferrite material impedance, the better the filtering effect. .

B) Influence of the current: When a large current flows through the ferrite passing wire, the low-frequency insertion loss of the filter becomes smaller and the high-frequency insertion loss does not change much. To avoid this, when using the power cord, you can pass the power cord and the power return cord through the ferrite simultaneously.

C) Selection of Ferrite Materials: Select ferrite materials with different magnetic permeability depending on the frequency at which interference is to be suppressed. The higher the permeability of the ferrite material, the greater the impedance of the low frequency and the lower the impedance of the high frequency.

D) Determination of Ferrite Ring Dimensions: The greater the difference between the inner and outer diameters of the magnetic ring, the longer the axial direction and the greater the impedance. However, the inner diameter must be wrapped tightly. Therefore, in order to obtain large attenuation, use a magnetic core with a large volume as much as possible in the premise that the inner diameter of the ferrite core is covered with a wire.

E) The number of turns of the common mode choke: Increasing the number of turns 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 frequency band that needs to be suppressed is wider, different turns can be wound around the two magnetic rings.

F) Number of ferrite rings on the cable: Increasing the number of ferrite rings on the cable can increase the impedance at low frequencies, but the impedance at high frequencies will decrease. This is because of increased parasitic capacitance.

G) Installation position of ferrite ring: generally as close as possible to the interference source. For the cables on the shielded chassis, the magnetic ring should be as close as possible to the inlet and outlet of the chassis cables.

H) It works better with capacitive filter connectors: Since the effect of the ferrite ring depends on the impedance of the circuit, the lower the impedance of the circuit, the more pronounced the effect of the magnetic ring. Therefore, when the original cable is equipped with a capacitive filter connector at both ends, its impedance is very low, and the effect of the magnetic ring is more pronounced.

Ferrite core coil is still an inductor when the frequency is low. For this single inductor filter circuit, the cutoff frequency is: Fco=1/(2πRsL), Rs is the original circuit impedance and load circuit The series value of the impedance.

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