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The anti-interference ferrite cores made of different materials have different optimum suppression frequency range. They are related to permeability. The higher the material’s permeability, the lower the best attenuation frequency is, And vice versa .Therefore Mn-Zn materials are more suitable for applications in low frequencies (such as 30M Hz or lower frequency), while Ni-Zn materials are more suitable for applications in high frequencies (such as 25 M Hz～200 M Hz or higher frequency). In the case of DC or low-frequency AC current bias, the materials with low permeability should be used in consideration of degradation and saturation on suppression performance.
Anti-interference ferrite cores are widely used in the suppression of printed boards, power cords and data lines.
Application in printed board
Interference on PCB is mainly from digital circuit, Its high-frequency switching current brings a strong interference between power cord and ground wire. When digital circuit is switched The high-frequency noise occurred will be transmitted in the way of conduction or radiation through power cord and data line. Normal method to suppress interference is to add a decoupling capacitor between power supply and the ground in order to short-circuit the high-frequency noise. However, decoupling capacitors can sometimes cause high frequency resonance, resulting in new interference. Adding suppression beads at the entrance of PCB will reduce the high frequency noise.
Application in power cord
Power cord will transmit the interference of external power grid and the noise of switching power supply into circuit. A ferrite core placed at outlet of power supply and inlet of the power supply of PCB can not only transmit the high-frequency interference between the power supply and the PCB, but also high-frequency noise/interference among all PCBs. Note that the application of ferrite core on power cord sometimes brings problem of bias current (ferrite core is sheathed through a power cord with direct circuit passing through), or even the saturation of ferrite core. Reducing permeability of ferrite can reduce the influence of bias current, so we should choose ferrite cores with low permeability and large cross-section to reduce interference on power cord (of course, the influence of bias current can also be solved by opening air gap to ferrite core). When the bias current is large, power input lines and return lines of AC and DC can also be sleeved in one ferrite core to avoid saturation, but the method is still effective to suppress common mode noise on the input and return lines.
Application in signal line
Anti-interference ferrite cores can also be used on signal lines to suppress noise transmission between components or equipments. However, the impedance of signal line is generally greater than that of power cord, so the effect of suppression will be less obvious than that of power cord.
Next：Shape /size selection of EMI ferrite core
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