Choose Magnetic Toroidal Ferrite Core

The common ring that we normally see on one or both ends of the power line or signal line of the electronic device is the common mode choke. The common mode choke can form a large impedance to the common mode interference current, but has no effect on the differential mode signal (the working signal is a differential mode signal), so it is easy to use without considering the signal distortion problem. And the common mode choke does not need grounding, and can be directly added to the cable.

Turn number selection of magnetic toroidal core

The whole cable passes through an ferrite ring and forms a common mode choke. According to the requirement, the cable can be wrapped around a few turns on the magnetic ring. The more the number of turns, the better the interference suppression effect on the lower frequency, while the higher frequency noise suppression is weaker. In practical engineering, the number of turns of the magnetic ring should be adjusted according to the frequency characteristics of the interference current. Usually, when the frequency band of the interference signal is wider, two magnetic rings can be sheathed on the cable, and each magnetic ring is surrounded by different turns, which can restrain the high frequency interference and the low frequency interference.

From the mechanism of the common mode choke, the greater the impedance, the more obvious the interference suppression effect. The impedance of the common mode choke comes from the common mode inductance Lcm=jwLcm, and it is not difficult to see from the formula that for a certain frequency noise, the greater the inductance of the ring, the better. But the actual situation is not so, because there is parasitic capacitance on the actual magnetic ring, the existence of it is parallel with inductance. When the high frequency interference signal when the capacitor capacitance is smaller, the magnetic inductor short, so that the common mode choke out.

According to the frequency characteristics of interference signals can be Ni Zn ferrite or Mn Zn ferrite with high frequency characteristics of the former is better than the latter. Mn-Zn ferrite magnetic permeability in the thousands - tens of thousands, and the nickel zinc ferrite for the hundreds of thousands. The higher the permeability of ferrite, the higher the impedance at low frequency and the smaller the impedance at high frequency. So, in the suppression of high frequency interference, should use nickel zinc ferrite; whereas with Mn Zn ferrite. Or in the same cable at the same time on Mn Zn and Ni Zn ferrite, which can

suppress the interference frequency band is wide. The greater the difference between the inner diameter and the outer diameter of the magnetic ring, the greater the longitudinal height, the greater the impedance, but the inner diameter of the ring must be wrapped cable, to avoid leakage. The installation position of the magnetic ring should be close to the interference source, that is,

close to the entrance and exit of the cable.

Operating principle of magnetic core

1, the longer the magnetic ring, the better

2, the closer the aperture is to the cable that goes through it, the better.

3, when the low frequency interference, it is recommended that the cable around the 2~3 turn, the high frequency interference can not turn around (because the existence of the distribution capacitor), choose a longer magnetic core.