Magnetic ring selection

The magnetic ring that we usually see at one or both ends of the power or signal line of an electronic device is a common mode choke. The common mode choke can form a large impedance to the common mode interference current, but has no influence on the differential mode signal (the working signal is a differential mode signal), so the use is simple without considering the signal distortion problem. And the common mode choke does not need to be grounded and can be directly applied to the cable.

The entire bundle of cables is passed through a ferrite bead to form a common mode choke, and the cable can be wound around the magnetic ring as needed. The more the number of turns, the better the interference suppression effect on the lower frequency, and the weaker the noise suppression on the higher frequency. In actual engineering, the number of turns of the magnetic ring should be adjusted according to the frequency characteristics of the interference current. Generally, when the frequency band of the interference signal is wide, two magnetic rings can be placed on the cable, and each magnetic circle surrounds different turns, so that high frequency interference and low frequency interference can be suppressed at the same time. From the mechanism of the common mode choke action, the greater the impedance, the more obvious the interference suppression effect. The impedance of the common mode choke is from the common mode inductance lcm=jwlcm. It is easy to see from the formula that for a certain frequency of noise, the inductance of the magnetic ring is as large as possible. But this is not the case, because the actual magnetic ring also has parasitic capacitance, which exists in parallel with the inductor. When a high-frequency interference signal is encountered, the capacitive reactance of the capacitor is small, and the inductance of the magnetic ring is short-circuited, thereby causing the common mode choke to lose its effect.

According to the frequency characteristics of the interference signal, nickel-zinc ferrite or manganese-zinc ferrite can be selected, and the former has higher frequency characteristics than the latter. The magnetic permeability of MnZn ferrite is several thousand---tens of thousands, and the nickel-zinc ferrite is several hundred---thousands. The higher the magnetic permeability of the ferrite, the higher the impedance at low frequencies and the lower the impedance at high frequencies. Therefore, in suppressing high-frequency interference, nickel-zinc ferrite should be used; otherwise, manganese-zinc ferrite is used. Or put manganese Zn and nickel zinc ferrite on the same cable at the same time, which can suppress the interference band is wider.

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