About the selection of magnetic core



The magnetic ring that we normally see at one or both ends of the power or signal line of an electronic device is the common-mode choke coil. The common mode choke coil can form a large impedance to the common mode interference current, but has no effect on the differential mode signal (the working signal is the differential mode signal), so it is simple to use without considering the signal distortion. And the common mode choke coil does not need to be grounded, can be directly added to the cable. The number of turns of the ring is selected by passing the entire bundle of cables through a ferrite ring to form a common-mode choke coil. The higher the number of turns, the better the interference suppression effect on the lower frequency, but the weaker the noise suppression effect on the higher frequency. In practical 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 relatively wide, two magnetic rings can be set on the cable, each magnetic ring surrounding a different number of turns, so as to suppress both high-frequency interference and low-frequency interference. From the mechanism of common-mode choke, the greater the impedance, the more obvious the effect of interference suppression. The impedance of the common mode choke coil comes from the common mode inductance Lcm=jwLcm. It is not difficult to see from the formula that, for a certain frequency of noise, the greater the inductance of the magnetic ring, the better. This is not the case because the actual magnetic ring also has parasitic capacitance, which exists in parallel with the inductance. When the high frequency interference signal is encountered, the capacitive reactance is small, the inductance of the magnetic ring is short-circuited, so that the common mode choke coil loses its function. According to the frequency characteristics of the interference signal, nickel - zinc ferrite or manganese - zinc ferrite can be used. The permeability of manganese-zinc ferrites is in the thousands -- tens of thousands, while that of nickel-zinc ferrites is in the hundreds -- thousands. The higher the permeability of ferrite, the greater the impedance at low frequency and the smaller the impedance at high frequency. Therefore, in the suppression of high frequency interference, should choose nickel zinc ferrite; Instead, use manganese zinc ferrite. Or in the same bundle of cables at the same time on the manganese zinc and nickel zinc ferrite, so that the interference band can be suppressed wider. The greater the difference between the inner and outer diameters of the magnetic ring, the greater the longitudinal height, and the greater the impedance. However, the inner diameter of the magnetic ring must include the cable to avoid magnetic leakage. The magnetic ring should be installed as close as possible to the source of interference, that is, it should be close to the cable's inlet and outlet.