Selection and application of ferrite magnetic ring

Absorbing magnetic ring, also known as ferrite magnetic ring, is called magnetic ring for short. It is an anti-interference component commonly used in electronic circuits and has a good weakening effect on high-frequency noise. It is generally made of ferrite material (Mn Zn). This material is characterized by high frequency loss and high permeability. The most important parameter is permeability μ And saturation flux density Bs.  

The magnetic ring solves the problem of high-frequency interference weakening of power lines, signal lines and connectors, and has a series of advantages such as simple, convenient, effective, and small space occupation. Using ferrite anti-interference magnetic core to weaken electromagnetic interference (EMI) is an economical, simple and effective method, which has been widely used in various military or civil electronic equipment such as computers.

Selection of magnetic ring

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

A common mode choke coil is formed by passing the whole cable through a ferrite magnetic ring. If necessary, the cable can also be wound on the magnetic ring for several turns. The more turns, the better the effect on the interference with lower frequency, while the weaker the effect on the noise with higher frequency. In practical projects, 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 interference signal is wide, two magnetic rings can be sleeved on the cable, and each magnetic ring is wound with different turns, so that high-frequency interference and low-frequency interference can be weakened at the same time. From the mechanism of common mode choke, the greater the impedance is, the more obvious the effect of interference reduction is. The impedance of the common mode choke is from the common mode inductance lcm=jwlcm. It is not difficult to see from the formula that, for the noise of a certain frequency, the larger the inductance of the magnetic ring is, the better. However, this is not the case, because there are parasitic capacitors on the actual magnetic ring, which exist in parallel with the inductor. When encountering high-frequency interference signals, the capacitive reactance of the capacitor is small, and the inductance of the magnetic ring is short circuited, thus making the common mode choke ineffective.

According to the frequency characteristics of the interference signal, nickel zinc ferrite or manganese zinc ferrite can be selected, and the high-frequency characteristics of the former is better than the latter. The magnetic permeability of manganese zinc ferrite is thousands to tens of thousands, while that of nickel zinc ferrite is hundreds to thousands. The higher the permeability of ferrite is, the greater the impedance at low frequency is, and the smaller the impedance at high frequency is. Therefore, nickel zinc ferrite should be used to weaken high-frequency interference; On the contrary, manganese zinc ferrite is used. Or, manganese zinc and nickel zinc ferrite are sheathed on the same bundle of cables at the same time, which can weaken the interference frequency band.

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 be tightly wrapped around the cable to avoid magnetic leakage.

The magnetic ring shall be installed as close to the interference source as possible, that is, close to the inlet and outlet of the cable.