Purpose of magnetic ring inductance

The electromagnetic waves emitted and leaked by electronic devices not only seriously interfere with the normal operation of other electronic devices, leading to device dysfunction, transmission errors, but also posing a threat to human health and safety, posing a great threat. Therefore, reducing electromagnetic interference (EMI) of electronic devices has become a necessary consideration. Absorbing magnetic ring, also known as ferrite magnetic ring, abbreviated as magnetic ring. It is a commonly used anti-interference component in electronic circuits, which has a good suppression effect on high-frequency noise. It is generally made of ferrite material (Mn-Zn).

The magnetic ring has different impedance characteristics at different frequencies, and the impedance is generally very small at low frequencies. When the signal frequency increases, the impedance of the magnetic ring increases sharply.

As we all know, the higher the signal frequency, the easier it is to radiate (to buy a high-quality computer case, it is also necessary to reduce electromagnetic leakage). Generally, signal lines do not have a shielding layer, so these signal lines become good antennas to receive various chaotic high-frequency signals in the surrounding environment. These signals are superimposed on the originally transmitted signals, and even change the useful signals that were originally transmitted. So under the action of the magnetic ring, normal and useful signals can pass well, and high-frequency interference signals can be effectively suppressed, and the cost is low.

So it's not surprising that you can see plastic lump-shaped integrated magnetic rings on display signal cables, USB connection cables, and even high-end keyboards and mice. Passing the entire bundle of cables through a ferrite magnetic ring forms a common mode choke, and the cable can also be wound several turns on top of the magnetic ring as needed. The more turns, the better the suppression effect on interference with lower frequencies, while the weaker the suppression effect on noise with higher frequencies. In practical engineering, the number of turns of the magnetic ring should be adjusted based on the frequency characteristics of the interference current. Usually, when the frequency band of the interference signal is wide, two magnetic rings can be placed on the cable, each with a different number of turns, which can simultaneously suppress high-frequency interference and low-frequency interference. From the mechanism of the action of the common mode choke, the larger its impedance, the more obvious its interference suppression effect. The impedance of the common mode choke 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 larger the inductance of the magnetic ring, the better. But this is not the case in reality, as there are parasitic capacitors on the actual magnetic ring, which exist in parallel with the inductance. When encountering high-frequency interference signals, the capacitive reactance of the capacitor is small, which shortens the inductance of the magnetic ring and makes the common mode choke ineffective.