The motherboard, CPU, power supply, and IDE data line in the computer case all work at a very high frequency, so there is a lot of space stray electromagnetic interference signal in the case, and the signal intensity is several times to tens of times of the case outside!

Absorption magnetic ring, also known as ferrite magnetic ring, is often used in removable separated magnetic ring, it is commonly used in electronic circuit anti-interference components, for high frequency noise has a good suppression, generally made of ferrite material (Mn-Zn). The magnetic ring has different impedance characteristics at different frequencies. Generally, the impedance is very small at low frequency, and the impedance of the magnetic ring increases sharply when the signal frequency increases. So that the normal useful signal is very good through, and can well suppress the high-frequency interference signal through, and the cost is low.

Ferrite is a ferromagnetic material with cubic lattice structure. Ferrite material is ferro-magnesium alloy or ferro-nickel alloy. Its manufacturing process and mechanical properties are similar to those of ceramics, and its color is gray black. Ferrite is a kind of magnetic core commonly used in emi filters. Many manufacturers provide ferrite materials specially used for EMI suppression. This material is characterized by very large loss at high frequency and high permeability, and he can be inductor between the winding of the coil in the case of high resistance at high frequency and small capacitance.

For ferrites used to suppress electromagnetic interference, the important performance parameters are permeability μ and saturation flux density Bs. The permeability μ can be expressed as a complex number, the real part constitutes the inductance, and the imaginary part represents the loss, which increases with increasing frequency.

Therefore, its equivalent circuit is a series circuit consisting of inductor L and resistance R, both of which are functions of frequency. When a wire passes through such a ferrite core, the form of the inductance impedance formed increases with increasing frequency, but the mechanism is completely different at different frequencies.

Different ferrite suppression elements have different good suppression frequency range. In general, the higher the permeability, the lower the frequency of inhibition. In addition, the larger the volume of ferrite, the better the inhibition effect. When the volume is constant, the long and thin shape is better than the short and thick shape, and the smaller the inner diameter is, the better the inhibition effect is. However, in the case of DC or AC bias, there is also the problem of ferrite saturation. The larger the cross section of the restraining element is, the less saturated it is, and the larger the allowable bias is.

When EMI absorbs the magnetic ring/bead to suppress the differential mode interference, the current value passing through it is proportional to its volume, which causes saturation and reduces the performance of the element. To suppress common-mode interference, the two wires of the power supply (positive and negative) pass through a magnetic ring at the same time. The effective signal is a differential mode signal, and EMI absorption of magnetic ring/bead has no influence on it, while for common-mode signals, it will show a large inductance. A better way to use a magnetic ring is to loop the wire through it several times to increase the inductance. It can be used reasonably according to its suppression principle of electromagnetic interference.

The ferrite suppression element should be installed near the interference source. For input/output circuits, the inlet and outlet of the shielding shell should be as close as possible. For the absorption filter composed of ferrite magnetic ring and magnetic bead, in addition to choosing the lossy material with high permeability, attention should be paid to its application. Their resistance to high frequency components in the circuit is in the order of ten to several hundred ohms, so their role in high impedance circuits is not obvious, instead, their use in low impedance circuits (such as power distribution, power supply, or rf circuits) will be very effective.