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The use of "little things" magnetic beads, magnetic rings and magnetic clips in the rectification of

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Issuing time:2020-06-05 09:00

The use of "little things" magnetic beads, magnetic rings and magnetic clips in the rectification of electromagnetic compatibility practice


There are two special types of inductance in electromagnetic compatibility rectification applications


Ferrite beads, ferrite magnetic rings, magnetic clips


Ferrite beads are single-ring inductors, which form a single ring by passing a single wire through a ferrite profile. The attenuation of this device is 10dB in the high-frequency range, and the attenuation is small at DC. Similar to ferrite beads, the common mode (CM) and differential mode (DM) attenuation in the frequency range up to GHz can reach 10-20dB.


The ferrite material is iron-magnesium alloy or iron-nickel alloy, which is a ferrite magnetic material with cubic lattice structure. This material has a very high magnetic permeability, which can minimize the capacitance generated between the coil windings of the inductor under high frequency and high resistance. Its manufacturing process and mechanical properties are similar to ceramics, and its color is gray-black. For ferrites for electromagnetic interference suppression, the most important performance parameters are permeability and saturation flux density. The permeability can be expressed as a complex number, the real part constitutes the inductance, and the imaginary part represents the loss, which increases as the frequency increases. Therefore, the equivalent circuit of the magnetic bead is a series circuit composed of inductance L and resistance R, where X and R are both functions of frequency, as shown in the following figure:


Ferrite materials are usually applied at high frequencies, because at low frequencies the ferrite mainly exhibits inductive characteristics, and the loss on the line is small; at high frequencies, the ferrite mainly exhibits resistive characteristics, which attenuates high-frequency interference Very useful, both inductance and resistance change with frequency. Therefore, in practical applications, ferrite materials can be used as high-frequency attenuators for radio frequency circuits.


Ferrite beads have better high-frequency filtering characteristics than ordinary inductors. Ferrite exhibits resistance at high frequencies, which is equivalent to an inductor with a very low quality factor, so it can maintain a relatively high impedance over a wide range, thereby improving filtering performance.


In the low frequency band, the impedance is formed by the inductive reactance of the inductor. At low frequencies, R is small, and the magnetic permeability of the magnetic core is high, so the inductance is large, and L plays a major role. The electromagnetic interference is reflected and suppressed; The loss of the core is small, and the entire device is an inductor with low loss and high Q characteristics. This inductance is likely to cause resonance. Therefore, in the low frequency band, sometimes interference may increase after using ferrite beads.


In the high frequency band, the impedance is composed of the resistance part. As the frequency increases, the magnetic permeability of the magnetic core decreases, resulting in a reduction in the inductance of the inductor and a reduction in the inductive reactance component. However, at this time, the loss of the magnetic core increases and the resistance component increases, resulting in an increase in the total impedance. When the high-frequency signal passes through the ferrite, the electromagnetic interference can be absorbed and converted into heat and dissipated.


Ferrite suppression components are widely used in printed circuit boards, power lines and data lines. For example, adding a ferrite suppression component to the entrance of the power line of the printed board can filter out high-frequency interference. Ferrite magnetic rings or magnetic beads are designed to suppress high-frequency interference and spike interference on signal lines and power lines. In addition, it also has the ability to absorb electrostatic discharge and pulse interference.


Different ferrite suppression components have different optimal suppression frequency ranges. Generally, the higher the permeability, the lower the frequency of suppression. In addition, the larger the volume of the ferrite, the better the suppression effect. In the case of a certain volume, the long and thin shape is generally better than the short and thick suppression effect, the smaller the inner diameter, the better the suppression effect. But in the case of DC or AC bias current, there is also the problem of ferrite saturation. The larger the cross-section of the component is, the more difficult it is to saturate, and the greater the bias current that can be tolerated.


Ferrite materials are generally installed close to the source of harassment. For the input/output circuit, it should be as close as possible to the entrance and exit of the shielding shell. It should also be noted that during installation, ferrite components are easily broken and cracked, and reliable fixing measures should be taken.


The use of chip beads or chip inductors depends on the actual application. Generally, chip inductors are used in resonant circuits, and when it is necessary to eliminate unwanted EMI noise, it is best to use chip beads s Choice.


In addition, when selecting the magnetic beads, pay attention to the flux of the magnetic beads. When using, they cannot exceed the rated current given in the device data sheet. Generally, they need to be derated by 80%. .


Finally, regardless of the magnetic beads or magnetic rings, whether they are patch or clip-on, good use of them is the problem. "It is better to teach fish than teach fish." The clever use of insignificant small magnetic beads and magnetic rings can often bring about a qualitative improvement in the spectrum. Use is not equal to use, use is not equal to use, only in the long-term experience accumulation and exploration, can you realize the mystery brought by the small ferrite.


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