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What is a magnetic bead and how to choose a magnetic bead

163.com | 08/06/2021

1. What are magnetic beads?

Magnetic beads have very high resistivity and permeability. They are equivalent to a series connection of resistance and inductance, but the resistance value and inductance value vary with frequency. It has better high-frequency filtering characteristics than ordinary inductors, and shows resistance at high frequencies, so it can maintain a higher impedance in a relatively wide frequency range, thereby improving the FM filtering effect.

As a power filter, an inductor can be used. The circuit symbol of a magnetic bead is an inductance, but it can be seen from the model that the magnetic bead is used in the circuit function. The principle of the magnetic bead and the inductance are the same, but the frequency characteristics are different.

The magnetic beads are composed of oxygen magnets, and the inductors are composed of magnetic cores and coils. The magnetic beads convert AC signals into heat energy, and the inductors store the AC and release them slowly.

The magnetic beads have a greater hindrance to high-frequency signals. The general specification is 100 ohms/100mMHZ. Its resistance is much smaller than inductance at low frequencies.

Ferrite Bead is a kind of anti-jamming component with rapid application development. It is cheap, easy to use, and has a significant effect on filtering high frequency noise.

As long as the wire passes through it in the circuit. When the current in the wire passes through, the ferrite has almost no impedance to the low-frequency current, but will have a greater attenuation effect on the higher-frequency current. The high-frequency current is dissipated in the form of heat. The equivalent circuit is an inductance and a resistor in series, and the values ​​of the two components are proportional to the length of the magnetic beads. There are many types of magnetic beads, and the manufacturer should provide technical specifications, especially the curve of the relationship between the impedance of the magnetic beads and the frequency.

Some magnetic beads have multiple holes. Using a wire to pass through can increase the impedance of the component (the square of the number of times the magnetic bead is passed), but the increased noise suppression ability at high frequencies may not be as much as expected, so use more series A few magnetic beads will be better.

Ferrite is a magnetic material, which will cause magnetic saturation due to excessive current passing through it, and its permeability will drop sharply. High-current filtering should use specially designed magnetic beads in structure, and pay attention to its heat dissipation measures.

Ferrite beads can be used not only to filter high frequency noise in power circuits (for DC and AC output), but also to be widely used in other circuits, and their volume can be made small. Especially in digital circuits, because the pulse signal contains high-frequency harmonics, it is also the main source of high-frequency radiation in the circuit, so it can play the role of magnetic beads in this kind of occasion.

2. The working principle of magnetic beads

The main raw material of magnetic beads is ferrite. Ferrite is a ferrimagnetic material with a cubic lattice structure. The ferrite material is iron-magnesium alloy or iron-nickel alloy. Its manufacturing process and mechanical properties are similar to ceramics, and its color is gray-black. One type of magnetic core often used in electromagnetic interference filters is ferrite materials, and many manufacturers provide ferrite materials specifically for electromagnetic interference suppression. This material is characterized by very large high-frequency loss and high magnetic permeability. It can be the smallest capacitance generated between the coil windings of the inductor under the condition of high-frequency and high-resistance. For ferrites used to suppress electromagnetic interference, the most important performance parameters are permeability μ and saturation magnetic flux density Bs. 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 the increase of frequency. Therefore, its equivalent circuit is a series circuit composed of inductance L and resistance R. Both L and R are functions of frequency. When the wire passes through the ferrite core, the inductive impedance formed increases in form with the increase of frequency, but the mechanism is completely different at different frequencies.

In the low frequency range, the impedance is composed of the inductive reactance of the inductance. At low frequencies, R is very small, and the permeability of the magnetic core is high, so the inductance is large. L plays the main role. The electromagnetic interference is reflected and suppressed. The core loss is small, and the entire device is a low-loss, high-Q inductor. This inductance is likely to cause resonance. Therefore, in low frequency bands, interference may increase after ferrite beads are used.

In the high frequency range, the impedance is composed of the resistance component. As the frequency increases, the magnetic permeability of the magnetic core decreases, resulting in a decrease in the inductance of the inductor and a decrease 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, electromagnetic interference is absorbed and converted into heat to be dissipated.

Ferrite suppression components are widely used in printed circuit boards, power lines and data lines. For example, adding ferrite suppression components at the entrance of the power cord of the printed board can filter out high-frequency interference. Ferrite bead or bead is specially used to suppress high-frequency interference and spike interference on signal lines and power lines. It also has the ability to absorb electrostatic discharge pulse interference.

The value of the two elements is proportional to the length of the magnetic bead, and the length of the magnetic bead has a significant influence on the suppression effect. The longer the length of the magnetic bead, the better the suppression effect.

3. the selection/selection of magnetic beads

The unit of the magnetic beads is ohms, not Hunter. This point must be paid special attention to. Because the unit of the magnetic bead is nominally based on the impedance it generates at a certain frequency, the unit of impedance is also ohms. The DATASHEET of the magnetic beads generally provides frequency and impedance characteristic curves. Generally, 100MHz is the standard, such as 1000R@100MHz, which means that the impedance of the magnetic beads is equivalent to 600 ohms at a frequency of 100MHz.

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