Understanding of magnetic beads used in high frequency absorption filters

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Understanding of magnetic beads used in high frequency absorption filters [Copy link]

The full name of magnetic beads is ferrite bead filter, which is a kind of anti-interference component with rapid application development. It is cheap, easy to use, and has a significant effect in filtering high-frequency noise. Another kind of magnetic beads is made of ultra-small amorphous alloy magnetic materials that have come out in recent years. It is not the same material as ferrite. (Note: Please distinguish it from the "insulating porcelain beads" in electrical technology-editor) The main raw material of magnetic beads is ferrite, which is a ferrimagnetic material with a cubic lattice structure. The characteristics of this material are very large high-frequency loss and high magnetic permeability, which minimizes the capacitance between the coil windings of the inductor under high frequency and high resistance conditions. When current passes through the wire, ferrite has almost no impedance to low-frequency current, but will produce greater attenuation for higher frequency current. For ferrite used to suppress electromagnetic interference, the most important performance parameters are magnetic permeability μ and saturation flux density Bs. Its equivalent circuit is an inductor and a resistor in series, and the values of both components are proportional to the length of the magnetic bead. When the wire passes through this ferrite core, the impedance of the inductor increases with the increase of frequency. High-frequency current is dissipated in it as heat. In the low frequency band, the impedance is composed of the inductive reactance of the inductor. At low frequencies, R is very small and the magnetic permeability of the magnetic core is high, so the inductance is large, L plays a major role, electromagnetic interference is reflected and suppressed, and the loss of the magnetic core is small at this time. The entire device is an inductor with low loss and high Q characteristics. This inductor is prone to resonance. Therefore, in the low frequency band, sometimes the interference may be enhanced after using ferrite beads. In the high frequency band, 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. At this time, the loss of the magnetic core increases, the resistance component increases, and the total impedance increases. When high-frequency signals pass through ferrite, electromagnetic interference is absorbed and converted into heat energy and dissipated. Ferrite suppression components are widely used in printed circuit boards. On power lines and data lines, such as adding ferrite suppression components at the power line entrance of the printed circuit board, high-frequency interference can be filtered out. Ferrite magnetic rings or magnetic beads are specially used to suppress high-frequency interference and spike interference on signal lines and power lines. They also have the ability to absorb electrostatic discharge pulse interference. Inductors are energy storage components, while magnetic beads are energy conversion (consumption) devices. Inductors are mostly used in power supply filter circuits, focusing on suppressing conducted interference; magnetic beads are mostly used in signal circuits, mainly for EMI (electromagnetic compatibility). Magnetic beads are used to absorb ultra-high frequency signals. For example, in some RF circuits, PLLs, oscillation circuits, and ultra-high frequency memory circuits, magnetic beads need to be added to the power input part. Inductance is an energy storage component, mostly used in LC oscillation circuits, medium and low frequency filter circuits, etc., and its application frequency range rarely exceeds 50MHz. 1 The unit of magnetic beads is ohms, not hunters. The unit of magnetic beads is nominal according to the impedance it produces at a certain frequency, and the unit of impedance is also ohms. The data parameter table (DATASHEET) of magnetic beads generally provides a characteristic curve of frequency and impedance. 100MHz is often used as the standard, such as 600R 100MHz, which means that the impedance of the magnetic beads is equivalent to 600 ohms at a frequency of 100MHz. For example, the parameters of a magnetic bead are 120ohm, 25%, 3A, 1206, where 120ohm means that at a frequency of 100MHz, the impedance value of the magnetic bead is 120 ohms; 25% means that the allowable error of the above impedance is ±25%; 3A refers to the maximum current allowed to flow through the magnetic bead; 1206 refers to the size of the magnetic bead, EIAl206 (English: inches) is equivalent to JIS/IEC3216 (International System of Units: millimeters), that is, 3.2mm long and 1.6mm wide. 2 Magnetic rings and magnetic beads absorb high-frequency components, also known as absorption filters. Ordinary filters are composed of lossless reactive elements. Their function in the line is to reflect the stopband frequency back to the signal source. This type of ordinary filter is also called a reflection filter. When the reflection filter does not match the impedance of the signal source, part of the energy will be reflected back to the signal source, causing the interference level to increase. To solve this problem, ferrite magnetic rings or magnetic beads can be used on the incoming line of the filter, and the eddy current loss of the magnetic rings or magnetic beads on the high-frequency signal can be used to convert the high-frequency component into heat loss. Therefore, the magnetic rings and magnetic beads actually absorb the high-frequency components. 3 Magnetic beads suppress switching noise and belong to the active suppression type. Magnetic beads are different from ordinary noise filters. Usually, noise filters can only absorb the noise that has occurred, which belongs to the passive suppression type. The role of magnetic beads is different. It can suppress the generation of switching noise. It belongs to the active suppression type, which is the fundamental difference between the two. Magnetic beads can be widely used in high-frequency switching power supplies, video recorders, electronic measuring instruments, and various circuits with very strict noise requirements. Different ferrite suppression components have different optimal suppression frequency ranges. Generally, the higher the magnetic permeability, the lower the suppression frequency. In addition, the larger the volume of the ferrite, the better the suppression effect. When the volume is constant, a long and thin shape has a better suppression effect than a short and thick one, and the smaller the inner diameter, the better the suppression effect. However, in the case of DC or AC bias current, there is still the problem of ferrite saturation. The larger the cross-section of the suppression element, the less likely it is to saturate, and the greater the bias current it can withstand.