The main raw material of magnetic beads is ferrite. Ferrite is a kind of ferrimagnetic material with cubic lattice structure. Ferrite material is Fe-Mg alloy or Fe-Ni alloy. Its manufacturing process and mechanical properties are similar to those of ceramics, and its color is gray-black. Ferrite material is a kind of magnetic core often used in electromagnetic interference filter. Many manufacturers provide ferrite materials specially used for electromagnetic interference suppression. This material is characterized by very high loss at high frequency and high permeability. It can be the smallest capacitance between the coil windings of the inductor under the condition of high resistance at high frequency. For ferrites used to suppress electromagnetic interference, the most important performance parameter is permeability μ And saturation magnetic flux density Bs. Permeability μ It can be expressed as a complex number. The real number part constitutes the inductance, and the imaginary number 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 inductance impedance formed by the wire increases with the increase of frequency in form, but its mechanism is completely different at different frequencies.
In the low frequency band, the impedance is composed of the inductive reactance of the inductor. At low frequency, R is very small, and the magnetic permeability of the magnetic core is high, so the inductance is large. L plays the main role, and the electromagnetic interference is suppressed by reflection. At this time, the loss of the magnetic core is small. The whole device is an inductor with low loss and high Q characteristics, which is easy to cause resonance. Therefore, in the low frequency band, sometimes the interference may be enhanced after the use of ferrite magnetic beads.
In the high frequency band, the impedance is composed of resistance components. With the increase of frequency, the permeability of the magnetic core decreases, resulting in the reduction of the inductance of the inductor and the reduction of the inductive reactance components. However, at this time, the loss of the magnetic core increases, and the resistance component increases, resulting in the increase of the total impedance. When the high frequency signal passes through the ferrite, the electromagnetic interference is absorbed and converted into heat energy for dissipation.
Ferrite suppression elements are widely used in printed circuit boards, power lines and data lines. If a ferrite suppression element is added at the power line inlet of the printed board, high-frequency interference can be filtered. Ferrite magnetic ring or bead is specially used to suppress high-frequency interference and spike interference on signal line and power line. It also has the ability to absorb electrostatic discharge pulse interference.
The numerical value of the two elements is proportional to the length of the magnetic bead, and the length of the magnetic bead has a significant impact on the suppression effect. The longer the length of the magnetic bead, the better the suppression effect
Difference between magnetic bead and inductance
Inductors are energy storage elements, while magnetic beads are energy conversion (consumption) devices. Inductance is mostly used in power filter circuit, focusing on restraining conductive interference; Magnetic beads are mostly used in signal circuits, mainly in EMI. Magnetic beads are used to absorb UHF signals, such as some RF circuits, PLL, oscillation circuits, and circuits containing UHF memory (DDR, RAMBUS, etc.). Magnetic beads need to be added to the power input part. Inductance is an energy storage element, which is used in LC oscillation circuits, medium and low frequency filter circuits, and its application frequency range rarely exceeds 50MHz.
1. Chip inductor: inductive elements and EMI filter elements are widely used in the board circuit of electronic equipment. These components include chip inductors and chip magnetic beads. The following describes the characteristics of these two devices and analyzes their common and special applications. The advantages of surface mount components are small package size and can meet the requirements of actual space. In addition to the difference in impedance value, current-carrying capacity and other similar physical characteristics, other performance characteristics of through-hole connectors and surface mount devices are basically the same. When chip inductors are needed, they are required to achieve the following two basic functions: circuit resonance and choke reactance. Resonance circuit includes resonance generation circuit, oscillation circuit, clock circuit, pulse circuit, waveform generation circuit, etc. The resonant circuit also includes a high Q bandpass filter circuit. To make the circuit produce resonance, there must be both capacitance and inductance in the circuit. There is parasitic capacitance at both ends of the inductor, which is caused by the fact that the ferrite body between the two electrodes of the device is equivalent to the capacitance medium. In the resonant circuit, the inductance must have high Q, narrow inductance deviation and stable temperature coefficient to meet the requirements of narrow band and low frequency temperature drift of the resonant circuit. High-Q circuit has sharp resonance peak. Narrow inductance bias ensures that the resonance frequency deviation is as small as possible. The stable temperature coefficient ensures that the resonant frequency has stable temperature change characteristics. The difference between standard radial lead out inductor and axial lead out inductor and chip inductor is only the difference in packaging. The inductance structure includes a coil wound on a dielectric material (usually alumina ceramic material), or a hollow coil and a coil wound on a ferromagnetic material. In power applications, when used as choke, the main parameters of inductance are DC resistance (DCR), rated current, and low Q value. When used as a filter, the wide bandwidth characteristic is desired, so the high Q characteristic of the inductor is not required. Low DCR can ensure the minimum voltage drop. DCR is defined as the DC resistance of the component without AC signal.
2. Chip magnetic beads: The function of chip magnetic beads is mainly to eliminate the RF noise existing in the transmission line structure (). RF energy is the AC sine wave component superimposed on the DC transmission level. The DC component is the required useful signal, while the RF energy is the useless electromagnetic interference transmission and radiation (EMI) along the line. To eliminate these unnecessary signal energy, chip magnetic beads are used to play the role of high-frequency resistance (attenuator). This device allows DC signals to pass through and filter AC signals. Usually, the high frequency signal is above 30MHz, however, the low frequency signal will also be affected by the chip magnetic beads.
The flake magnetic beads are composed of soft ferrite materials, forming a monolith structure with high volume resistivity. Eddy current loss is inversely proportional to the resistivity of ferrite material. Eddy current loss is proportional to the square of signal frequency. Advantages of using chip magnetic beads:
Miniaturization and lightweight. It has high impedance in the noise frequency range to eliminate electromagnetic interference in the transmission line. The closed magnetic circuit structure can better eliminate the serial winding of the signal. Excellent magnetic shielding structure. Reduce the DC resistance to avoid excessive attenuation of useful signals.
Significant high-frequency characteristics and impedance characteristics (better energy elimination). Eliminate parasitic oscillation in high-frequency amplification circuit. It works effectively in the frequency range of several MHz to hundreds of MHz. To select the magnetic beads correctly, we must pay attention to the following points: what is the frequency range of unwanted signals. Who is the noise source. How much noise attenuation is required. What are the environmental conditions (temperature, DC voltage, structural strength). What is the circuit and load impedance. Whether there is space to place magnetic beads on the PCB board. The first three can be judged by observing the impedance frequency curve provided by the manufacturer. In the impedance curve, three curves are very important, namely resistance, inductive impedance and total impedance. The total impedance is described by ZR22 π fL() 2+:=fL. See DATASHEET of magnetic beads for typical impedance curve.
Through this curve, select the magnetic bead model that has the maximum impedance in the frequency range of the desired noise atte nuation and the signal attenuation is as small as possible at low frequency and DC. The impedance characteristics of chip magnetic beads will be affected under excessive DC voltage. In addition, if the operating temperature rise is too high or the external magnetic field is too large, the impedance of magnetic beads will be adversely affected.
The reason for using chip magnetic beads and chip inductors: whether to use chip magnetic beads or chip inductors mainly depends on the application. Chip inductors are needed in resonant circuits. When it is necessary to eliminate unwanted EMI noise, the use of chip magnetic beads is the best choice. Applications of chip magnetic beads and chip inductors: chip inductors: radio frequency (RF) and wireless communication, information technology equipment, radar sensors, automotive electronics, cellular phones, pagers, audio equipment, PDAs (personal digital assistants), wireless remote control systems and low-voltage power supply modules. Chip magnetic beads: filtering between clock generation circuit, analog circuit and digital circuit, I/O input/output internal connectors (such as serial port, parallel port, keyboard, mouse, long-distance telecommunications, local area network), radio frequency (RF) circuit and logic equipment susceptible to interference, filtering high-frequency conducted interference in power supply circuit, EMI noise suppression in computers, printers, video recorders (VCRS), television systems and mobile phones.
Selection of magnetic beads
1. The unit of magnetic beads is ohm, not Hunter, which should be paid special attention to. Because the unit of magnetic bead is nominal according to its impedance generated at a certain frequency, and the unit of impedance is also ohm. The DATASHEET of magnetic beads generally provides the characteristic curves of frequency and impedance, generally taking 100MHz as the standard, such as 1000R@100MHz It means that the impedance of the magnetic bead is equivalent to 600 ohms at 100 MHz.
2. Ordinary filter is composed of lossless reactance elements. Its function in the circuit is to reflect the stopband frequency back to the signal source, so this type of filter is also called reflection filter. When the reflection filter does not match the impedance of the signal source, a part of the energy will be reflected back to the signal source, resulting in the enhancement of the interference level. To solve this problem, ferrite magnetic ring or magnetic bead sleeve can be used on the inlet line of the filter, and the eddy current loss of the high frequency signal caused by the magnetic ring or bead can be used to convert the high frequency component into heat loss. Therefore, magnetic rings and beads actually absorb high-frequency components, so they are sometimes called absorption filters.
Different ferrite suppression elements have different optimal suppression frequency range. Generally, the higher the permeability, the lower the suppression frequency. In addition, the larger the volume of ferrite, the better the inhibition effect. When the volume is fixed, the long and thin shape has better inhibition effect than the short and thick shape. The smaller the inner diameter is, the better the inhibition effect is. However, in the case of DC or AC bias current, there is also the problem of ferrite saturation. The larger the cross section of the suppression element, the more difficult it is to be saturated, and the greater the allowable bias current.
When EMI absorbs the magnetic ring/bead to suppress the differential mode interference, its current value is proportional to its volume, and the misalignment of the two causes saturation, which reduces the component performance; When suppressing common-mode interference, the two lines (positive and negative) of the power supply pass through a magnetic ring at the same time. The effective signal is a differential mode signal. The EMI absorption magnetic ring/bead has no effect on it, while the common-mode signal will show a large amount of inductance. Another good way to use the magnetic ring is to repeatedly wind the wire through the magnetic ring several times to increase the inductance. According to its suppression principle of electromagnetic interference, its suppression effect can be reasonably used.
The ferrite suppression element shall be installed near the interference source. For input/output circuits, they should be as close as possible to the inlet and outlet of the shielding shell. For the absorption filter composed of ferrite magnetic ring and magnetic bead, in addition to selecting the lossy material with high permeability, attention should also be paid to its application. Their resistance to high-frequency components in the circuit is about ten to several hundred Ω, so their role in high-impedance circuits is not obvious. On the contrary, they will be very effective in low-impedance circuits (such as power distribution, power supply or RF circuits).
conclusion
Because ferrite can attenuate higher frequencies and let lower frequencies pass almost unimpeded, it has been widely used in EMI control. Magnetic rings/beads for EMI absorption can be made into various shapes and widely used in various occasions. For example, it can be added to DC/DC module, data line, power line, etc. It absorbs high-frequency interference signals on the line where it is located, but it will not generate new zeros and poles in the system and will not destroy the stability of the system. It can be used in conjunction with power filter to supplement the performance of the high frequency end of the filter and improve the filtering characteristics in the system.
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