"I Principle of magnetic beads
The main raw material of magnetic beads is ferrite. Ferrite is a kind of ferromagnetic material with 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. A kind of magnetic core often used in EMI filter is ferrite material. Many manufacturers provide ferrite materials specially used for EMI suppression. This material is characterized by very large high frequency loss and high permeability. It can be the smallest capacitance between the coil windings of the inductor under the condition of high frequency and high resistance. For ferrite used to suppress electromagnetic interference, the most important performance parameter is permeability μ And saturated flux density BS. Permeability μ It 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 of which are functions of frequency. When the conductor passes through the ferrite core, the inductance impedance increases with the increase of frequency, but the 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, the permeability of the magnetic core is high, so the inductance is large. L plays a major role, and the electromagnetic interference is reflected and suppressed. At this time, the loss of the magnetic core is small. The whole device is an inductor with low loss and high Q characteristics. This inductor is easy to cause resonance. Therefore, in the low frequency band, Sometimes, the interference may be enhanced after the use of ferrite beads.
In the high frequency band, the impedance is composed of resistance component. With the increase of frequency, the permeability of the magnetic core decreases, resulting in the decrease of inductance and inductive reactance component. However, at this time, the loss of the magnetic core and the resistance component increase, 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.
Ferrite suppression elements are widely used in printed circuit boards, power lines and data lines. If a ferrite suppression element is added at the inlet of the power line of the printed board, the high-frequency interference can be filtered out. Ferrite magnetic rings or beads are specially used to suppress high-frequency interference and peak interference on signal lines and power lines. It also has the ability to absorb electrostatic discharge pulse interference.
The numerical value of the two elements is directly 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.
II Structural characteristics of magnetic beads
When the current in the conductor passes through, ferrite has little impedance to the low-frequency current, but has a great attenuation effect to the higher frequency current. The high-frequency current is dissipated in the form of heat. Its equivalent circuit is an inductor and a resistor in series. The values of the two components are proportional to the length of the magnetic bead. There are many kinds of magnetic beads. The manufacturer shall provide technical specifications, especially the curve of the relationship between impedance and frequency of magnetic beads. Some magnetic beads have multiple holes, which can increase the component impedance (the square of the times of passing through the magnetic beads) by passing through the wire. However, the increased noise suppression ability at high frequency cannot be as expected, and it is better to use more magnetic beads in series.
Ferrite is a magnetic material, which will produce magnetic saturation due to excessive current, and the permeability will decrease sharply. Magnetic beads specially designed in structure shall be used for high current filtering, and its heat dissipation measures shall be paid attention to. Ferrite magnetic beads can not only be used to filter high-frequency noise in power supply circuits (DC and AC output), but also be widely used in other circuits, and its volume can be made very small. Especially in digital circuits, because the pulse signal contains high-frequency harmonics, which is also the main source of high-frequency radiation of the circuit, it can play the role of magnetic beads in this occasion. Ferrite beads are also widely used in noise filtering of signal cables.
III Main characteristic parameters of magnetic bead
1. DC resistance (MOhm): the resistance value of the magnetic bead when DC current passes through the magnetic bead.
2. Rated current (MA): indicates the maximum allowable current when the magnetic bead works normally.
3. Impedance [Z] @ 100MHz (Ohm): here refers to AC impedance.
4. Impedance frequency characteristic: a curve describing the change of impedance value with frequency.
5. Resistance frequency characteristic: a curve describing the change of resistance value with frequency
6. Inductive reactance frequency characteristic: describes the curve of inductive reactance varying with frequency.
The figure below shows the characteristic parameters and frequency characteristic curve of magnetic beads of a manufacturer:
IV Difference between magnetic bead and inductance
Inductance is an energy storage element, and magnetic beads are energy conversion (consumption) devices. Inductors are mostly used in power filter circuits, focusing on restraining conductive interference; Magnetic beads are mostly used in signal circuits, mainly in EMI. Magnetic beads are used to absorb UHF signals. For example, some RF circuits, PLL, oscillation circuits and UHF memory circuits (DDR, SDRAM, Rambus, etc.), magnetic beads need to be added to the power input part, while inductance is an energy storage element, which is used in LC oscillation circuit, medium and low frequency filter circuit, and its application frequency range rarely exceeds 50MHz.
1. Chip inductor: inductive elements and EMI filter elements will be widely used in PCB 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 general and special applications. The advantages of surface mount components are small package size and can meet the requirements of actual space. In addition to different 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 realize 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 and so on. The resonant circuit also includes a high-Q band-pass filter circuit. To make the circuit resonate, capacitance and inductance must exist in the circuit at the same time. Parasitic capacitance exists at both ends of the inductor, which is due to the fact that the ferrite body between the two electrodes of the device is equivalent to a capacitive medium. In the resonant circuit, the inductance must have high Q, narrow inductance deviation and stable temperature coefficient in order 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 variation characteristics. The difference between the standard radial lead-out inductor, axial lead-out inductor and chip inductor is only that the package is different. 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 a choke, the main parameters of inductance are DC resistance (DCR), rated current, and low Q value. When used as a filter, a 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 (PCB circuit). RF energy is the AC sine wave component superimposed on the DC transmission level. DC component is the required useful signal, while RF RF energy is useless. Electromagnetic interference is transmitted and radiated (EMI) along the line. To eliminate these unwanted signal energy, chip magnetic beads are used as high-frequency resistors (attenuators), which allow DC signals to pass through and filter AC signals. Usually, the high-frequency signal is more than 30MHz. However, the low-frequency signal will also be affected by chip magnetic beads.
The chip 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 materials. Eddy current loss is proportional to the square of signal frequency. Benefits of using chip magnetic beads:
Miniaturization and lightweight. It has high impedance in the frequency range of RF noise to eliminate electromagnetic interference in the transmission line. The closed magnetic circuit structure can better eliminate the series winding of signals. Excellent magnetic shielding structure. Reduce the DC resistance to avoid excessive attenuation of useful signals.
Significant high frequency characteristics and impedance characteristics (better elimination of RF energy). Eliminate parasitic oscillation in high frequency amplification circuit. It works effectively in the frequency range of several MHz to hundreds of MHz. To correctly select magnetic beads, 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 PCB. 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 reactance and total impedance. The total impedance is described by zr22 π FL () 2 +: = fl. Typical impedance curves can be found in the datasheet of magnetic beads.
Through this curve, the magnetic bead model with the maximum impedance in the frequency range where the noise is expected to be attenuated and the signal attenuation as small as possible under low frequency and DC is selected. The impedance characteristics of chip magnetic beads will be affected under excessive DC voltage. In addition, if the working temperature rise is too high or the external magnetic field is too large, the impedance of magnetic beads will be adversely affected.
Reasons for using chip magnetic beads and chip inductors: whether to use chip magnetic beads or chip inductors mainly depends on the application. Chip inductors need to be used 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 detector, automotive electronics, cellular telephone, pager, audio equipment, PDAs (personal digital assistant), wireless remote control system and low-voltage power supply module, etc. 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), between radio frequency (RF) circuit and easily disturbed logic equipment, filtering high-frequency conducted interference in power supply circuit, computer, printer, video recorder (VCRs), EMI noise suppression in TV systems and mobile phones.
V Selection of magnetic beads
one 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 the impedance it produces at a certain frequency, and the unit of impedance is also ohm. The characteristic curves of frequency and impedance are generally provided on the datasheet of the magnetic beads. Generally, 100MHz is used as the standard, such as 1000R@100MHz At 100MHz, the impedance of the magnetic bead is equivalent to 600 ohms.
two Ordinary filter is composed of lossless reactance elements. Its function in the line is to reflect the stopband frequency back to the signal source, so this kind of filter is also called reflection filter. When the impedance of the reflection filter does not match the signal source, some energy will be reflected back to the signal source, resulting in the enhancement of the interference level. In order to solve this problem, ferrite magnetic ring or magnetic bead sleeve can be used on the inlet line of the filter to convert the high-frequency component into heat loss by using the eddy current loss of the ring or magnetic bead on the high-frequency signal. 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 ranges. 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, and the smaller the inner diameter, the better the inhibition effect. 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 less easy it is to saturate, and the greater the bias current it can bear.
When EMI absorbing magnetic ring / bead suppresses differential mode interference, its current value is directly proportional to its volume. The imbalance between them causes saturation and reduces the performance of the element; When suppressing common mode interference, two wires (positive and negative) of the power supply pass through a magnetic ring at the same time. The effective signal is the differential mode signal. The EMI absorption magnetic ring / bead has no effect on it, but it will show a large inductance for the common mode signal. Another better way to use the magnetic ring is to make the wires passing through the magnetic ring wind several times repeatedly to increase the inductance. According to its suppression principle of electromagnetic interference, its suppression effect can be used reasonably.
The ferrite suppression element shall be installed close to the interference source. For the input / output circuit, it shall 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 beads, we should not only select the consumable material with high permeability, but also pay attention to its application. Their resistance to high-frequency components in the line is about ten to hundreds of Ω, so its role in high impedance circuits is not obvious. On the contrary, it will be very effective in low impedance circuits (such as power distribution, power supply or RF circuits).
Vi conclusion
Ferrite is widely used in EMI control because it can attenuate higher frequency and let lower frequency pass almost unimpeded. Magnetic rings / beads for EMI absorption can be made into various shapes and are widely used in various occasions. For example, it can be added to DC / DC module, data line, power line, etc. It absorbs the high-frequency interference signal on the line, but it will not produce new zeros and poles in the system and will not destroy the stability of the system. It is used together with the power filter, which can well supplement the performance of the high-frequency end of the filter and improve the filtering characteristics of the system.
Magnetic beads have high resistivity and permeability. They are equivalent to resistance and inductance in series, but both resistance and inductance change with frequency. Compared with ordinary inductors, it has better high-frequency filtering characteristics and shows resistance at high frequency, so it can maintain high impedance in a wide frequency range, so as to improve the effect of FM filtering.
As a power filter, inductance can be used. The circuit symbol of magnetic beads is inductance, but it can be seen from the model that magnetic beads are used. In terms of circuit function, the principle of magnetic beads and inductance is the same, but the frequency characteristics are different.
The magnetic bead is composed of oxygen magnet, and the inductance is composed of magnetic core and coil. The magnetic bead converts the AC signal into heat energy, and the inductance stores the AC and releases it slowly.
The magnetic bead has a great obstacle to the high-frequency signal. The general specification is 100 Ω / 100mmhz. Its resistance is much smaller than the inductance at low frequency.
Ferrite bead is an anti-interference component with rapid application and development. It is cheap, easy to use and has remarkable effect of filtering high-frequency noise.
In the circuit, as long as the wire passes through it (what I use is like an ordinary resistor. The wire has been passed through and glued, and there are also surface mount forms, but I rarely see them sold). When the current in the conductor passes through, ferrite has little impedance to the low-frequency current, but has a great attenuation effect to the higher frequency current. The high-frequency current is dissipated in the form of heat. Its equivalent circuit is an inductor and a resistor in series. The values of the two components are proportional to the length of the magnetic bead. There are many kinds of magnetic beads. The manufacturer shall provide technical specifications, especially the curve of the relationship between impedance and frequency of magnetic beads.
Some magnetic beads have multiple holes, which can increase the component impedance (the square of the times of passing through the magnetic beads) by passing through the wire. However, the increased noise suppression ability at high frequency cannot be as expected, and it is better to use more magnetic beads in series.
Ferrite is a magnetic material, which will produce magnetic saturation due to excessive current, and the permeability will decrease sharply
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