1. A magnetic bead is an inductor with a relatively small inductance, and in addition to being easy to make, he has features that ordinary inductors do not have.
For ordinary inductors, due to the existence of distributed capacitance, in the case of very high frequency, especially for the high harmonics, may be equivalent to a capacitor, can not play the role of inductance, but the magnetic beads of inductors do not have this drawback, so his high frequency characteristics are extremely good.
A component that corresponds to it is the through-center capacitor, which has a similar effect.
2. The role of magnetic beads2006-11-19 18:05Magnetic beads are used to suppress signal lines, power lines on the high-frequency noise and spike interference, but also has the ability to absorb static electricity pulse.
Magnetic beads are used to absorb ultra-high-frequency signals, such as some RF circuits, PLL, oscillator circuits, including ultra-high-frequency memory circuits (DDR SDRAM, RAMBUS, etc.) need to add magnetic beads in the power supply input part of the magnetic beads, and the inductance is a kind of energy storage components, used in the LC oscillator circuits, low-frequency filtering circuits, etc., and the application of the frequency range is rarely more than wrong 50MHZ.
The function of beads is mainly used to suppress signal lines, power lines and spike interference, also has the ability to absorb electrostatic pulses.
The function of the bead is mainly to eliminate the existence of RF noise in the transmission line structure (circuit), RF energy is superimposed on the DC transmission level of the AC sine wave component, DC component is needed for useful signals, while RF RF energy is useless electromagnetic interference along the line transmission and radiation (EMI). To eliminate this unwanted signal energy, a chip bead is used to play the role of a high-frequency resistor (attenuator), a device that allows DC signals to pass through while filtering out AC signals. Typically, high-frequency signals are above 30MHz, however, low-frequency signals can also be affected by the chip beads.
The chip bead consists of a soft ferrite material that forms a monolithic structure with high volume resistivity. The eddy current loss is inversely proportional to the resistivity of the ferrite material. Eddy current loss is proportional to the square of the signal frequency. Benefits of using chip beads: Miniaturization and light weight High impedance in the RF noise frequency range, eliminating electromagnetic interference in the transmission line. Closed magnetic circuit structure to better eliminate signal crosstalk. Excellent magnetic shielding. Lower DC resistance to avoid excessive attenuation of useful signals. Remarkable high frequency characteristics and impedance characteristics (better elimination of RF energy). Elimination of parasitic oscillations in high frequency amplification circuits. Effectively operate in the frequency range of a few MHz to several hundred MHz.
To properly select a bead, the following points must be noted:
1, what is the frequency range of the unwanted signal;
2, who is the source of the noise;
3, how much noise attenuation is needed;
4, what are the ambient conditions (temperature, dc voltage, structural strength);
5, circuit and load impedance What is it;
6, whether there is room for magnetic beads placed on the PCB board;
The first three can be judged by observing the impedance frequency curve provided by the manufacturer. In the impedance curve in three curves are very important, namely resistance, inductive impedance and total impedance. The total impedance is described by ZR22πfL()2+:=fL. From this curve, a bead type is selected that has maximum impedance in the frequency range where noise attenuation is desired, while minimizing signal attenuation at low frequencies and DC. The impedance characteristics of chip beads are affected by excessive DC voltages, and the impedance of the beads can be adversely affected by high operating temperatures or excessive external magnetic fields. Reasons for using chip beads and chip inductors: Whether to use chip beads or chip inductors depends mainly on the application. Chip inductors are needed in resonant circuits. When unwanted EMI noise needs to be eliminated, the use of chip beads is the best choice. Applications for Chip Beads and Chip Inductors:
Chip Inductors: Radio Frequency (RF) and Wireless Communications, Information Technology Equipment, Radar Detectors, Automotive, Cellular Phones, Pagers, Audio Devices, PDAs (Personal Digital Assistants), Wireless Remote Controls, and Low Voltage Power Modules.
Chip beads: Clock generation circuits, filtering between analog and digital circuits, internal I/O connectors (e.g., serial ports, parallel ports, keyboards, mice, long-distance telecommunication, local local area networks), filtering of high-frequency conducted interference in radio-frequency (RF) circuits and interference-susceptible logic devices, power supply circuits, EMI filtering in computers, printers, video cassette recorders (VCRS), television systems, and cellular phones. EMI noise suppression in computers, printers, VCRs, TVs and cell phones.
Electronic PCB
Magnetic beads consist of oxygen magnets, inductors consist of cores and coils, magnetic beads convert AC signals into thermal energy, inductors store AC and release it slowly. Magnetic beads on the high-frequency signals only have a greater impediment to the role of the general specifications of 100 ohms / 100MHZ , it is much smaller than the resistance of the inductor at low frequencies. The equivalent resistance of an inductor can be found as Z=2X3.14xf. Ferrite bead (Ferrite Bead) is a kind of anti-jamming element with fast development of current application, cheap, easy to use, filtering out high-frequency noise effect is remarkable. In the circuit as long as the wire through it can be (I use are like ordinary resistors like, the wire has been through and glued, there are also surface mounted form, but rarely seen for sale). When a current is passed through the wire, the ferrite has little impedance to low frequency currents, while higher frequency currents are more attenuated. High frequency currents are dissipated in them as heat, and the equivalent circuit is an inductor and a resistor in series, with the values of both components proportional to the length of the magnetic bead.
There are many types of magnetic beads, and the manufacturer should provide a description of the specifications, especially the curve of the bead's impedance versus frequency.
Some beads have multiple holes in them, which can be passed through with a wire to increase the component impedance (the square of the number of times the bead has been passed through), but the increased noise suppression at high frequencies is not likely to be as much as might be expected, and the use of a few more beads in series would be a better approach.
Ferrite is a magnetic material, will be through the current is too large and produce magnetic saturation, permeability drops sharply. High-current filtering should be used in the structure of the specially designed beads, but also pay attention to its heat dissipation measures.
Ferrite beads are not only used in power supply circuits to filter out high-frequency noise (which can be used for both DC and AC outputs), but can also be used in a wide range of other circuits, and their size can be made very small. Especially in digital circuits, because the pulse signal contains high frequency high harmonics, is also the main source of high-frequency radiation of the circuit, so can play a role in this kind of occasions magnetic beads.
Ferrite beads are also widely used in signal cable noise filtering. Commonly used in power supply filtering HH-1H3216-500 as an example, the model number of the meaning of each field in turn: HH is a series, mainly used for power supply filtering, for the signal line is the HB series; 1 that a component encapsulated in a bead, if 4 is a side-by-side encapsulation of the four; H indicates that the composition of the substance, H, C, M for the mid-frequency applications (50-200MHz), T low-frequency applications (50-200MHz), T low-frequency applications (50-200MHz), T low-frequency applications (50-200MHz). 200MHz), T low-frequency applications (<50MHz), S high-frequency applications (>200MHz); 3216 package size, length 3.2mm, width 1.6mm, i.e., 1206 packages; 500 impedance (generally 100MHz), 50 ohm.
3.I. Principle of Magnetic Beads
The main raw material of magnetic beads is Ferrite. Ferrite is a cubic lattice structure of ferrous magnetic material. Ferrite material for the iron and magnesium alloy or iron-nickel alloy, its manufacturing process and mechanical properties similar to ceramics, the color is gray-black. EMI filters are often used in a class of magnetic core is the ferrite material, many manufacturers provide ferrite materials specifically for EMI suppression. This material is characterized by high frequency loss is very large, has a high permeability, it can be between the coil windings of the inductor in the case of high-frequency high-resistance capacitance generated by the smallest. For ferrite for EMI suppression, the most important performance parameters are the permeability μ and the saturation flux density Bs. The permeability μ can be expressed as a complex number, with the real part constituting the inductance and the imaginary part representing the losses, which increase with frequency. Thus, its equivalent circuit is a series circuit consisting of an inductor L and a resistor R. Both L and R are functions of frequency. When the wire passes through this ferrite core, the inductive impedance formed is formally increased with frequency, but the mechanism is completely different at different frequencies.
In the low-frequency band, the impedance by the inductance of the inductance, low-frequency R is very small, the permeability of the core is high, so the inductance is large, L plays a major role in the electromagnetic interference is reflected and suppressed, and this time, the core loss is small, the entire device is a low-loss, high-Q characteristics of the inductance, which is prone to resonance so that in the low-frequency band, there may be a ferrite beads after the use of the phenomenon of enhanced interference. The inductor has a low loss, high Q characteristic.
In the high-frequency band, the impedance is composed of a resistive component. As the frequency rises, the permeability of the core decreases, resulting in a decrease in the inductance of the inductor, and a decrease in the inductive component. However, the core loss increases, and the resistive component increases, resulting in an increase in the total impedance. When a high-frequency signal passes through the ferrite, the EMI is absorbed and converted to a form of thermal energy that is dissipated.
Ferrite suppression components are widely used in printed circuit boards, power lines and data lines. If you add a ferrite suppression element to the power line inlet of a printed circuit board, you can filter out high frequency interference. Ferrite magnetic ring or bead dedicated to the suppression of signal lines, power lines on the high-frequency interference and spike interference, it also has the ability to absorb electrostatic discharge pulse interference. The size of the value of the two components and the length of the bead is directly proportional to the length of the bead, and the length of the bead has a significant impact on the suppression effect, the longer the length of the bead the better the suppression effect. The difference between magnetic beads and inductors
Inductors are energy storage components, while magnetic beads are energy conversion (consumption) devices. Inductors are mostly used in power supply filtering circuits, focusing on the suppression of conductive interference; magnetic beads are mostly used in signaling circuits, mainly for EMI. Magnetic beads are used to absorb ultra-high-frequency signals, such as some RF circuits, PLL, oscillator circuits, including ultra-high-frequency memory circuits (DDR, SDRAM, RAMBUS, etc.) need to add magnetic beads in the power supply input portion of the magnetic beads, while inductors are a kind of energy storage components used in the LC oscillator circuits, low-frequency and medium-frequency filtering circuits, etc., and their application frequency range is rarely more than 50 MHz.
1.chip type Inductors: Inductive components and EMI filters are used extensively in PCB circuits of electronic devices. These components include chip inductors and chip beads. The following describes the characteristics of these two devices and analyzes their common applications as well as special applications. The advantages of surface mount components are the small package size and the ability to meet physical space requirements. Except for differences in impedance, current-carrying capacity, and other similar physical characteristics, the performance characteristics of through-hole connectors and surface-mount devices are essentially the same. Where chip inductors are used, they are required to fulfill two basic functions: circuit resonance and choke reactance. Resonant circuits include resonant generator circuits, oscillator circuits, clock circuits, pulse circuits, waveform generator circuits, and so on. Resonant circuits also include high-Q bandpass filter circuits. For a circuit to resonate, both a capacitor and an inductor must be present in the circuit. Parasitic capacitance exists at the 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 dielectric. In resonant circuits, the inductor must have high Q, narrow inductance deviation, and a stable temperature coefficient to achieve the narrow bandwidth and low frequency temperature drift required for resonant circuits. High Q circuits have sharp resonant peaks. Narrow inductor bias ensures that the resonant frequency deviation is as small as possible. Stable temperature coefficient ensures that the resonant frequency has a stable temperature variation characteristic. Standard radial leaded inductors differ from axial leaded inductors and chip inductors only in their packaging. Inductor construction consists of coils wound on a dielectric material, usually alumina ceramic, or hollow coils and coils wound on ferromagnetic materials. In power applications, when used as a choke, the main parameters of the inductor are DC resistance (DCR), current rating, and low Q. When used as a filter, the main parameters are DC resistance (DCR) and current rating. When used as a filter, a wide bandwidth characteristic is desired, therefore, a high Q characteristic of the inductor is not required. A low DCR ensures minimal voltage drop, and the DCR is defined as the DC resistance of the component in the absence of an AC signal.
2. Chip beads: The function of chip beads is mainly to eliminate RF noise present in the transmission line structure (PCB circuit), RF energy is superimposed on the AC sine wave component of the DC transmission level, the DC component is needed for useful signals, while the RF energy is useless electromagnetic interference along the line transmission and radiation (EMI). To eliminate this unwanted signal energy, a chip bead is used to play the role of a high-frequency resistor (attenuator), a device that allows DC signals to pass through while filtering out AC signals. Typically high frequency signals are above 30MHz, however, low frequency signals can also be affected by chip beads.
Chip beads consist of a soft ferrite material that forms a monolithic structure with high volume resistivity. The eddy current loss is inversely proportional to the resistivity of the ferrite material. Eddy current loss is proportional to the square of the signal frequency. Benefits of using chip beads: u Miniaturization and light weight. High impedance in the RF noise frequency range, eliminating electromagnetic interference in the transmission line. u Closed magnetic circuit structure to better eliminate signal crosstalk. Excellent magnetic shielding structure. Reduced DC resistance to avoid excessive attenuation of useful signals.
uSignificant high frequency characteristics and impedance characteristics (better elimination of RF energy). Eliminates parasitic oscillations in high frequency amplification circuits. Effectively operates in the frequency range of a few MHz to several hundred MHz. To properly select a bead, the following points must be noted: What is the frequency range of the unwanted signal. Who the noise source is. How much noise attenuation is needed. What are the environmental conditions (temperature, DC voltage, structural strength). What is the circuit and load impedance. Whether there is room for a magnetic bead on the PCB. The first three can be determined by looking at the impedance frequency curve provided by the manufacturer. All three curves are very important in the impedance curve, i.e. resistance, inductive impedance and total impedance. The total impedance is described by ZR22πfL()2+:=fL. A typical impedance curve can be found in the DATASHEET of a magnetic bead.
From this curve, the bead type with the greatest impedance in the frequency range where noise attenuation is desired and the least possible signal attenuation at low frequencies and DC is selected. The impedance characteristics of chip beads are affected by excessive DC voltages, and in addition, if the operating temperature rise is too high, or if the external magnetic field is too large, the impedance of the beads will be adversely affected.
uReasons for using chip beads and chip inductors: Whether to use chip beads or chip inductors depends on the application. Chip inductors are needed in resonant circuits. And the need to eliminate unwanted EMI noise, the use of chip beads is the best choice. Applications for Chip Beads and Chip Inductors: Chip Inductors: Radio Frequency (RF) and wireless communications, information technology equipment, radar detectors, automotive electronics, cellular telephones, pagers, audio equipment, PDAs (Personal Digital Assistants), wireless remote control systems, and low-voltage power supply modules. Chip beads: Clock generation circuits, filtering between analog and digital circuits, I/O input/output internal connectors (e.g., serial ports, parallel ports, keyboards, mice, long-distance telecommunication, local local area networks), RF circuits and interference-susceptible logic devices, high-frequency conducted interference filtering in power supply circuits, EMI noise suppression in computers, printers, VCRs, TV systems, and cellular telephones. EMI noise suppression in computers, printers, VCRs, TV systems and cell phones.
Three, the selection of magnetic beads
1. The unit of magnetic beads is Ohm, not Hunt, which should be particularly careful. Because the unit of the magnetic bead is in accordance with the impedance it produces at a certain frequency to nominalize, the unit of impedance is also ohm. The DATASHEET of the magnetic bead will usually provide a graph of the characteristics of frequency and impedance, usually with 100MHz as the standard, such as 1000R@100MHz, meaning that the impedance of the bead is equivalent to 600 ohms at a frequency of 100MHz.
2. Ordinary filters are composed of lossless reactance elements, which are used in the line to reflect the blocking band frequency back to the signal source, so these filters are also called reflection filters. When the impedance of the reflection filter and the signal source do not match, a portion of the energy will be reflected back to the signal source, resulting in an enhanced interference level. To solve this drawback, a ferrite ring or bead set can be used on the filter feed line, using the eddy current loss of the ring or bead on the high frequency signal to convert the high frequency component into heat loss. Therefore, the magnetic ring and magnetic beads actually absorb the high frequency components, so it is sometimes called absorption filters.
Different ferrite suppression components have different optimal suppression frequency ranges. Typically, the higher the permeability, the lower the suppressed frequency. In addition, the larger the size of the ferrite, the better the suppression. For a given volume, a long, thin shape is better than a short, thick one, and the smaller the inner diameter, the better the suppression. But in the case of DC or AC bias current, there is also the problem of ferrite saturation, inhibit the larger the cross-section of the component, the less likely to be saturated, the greater the bias current can withstand.EMI absorbing ring / bead suppression of differential-mode interference, the value of the current through it is proportional to the volume of both the two out of phase caused by the saturation, reduces the performance of the component; suppression ****-mode interference, will be the power supply of the two wires (positive and negative) at the same time through a magnetic ring, the effective signal is a differential-mode signal, the effective signal is a differential-mode signal, the signal is the same, and the signal is the same. The effective signal is a differential mode signal on which the EMI-absorbing magnetic ring/bead has no effect, whereas for *** mode signals a larger inductance is exhibited. Another better method in the use of magnetic rings is to have the wire that goes through the ring wound several times to increase the inductance. It can be used wisely according to its principle of suppression of electromagnetic interference.
The ferrite suppression element should be installed close to the source of the interference. For input/output circuits, they should be located as close as possible to the inlet and outlet of the shielded case. Ferrite rings and beads constitute the absorption filter, in addition to the high permeability of the consumable material should be selected, but also pay attention to its application. They are in the line to the high-frequency components of the resistance presented by about ten to several hundred Ω, so it is not obvious in the role of high-impedance circuits, on the contrary, in the low-impedance circuits (such as power distribution, power supply, or radio frequency circuits) in the use of the circuit will be very effective. Conclusion
Because ferrite can attenuate higher frequencies while allowing lower frequencies to pass through almost unimpeded, it has been widely used in EMI control. Magnetic rings/beads for EMI absorption can be made into a variety of shapes and are widely used in various applications. Such as on the PCB board, can be added to the DC/DC module, data lines, power lines and so on. It absorbs high-frequency interference signals on the line where it is located, but does not create new zero poles in the system, and does not damage the stability of the system. It is used in conjunction with power filters to complement the performance of the high-frequency end of the filter and improve the filtering characteristics of the system.
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