The main raw material of the magnetic beads is ferrite. Ferrite is a ferrimagnetic material of a cubic lattice structure. The ferrite material is iron-magnesium alloy or iron-nickel alloy, and its manufacturing process and mechanical properties are similar to those of ceramics, and the color is gray-black. One type of magnetic core that is often used in electromagnetic interference filters is a ferrite material, and many manufacturers provide ferrite materials specifically for electromagnetic interference suppression. This material is characterized by a very high frequency loss and a high magnetic permeability. It can be the smallest capacitance generated by the high-frequency high-resistance between the coil windings of the inductor. For ferrites for suppressing electromagnetic interference, the most important performance parameters are magnetic permeability μ and saturation magnetic flux density Bs. The magnetic permeability μ can be expressed as a complex number, the real part constitutes the inductance, and the imaginary part represents the loss, which increases as the frequency increases. Therefore, its equivalent circuit is a series circuit consisting of an inductor L and a resistor R, both L and R being a function of frequency. When a wire passes through such a ferrite core, the resulting inductive impedance increases in form as the frequency increases, but the mechanism is completely different at different frequencies. In the low frequency range, the impedance is composed of the inductive reactance of the inductor. R is low at low frequencies, and the magnetic permeability of the core is high. Therefore, the inductance is large, L plays a major role, electromagnetic interference is reflected and suppressed, and magnetic The loss of the core is small, and the whole device is a low-loss, high-Q inductor. This kind of inductance is easy to cause resonance. Therefore, in the low frequency range, the interference enhancement after the use of the ferrite bead may sometimes occur. In the high frequency band, the impedance is composed of a 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 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, the electromagnetic interference is absorbed and converted into heat energy to be dissipated. Ferrite suppression components are widely used in printed circuit boards, power lines, and data lines. High frequency interference can be filtered out by adding a ferrite suppression element to the input end of the power line of the printed board. Ferrite magnetic rings or magnetic beads are designed 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. The numerical value of the two components is proportional to the length of the magnetic beads, and the length of the magnetic beads has a significant effect on the suppression effect. The longer the length of the magnetic beads, the better the suppression effect.
Second, the structural characteristics of the magnetic beadsWhen the current in the wire passes, the ferrite has little resistance to the low-frequency current, and the higher-frequency current has a large attenuation. The high-frequency current is radiated in the form of heat, and the equivalent circuit is an inductor 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 manufacturers should provide technical specifications, especially the relationship between impedance and frequency of magnetic beads. Some magnetic beads have a plurality of holes, and the passage of wires can increase the impedance of the component (the square of the number of passes through the beads), but the increased noise suppression capability at high frequencies is not as expected, but multiple series A few magnetic beads will work better. Ferrite is a magnetic material that is magnetically saturated due to excessive current passing, and the magnetic permeability drops sharply. High-current filtering should use magnetic beads specially designed for the structure, and pay attention to the heat dissipation measures. Ferrite beads can be used not only for filtering high-frequency noise in power circuits (for DC and AC outputs), but also for other circuits, and their size can be made small. Especially in digital circuits, since the pulse signal contains high-order harmonics with high frequency, it is also the main source of high-frequency radiation of the circuit, so it can play the role of magnetic beads in this case. Ferrite beads are also widely used for noise filtering of signal cables.
Third, the main characteristics of the magnetic beads1. DC resistance DCResistance (mohm): The resistance value exhibited by the magnetic bead when the DC current passes through the magnetic bead. 2. Rated current RatedCurrent (mA): indicates the maximum allowable current when the magnetic beads are working normally. 3. Impedance [Z]@100MHz(ohm): This refers to the AC impedance. 4. Impedance-frequency characteristics: A curve describing the impedance value as a function of frequency. 5. Resistance-frequency characteristics: A curve describing the resistance value as a function of frequency. 6. Inductance-frequency characteristics: A curve describing the inductive reactance as a function of frequency. The figure below shows the characteristic parameters and frequency characteristics of a magnetic bead of a manufacturer:
Fourth, the difference between magnetic beads and inductanceThe inductor is an energy storage component and the magnetic beads are energy conversion (consumption) devices. Inductors are mostly used in power supply filter loops, focusing on suppressing conducted interference; magnetic beads are mostly used in signal loops, mainly for EMI. Magnetic beads are used to absorb ultra-high frequency signals, such as some RF circuits, PLLs, oscillator circuits, and ultra-high frequency memory circuits (DDR, SDRAM, RAMBUS, etc.), which need to add magnetic beads to the input part of the power supply, and the inductor is a kind of storage. The energy components are used in LC oscillation circuits, low-frequency filter circuits, etc., and their application frequency ranges rarely exceed 50 MHz. 1. Chip inductor: Inductive components and EMI filter components are widely used in the PCB circuit of electronic equipment. These components include chip inductors and chip beads. The characteristics of these two devices are described below and analyzed for their general application and special applications. The benefits of surface mount components are small package sizes and the ability to meet real space requirements. In addition to impedance values, current carrying capabilities, and other similar physical characteristics, the other performance characteristics of through-hole connectors and surface mount devices are essentially the same. Where chip inductors are required, the inductor is required to implement two basic functions: circuit resonance and choke reactance. The resonant circuit includes a resonance generating circuit, an oscillating circuit, a clock circuit, a pulse circuit, a waveform generating circuit, and the like. The resonant circuit also includes a high Q band pass filter circuit. In order for the circuit to resonate, both the capacitor and the inductor must exist in the circuit. There is parasitic capacitance at both ends of the inductor 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 inductor must have high Q, narrow inductance deviation, and stable temperature coefficient to achieve the narrow band of the resonant circuit and the low frequency temperature drift requirement. High Q circuits have sharp resonant peaks. The narrow inductor bias ensures that the resonant 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 and the axial lead-out inductor and the chip inductor is only that the package is different. The inductive structure includes a coil wound on a dielectric material (typically an alumina ceramic material), or an air-core coil and a coil wound on a ferromagnetic material. In power applications, when used as a choke, the main parameters of the inductor are DC resistance (DCR), rated current, and low Q. When used as a filter, a wide bandwidth characteristic is desired, and therefore, a high Q characteristic of the inductor is not required. A low DCR guarantees a minimum voltage drop, and a DCR is defined as the DC resistance of the component without an AC signal. 2. Chip type magnetic beads: The function of the chip type magnetic beads is mainly to eliminate the RF noise existing in the transmission line structure (PCB circuit). The RF energy is an AC sine wave component superimposed on the DC transmission level, and the DC component is required. Useful signals, while RF RF energy is useless electromagnetic interference along the line transmission and radiation (EMI). To eliminate these unwanted signal energies, use a chip bead to act as a high frequency resistor (attenuator) that allows the DC signal to pass through and filters out the AC signal. Usually the high frequency signal is above 30MHz, however, the low frequency signal will also be affected by the chip bead. The chip magnetic beads are composed of a soft ferrite material and constitute a monolithic structure with a high volume resistivity. The eddy current loss is inversely proportional to the resistivity of the ferrite material. The eddy current loss is proportional to the square of the signal frequency. Benefits of using chip beads: Miniaturization and lightweight. High impedance in the RF noise frequency range eliminates electromagnetic interference in the transmission line. Close the magnetic circuit structure to better eliminate the crosstalk of the signal. Excellent magnetic shielding structure. Reduce the DC resistance to avoid excessive attenuation of the wanted signal. Significant high frequency and impedance characteristics (better elimination of RF energy). Parasitic oscillations are eliminated in the high frequency amplifying circuit. Effective operation ranges from a few MHz to a few hundred MHz. To properly select a bead, you must be aware of the following: What is the frequency range of the unwanted signal? Who is the source of the noise. How much noise attenuation is needed. What are the environmental conditions (temperature, DC voltage, structural strength). What is the circuit and load impedance? Is there room for placing magnetic beads on the PCB? The first three can be judged by observing the impedance frequency curve provided by the manufacturer. All three curves in the impedance curve are very important, namely resistance, inductive reactance and total impedance. The total impedance is described by ZR22Ï€fL()2+:=fL. A typical impedance curve can be found in the DATASHEET of the bead. From this curve, select the type of bead that has the largest impedance in the frequency range where attenuation is desired and the signal attenuation is as small as possible at low frequencies and DC. The impedance of the chip bead will be affected by the excessive DC voltage. In addition, if the operating temperature rises too high or the external magnetic field is too large, the impedance of the bead will be adversely affected. Reasons for using chip beads and chip inductors: Whether to use chip beads or chip inductors is mainly in applications. A chip inductor is required in the resonant circuit. The use of chip beads is the best choice when eliminating unwanted EMI noise. Application of Chip Beads and Chip Inductors: Chip Inductors: Radio Frequency (RF) and Wireless Communications, Information Technology Equipment, Radar Detectors, Automotive Electronics, Cellular Phones, Pagers, Audio Equipment, PDAs (Personal Digital Assistants), Wireless remote control system and low voltage power supply module. Chip Beads: Clock generation circuit, filtering between analog circuit and digital circuit, I/O input/output internal connector (such as serial port, parallel port, keyboard, mouse, long distance telecommunication, local area network), radio frequency (RF) circuit Between high-frequency conducted interference in the power supply circuit and EMI noise suppression in computers, printers, video recorders (VCRS), television systems and mobile phones, and interference-prone logic devices.
Fifth, the selection of magnetic beads1. The unit of the magnetic beads is ohms, not Hunt, which requires special attention. Since the unit of the magnetic bead is nominally based on the impedance it produces at a certain frequency, the unit of impedance is also ohms. The magnetic field DATASHEET generally provides a characteristic curve of frequency and impedance, generally based on 100MHz, such as 1000R@100MHz, which means that the impedance of the magnetic bead is equivalent to 600 ohms at 100MHz. 2. The ordinary filter is composed of lossless reactance components. Its role in the line is to reflect the stopband frequency back to the signal source. Therefore, this type of filter is also called a reflection filter. When the reflection filter does not match the source impedance, a portion of the energy is reflected back to the source, causing an increase in the level of interference. In order to solve this drawback, a ferrite magnetic ring or a magnetic bead sleeve can be used on the incoming line of the filter, and the eddy current loss of the high frequency signal is utilized by the nuzzle or the magnetic bead to convert the high frequency component into heat loss. Therefore, the magnetic ring and the magnetic beads actually absorb the high-frequency components, so they are sometimes referred to as absorption filters. Different ferrite suppression elements have different optimal suppression frequency ranges. Generally, the higher the magnetic permeability, the lower the frequency of suppression. In addition, the larger the volume of the ferrite, the better the suppression effect. When the volume is constant, the long and thin shape is better than the 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 also a problem of ferrite saturation. The larger the cross-section of the suppressing element, the less likely it is to saturate and the greater the bias current that can be withstood. When the EMI absorption magnetic ring/bead suppresses the differential mode interference, the current value through it is proportional to its volume, and the two are out of regulation, causing saturation, which reduces the performance of the component; when suppressing common mode interference, the two wires of the power supply (positive and negative) At the same time, through a magnetic ring, the effective signal is a differential mode signal, the EMI absorption magnetic ring/magnetic bead has no influence on it, and for the common mode signal, it shows a large inductance. A better method of using the magnetic ring is to repeatedly circulate the wire passing through the magnetic ring to increase the inductance. According to its suppression principle of electromagnetic interference, its suppression can be reasonably used. The ferrite suppression element should be installed close to the source of the interference. For the input/output circuit, it should be as close as possible to the entrance and exit of the shield case. For the absorption filter composed of the ferrite magnetic ring and the magnetic beads, in addition to the use of high magnetic permeability consumable materials, it is also necessary to pay attention to its application. They exhibit a resistance of about ten to several hundred ohms in the line for high frequency components, so its role in high impedance circuits is not obvious. Instead, in low impedance circuits such as power distribution, power or RF circuits. Use will be very effective.
ConclusionSince ferrite can attenuate higher frequencies while allowing lower frequencies to pass almost unimpeded, it has been widely used in EMI control. The magnetic ring/magnetic beads for EMI absorption can be made into various shapes and are widely used in various applications. For example, on the PCB board, it can be added to DC/DC modules, data lines, power lines, etc. It absorbs high-frequency interference signals on the line, but does not generate new poles in the system, and does not damage the stability of the system. It is used in conjunction with the power filter to complement the high-frequency performance of the filter and improve the filtering characteristics of the system. Magnetic beads have high resistivity and permeability, which is equivalent to the series connection of resistance and inductance, but the resistance value and inductance value change with frequency. He has better high-frequency filtering characteristics than ordinary inductors and resistive at high frequencies, so it can maintain high impedance over a wide frequency range, thus improving the frequency modulation filtering effect. As a power supply filter, an inductor can be used. The circuit symbol of the magnetic bead is the inductance. However, it can be seen that the magnetic bead is used in the circuit function. The magnetic bead and the inductor are the same principle, but the frequency characteristics are different. The magnetic beads are composed of an oxygen magnet. The inductance is composed of a core and a coil. The magnetic beads convert the AC signal into heat energy, and the inductor stores the AC and slowly releases it. Magnetic beads have a great hindrance to high-frequency signals. The general specification is 100 ohms/100mMHZ, which is much smaller than the inductor at low frequencies. Ferrite Bead is a kind of anti-jamming component that has been developed rapidly. It is cheap, easy to use, and has a significant effect on filtering high frequency noise. In the circuit, as long as the wire passes through it (I use the same as the ordinary resistance, the wire has been passed through and glued, but also in the form of surface mount, but rarely seen sold). When the current in the wire passes, the ferrite has little resistance to the low-frequency current, and the higher-frequency current has a large attenuation. The high-frequency current is radiated in the form of heat, and the equivalent circuit is an inductor 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 manufacturers should provide technical specifications, especially the relationship between impedance and frequency of magnetic beads. Some magnetic beads have a plurality of holes, and the passage of wires can increase the impedance of the component (the square of the number of passes through the beads), but the increased noise suppression capability at high frequencies is not as expected, but multiple series A few magnetic beads will work better. Ferrite is a magnetic material that is magnetically saturated due to excessive current passing, and the magnetic permeability drops sharply. High-current filtering should use magnetic beads specially designed for the structure, and pay attention to the heat dissipation measures. Ferrite beads can be used not only for filtering high-frequency noise in power circuits (for DC and AC outputs), but also for other circuits, and their size can be made small. Especially in digital circuits, since the pulse signal contains high-order harmonics with high frequency, it is also the main source of high-frequency radiation of the circuit, so it can play the role of magnetic beads in this case. Ferrite beads are also widely used for noise filtering of signal cables. Taking the HH-1H3216-500, which is commonly used for power supply filtering, as an example, the meaning of each field of the model is: HH is a series thereof, mainly used for power supply filtering, for the signal line is HB series; 1 means that one component encapsulates a magnetic Beads, if 4, are packaged side by side; H is the composition of matter, H, C, M are for intermediate frequency applications (50-200MHz), T low frequency applications (<50MHz), S high frequency applications (>200MHz); 3216 Package size, 3.2mm long, 1.6mm wide, ie 1206 package; 500 impedance (typically 100MHz), 50 ohms. There are three main product parameters: impedance [Z]@100MHz (ohm): Typical 50, Minimum 37; DC resistance DC Resistance (m ohm): Maximum 20; rated current Rated Current (mA): 2500.
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