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Mar 19 2019

The Working Principle and Application of Magnetic Beads

Warm hints: This article contains about 5000 words and reading time is about 20 mins.

Introduction

Due to the urgent requirements of electromagnetic compatibility, electromagnetic interference (EMI) suppression components have been widely used. However, the electromagnetic compatibility problem in practical applications is very complicated. It is totally insufficient to rely solely on theoretical knowledge. It relies more on the actual experience of the majority of electronic engineers. In order to better solve the problem of electromagnetic compatibility of electronic products, we must also consider grounding, circuit and PCB design, cable design, shielding design and other issues. This paper introduces the basic principle and characteristics of magnetic beads to illustrate its importance and application in the electromagnetic compatibility design of switching power supply. The following is the designer's necessary reference when designing new products.

Article Core

Magnetic Bead

Purpose

Introduce what the working principle and application of magnetic beads are.

Application

Semiconductor industry.

Keywords

Magnetic Bead

Catalog

Introduction

 

 

ⅠMagnetic Beads

1.1 Conception of Magnetic Beads

1.2 Magnetic Beads Naming Rules

1.3 Working Principle of Magnetic Beads

1.4 Magnetic Beads Structure

1.5 Magnetic Beads Main Characteristic Parameters

 

Ⅱ Magnetic Beads and Inductors

2.1 The Difference Between Magnetic Beads and Inductors

2.1 The Difference Between Magnetic Beads and Inductors

2.2.1 Chip Inductors

2.2.1 Chip Inductors

2.3 Chip Inductors and the Use of Chip Beads

Ⅲ Selection and Application of Magnetic Bead

Ⅳ Conclusion


ⅠMagnetic Beads

1.1 Conception of Magnetic Beads

Magnetic beads belong to EMI noise element and also belong to noise filter. Their scientific name is sheet ferrite beads. Their effect is equivalent to that of resistance and inductance connected in series in the circuit. Their resistance and inductance are changed by the frequency of the circuit. When high frequency passes through, they show resistance, thus playing the role of filtering high frequency filter. The main raw material of magnetic beads is ferrite, the main component is iron-magnesium alloy or iron-nickel alloy, and the appearance is gray-black.

The high frequency signal in the circuit consumes a lot of high frequency signal when it passes through ferrite. It is precisely because of the high frequency characteristic of ferrite that magnetic beads play the role of resisting high frequency and low frequency in the circuit.


1.2 Magnetic Beads Naming Rules

The following figure shows the naming rules of magnetic beads with the model BLM18AG331SN1D as an example.

Magnetic Beads Naming Rules


1.3 Working Principle of Magnetic Beads

The main raw material of the magnetic beads is ferrite. The ferrite is a cubic ferromagnetic material with a cubic lattice structure. The ferrite material is iron-magnesium alloy or iron-nickel alloy. Its manufacturing process and mechanical properties are similar to those of ceramics. The color is grayish black. One type of magnetic core 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, which minimizes the capacitance generated by the high-frequency high-resistance between the coil windings of the inductor. Ferrite materials are commonly used in high frequency situations because they exhibit predominantly inductive properties at low frequencies, resulting in low losses. At high frequencies, they mainly exhibit reactance characteristics and change with frequency. In practical applications, ferrite materials are used as high frequency attenuators for RF circuits. In fact, the ferrite can be better equivalent to the parallel connection of the resistor and the inductor. The resistor at the low frequency is short-circuited by the inductor, and the impedance of the inductor at a high frequency becomes so high that the current passes through the resistor. Ferrite is a consumer device in which high-frequency energy is converted into thermal energy, which is determined by its electrical resistance characteristics.

For ferrites that suppress electromagnetic interference, the most important performance parameters are permeability and saturation flux density. The magnetic permeability 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 composed of an inductor L and a resistor R. As shown in FIG. 1, the inductor L and the resistor R are both functions of frequency. When a wire passes through such a ferrite core, the resulting impedance of the inductor increases in form as the frequency increases, but the mechanism is completely different at different frequencies.

In the high frequency band, the impedance is mainly 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, but at this time, the loss of the core increases. The increase in the resistance component causes the total impedance to increase, and when the high frequency signal passes through the ferrite, the electromagnetic interference is absorbed and converted into heat energy. In the low frequency band, the impedance is mainly composed of the inductive reactance of the inductor. When the low frequency is small, the magnetic permeability of the magnetic core is high, so the inductance is large, the inductance L plays a major role, and the electromagnetic interference is reflected and suppressed, and this When the loss of the core is small, the whole device is a low-loss, high-quality factor Q-inductive inductor, which is easy to cause resonance, so the interference enhancement after the use of the ferrite bead may occur at a low frequency band.

There are many types of magnetic beads, and the manufacturer will provide a description of the technical indicators, especially the relationship between the impedance of the magnetic beads and the frequency. 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 noise suppression capability at high frequencies may not be as much as expected, and multiple series connections may be used. A method of magnetic beads.

It is worth noting that the energy of the high-frequency noise is converted into heat energy by the coupling of the ferrite magnetic moment and the lattice, instead of introducing the noise into the ground or blocking it back, such as a bypass capacitor. Therefore, when the ferrite bead is mounted in the circuit, it is not necessary to set a grounding point for it. This is a prominent advantage of ferrite beads.

The figure below shows an example of the impedance frequency characteristics of the sheet-like ferrite bead. The basic principle involved is as follows: As the frequency rises, the impedance increases proportionally with the increase of the inductor, so by connecting these beads in series in the circuit, they act as low-pass filters. For conventional inductors, the main feature in the impedance (Z) value is the reactance component (X).

On the other hand, since the sheet-like ferrite bead uses a ferrite material having high loss at a high frequency, the main characteristic in the high frequency range is the resistance component (R). The reactance component is not accompanied by loss, but the resistance component is. This means that the sheet ferrite beads have better absorption of noise energy than conventional inductors.

Magnetic Beads

Chip ferrite beads are typically normalized by impedance values at a frequency of 100 MHz. However, a variety of products having the same impedance value can be used. This is to be able to select the sharpness of the impedance curve.

The figure below shows an example of a curve change. Both BLM18AG601SN1 and BLM18BD601SN1 are chip ferrite beads with an impedance of 600Ω at 100 MHz, but the BLM18BD601SN1 has a sharper impedance curve, while the BLM18AG601SN1 has a more gradual rise.

Magnetic Beads

For a type in which the impedance curve rises gently, the impedance starts to increase at a lower frequency level, so noise can be suppressed on a wide frequency band from a very low frequency to a high frequency. However, if the signal frequency is relatively high, the frequency may also decay.

On the contrary, for the type in which the impedance curve rises sharply, the impedance increases only in the high frequency range, so even if a signal having a relatively high frequency is used, the noise can be suppressed without affecting the signal. Therefore, when selecting a sheet-like ferrite bead, it is important to consider the signal frequency at which noise is suppressed.


1.4 Magnetic Beads Structure

When the current in the wire passes through, 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.

The figure below shows the typical structure of a sheet-like ferrite bead. A coil pattern is formed between the original ferrite sheets, and a three-dimensional coil structure is produced by the process of integration and firing.

Magnetic Beads Structure


1.5 Magnetic Beads Main Characteristic Parameters

DC resistance DCResistance (mohm): The resistance value exhibited by this magnetic bead when a DC current passes through the magnetic bead.

Rated Current RatedCurrent (mA): Indicates the maximum allowable current when the bead is operating normally.

Impedance [Z]@100MHz(ohm): This refers to the AC impedance.

Impedance-Frequency Characteristics: A curve describing the impedance value as a function of frequency.

Resistance-Frequency Characteristics: A curve describing the resistance value as a function of frequency

Inductive reactance-frequency characteristics: A curve describing the inductive reactance as a function of frequency.


 Magnetic Beads and Inductors

2.1 The Difference Between Magnetic Beads and Inductors

The 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. To make the circuit 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, stable temperature coefficient, in order to achieve the narrow band of the resonant circuit, low frequency temperature drift requirements. 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 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 unwanted 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 filter out the AC signal. Usually the high frequency signal is above 30 MHz, however, the low frequency signal is also 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 noise source? 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 to place 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 ZR22πfL()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.


2.2 Chip Magnetic Beads and Chip Inductors

2.2.1 Chip Inductors

Inductive components and EMI filter components are used extensively in the PCB circuit of electronic devices, including chip inductors and chip beads. Where chip inductors are required, the inductor is required to implement two basic functions: circuit resonance and choke reactance. The resonance circuit includes a resonance generation circuit, an oscillation circuit, a clock circuit, a pulse circuit, a waveform generation circuit, and the like. The resonant circuit also includes a high Q band pass filter circuit. To make the circuit 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 a high quality factor Q, a narrow inductance deviation, and a stable temperature coefficient to achieve the narrowband 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, defined as the DC resistance of the component without an AC signal), rated current, and low Q. When used as a filter, a wide bandwidth characteristic is desired, so high Q characteristics of the inductor are not required, and a low DC resistance (DCR) can ensure a minimum voltage drop.


2.2.2 Chip Magnetic Beads

The chip type magnetic bead is an anti-interference component that is currently applied and developed rapidly. It is cheap and easy to use, and the effect of filtering high frequency noise is remarkable. The chip type magnetic bead is composed of a soft ferrite material, and the structure and equivalent circuit of the chip ferrite bead are as shown in FIG. 2, which is essentially a laminated chip inductor which is made of ferrite. A laminated monolithic structure composed of a magnetic material and a conductor coil. Because it is sintered at high temperature, it has the advantages of good compactness and high reliability. The electrodes at both ends are made of 3 layers of silver/nickel/solder to meet the requirements of reflow and wave soldering. In the equivalent circuit shown in Fig. 2, R represents the equivalent resistance due to the loss of the ferrite material (mainly magnetic loss) and the EU loss of the conductor coil, and C is the parasitic capacitance of the conductor coil.

Chip Magnetic Beads

The function of the chip bead is to eliminate the RF noise present in the transmission line structure (PCB circuit). The RF energy is the AC sine wave component superimposed on the DC transmission level. The DC component is the useful signal required, and the RF RF energy. It 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 filter out the AC signal. Usually the high frequency signal is above 30MHz, but the low frequency signal will also be affected by the chip bead.

The chip beads not only have the advantages of miniaturization and light weight, but also have high impedance characteristics in the RF noise frequency range, which can eliminate electromagnetic interference in the transmission line. Chip beads reduce the DC resistance to avoid excessive attenuation of the wanted signal. The chip beads also have significant high frequency and impedance characteristics to better eliminate RF energy. Parasitic oscillations can also be eliminated in the high frequency amplifying circuit. Effective operation ranges from a few MHz to a few hundred MHz.

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.


2.3 Chip Inductors and the Use of Chip Beads

Whether to use a chip bead or a chip inductor is mainly in the application. Chip inductors are required in resonant circuits, and chip beads are the best choice when it is necessary to eliminate unwanted EMI noise. The applications of chip inductors are: radio frequency (RF) and wireless communication, information technology equipment, radar detectors, automotive electronics, cellular phones, pagers, audio equipment, PDAs (personal digital assistants), wireless remote control systems and low voltage power supply modules. Wait. The application of chip beads mainly includes: 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, etc.) Between the radio frequency (RF) circuit and the susceptible logic device, the power supply circuit filters out high frequency conducted interference, and the EMI noise in computers, printers, video recorders, television systems, and mobile phones is suppressed.


 Selection and Application of Magnetic Beads

Since the ferrite bead is used in the circuit to increase the high frequency loss without introducing DC loss, and is small in size and easy to be mounted on the lead wires or wires of the interval, the suppression effect on the noise signal above 1 MHz is very obvious, so Decoupling, filtering, and suppression of parasitic oscillations in high-frequency circuits. In particular, it is effective to eliminate the sudden change of current caused by the switching device inside the circuit and the high frequency noise interference of the filter power line or other wire lead-in circuit. Low-impedance power supply circuits, resonant circuits, Class C power amplifiers, and thyristor switching circuits are all very effective using ferrite beads for filtering. Ferrite magnetic beads can be generally divided into two types, resistive and inductive, which can be selected according to needs when used. The impedance of a single magnetic bead is generally ten to several hundred ohms. If a single amount of attenuation is insufficient, a plurality of magnetic beads can be used in series, but usually three or more effects are not significantly increased [7]. FIG. 3 shows a high frequency LC filter circuit constructed by using two inductive ferrite beads, which can effectively absorb the oscillating signal generated by the high frequency oscillator without breaking into the load and without reducing the load. DC voltage on.

Selection and Application of Magnetic Beads

Since any transmission line inevitably has lead resistance, lead inductance and stray capacitance, a standard pulse signal is prone to overshoot and ringing after a long transmission line. A large number of experiments have shown that the lead resistance can reduce the average amplitude of the pulse, while the presence of lead inductance and stray capacitance is the root cause of the overshoot and ringing. Under the same condition that the pulse front rise time is the same, the larger the lead inductance, the more serious the overshoot and ringing phenomenon. The larger the stray capacitance, the longer the rise time of the waveform, and the increase of the lead resistance will make the pulse The amplitude is reduced. In practical circuits, series resistors can be used to reduce and suppress overshoot and ringing.

Ferrite suppression components are also widely used in printed circuit boards, power lines, and data lines. If ferrite beads are added to the input end of the power line of the printed board, high frequency interference can be filtered out. 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.

The ordinary filter is composed of a lossless reactance element, and its role in the line is to reflect the stop band frequency back to the signal source, so 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 high frequency component is converted into heat loss by the eddy current loss of the high frequency signal by the magnetic ring or the magnetic bead. 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 suppression 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, 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 the differential mode signal, the EMI absorption magnetic ring/magnetic bead has no influence on it, and the common mode signal shows a large inductance. A better method of using the magnetic ring is to repeatedly circulate the wire of the magnetic ring that passes through 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. Their resistance to high-frequency components in the line is about ten to several hundred ohms, 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.

Magnetic Beads

Magnetic Beads

Magnetic Beads Application Case

smart phone

smart phone


 Conclusion

Ferrite is widely used in EMI control because it can attenuate higher frequencies while allowing lower frequencies to pass almost unimpeded. 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 a high electrical 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 consists 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 pattern as the ordinary resistor, 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 through, 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


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