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Mar 27 2019

Role of Magnetic Beads

Warm hints: This article contains about 6000 words and reading time is about 20 mins.

Introduction

The magnetic beads belong to the EMI squelch component, and also belong to the squelch filter. The scientific name is the chip ferrite bead. The function is equivalent to the resistor and the inductor connected in series in the circuit. The resistance and inductance values are changed by the frequency in the circuit. The change, when there is a high frequency, exhibits a resistance, thereby exerting a filter function of filtering high frequencies.

Article Core

Magnetic Beads

Purpose

Introduce what the roles of magnetic beads are.

Application

Semiconductor industry.

Keywords

Magnetic Beads

Catalog

Introduction


ⅠMagnetic Beads


Ⅱ Magnetic Bead Function


Ⅲ Magnetic Bead Unit


Ⅳ Magnetic Bead Parameters

4.1 Magnetic Flux

4.2 Curie Temperature

4.3 Working Frequency

Ⅴ Magnetic Bead Principle


Ⅵ Magnetic Bead Difference

6.1 Chip Inductor

6.2 Chip Beads

Ⅶ High Current Patch


Ⅷ Magnetic Bead Selection


Ⅸ Magnetic Bead Application


Ⅹ Precautions


Ⅺ Common Type



ⅠMagnetic Beads

Magnetic beads are designed to suppress high-frequency noise and spikes on signal lines, power lines, and the ability to absorb electrostatic pulses. Magnetic beads are used to absorb ultra-high frequency signals. Some RF circuits, PLLs, and oscillating circuits, including UHF memory circuits, require magnetic beads to be added to the power input section. The inductor is an energy storage component used in LC oscillation. Circuits, low-frequency filter circuits, etc., whose application frequency range rarely exceeds 50 MHz. Magnetic beads have high resistivity and permeability, equivalent to series connection of resistance and inductance, but both resistance and inductance vary with frequency.


Ⅱ Magnetic Bead Function

Magnetic component symbo

Magnetic component symbo

Magnetic beads

Magnetic beads

The function of the magnetic beads is mainly to eliminate the RF noise existing in the transmission line structure (circuit). The RF energy is an AC sine wave component superimposed on the DC transmission level, and the DC component is a useful signal required. To eliminate these unwanted signal energies, use a chip bead to act as a high frequency resistor (attenuator).

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 but the model can be seen to use the magnetic beads. In terms of circuit function, the magnetic beads and the inductor are identical in principle, but the frequency characteristics are different.

The DC component is a useful signal that is needed, while the 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. Magnetic beads have a high electrical resistivity and permeability, equivalent to the series connection of resistance and inductance. As long as the wire passes through it in the circuit. 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. 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). 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.


Ⅲ Magnetic Bead Unit

The magnetic beads have a greater impediment to high frequency signals. The general specification is 100 Euro/100MHZ. The resistance of magnetic beads is much smaller than that of inductance at low frequency.

Taking the H1 H3216-500 commonly used for power filter as an example, the meanings of each field of the model are as follows:

HH is one of its series, which is mainly used for power filter and HB series for signal line.

1 means that a component encapsulates a magnetic bead, and if it is 4, it encapsulates four beads side by side.

H denotes constituents, H, C and M are intermediate frequency applications (50-200 MHz).

T low frequency application (50MHz), S high frequency application (200MHz);

3216 package size, 3.2 mm long and 1.6 mm wide, i.e. 1206 package;

500 impedance (usually 100 MHz), 50 ohm.

Notice: The unit of the magnetic beads is ohms, not Henry, so pay special attention to this. Because the unit of the bead is nominally based on the impedance it produces at a certain frequency, the unit of impedance is also ohms.

Ferrite Bead is an anti-jamming component that has been developed rapidly. It is cheap, easy to use, and has a significant effect on filtering high frequency noise. Ferrite beads are also widely used for noise filtering of signal cables.

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.


Ⅳ Magnetic Bead Parameters

The magnetic bead parameters mainly include: initial magnetic flux (U value), Curie temperature, operating frequency.


4.1 Magnetic Flux

The high U has a low magnetic saturation, that is, the magnetic pole can withstand the maximum current at a low frequency, and the inductive reactance is capacitive with the change of current. The heating of the magnetic beads means that the core loss is too large, and the power is converted into heat energy, but not converted into magnetic energy, and the energy is consumed. Generally, the magnetic core bandwidth of the nickel material has a balance relationship between the Q value and the U. The higher the U value, the lower the Q value, and vice versa. U-value low-frequency operation is difficult, but the loss is small, U-value high-low frequency operation is easy, but the core loss is too large, the power loss is also large, and it is basically difficult to work continuously. Using a magnetic ring with a U value of 400 should significantly reduce magnetic losses. Although the inductance is lower, it can be solved by increasing the number of coils. Taking the 1:4 transformer as an example, the primary of one turn is changed to two turns; the secondary of two turns is changed to four turns. In this way, the total length of the winding is doubled, and the maximum transmission frequency is also reduced accordingly.


4.2 Curie Temperature

Generally, the magnetic ring has a Curie temperature of 110 ° C. After reaching this temperature, it loses its magnetic properties immediately, like an air medium. After returning to room temperature, the magnetic properties are permanently changed, and the magnetic permeability is reduced by 10%. The magnetic material applied to the output transformer of the power amplifier can burn out the output power tube if the operating temperature exceeds the Curie temperature. The point at which the output power begins to drop is taken as the temperature limit of the magnetic ring. From the past experimental results, at 55 degrees, those EMI magnetic rings have not experienced a drop in output power.


4.3 Working Frequency

The material of each core determines its optimum operating frequency, so the material of the core must be selected based on the specific frequency. The magnetic ring with low working frequency is forced to work at high frequency, and there will be great loss and heat. When the magnetic ring heat exceeds the Curie temperature, the electrical performance will be abrupt and it will not work properly.

In general, to select the magnetic ring, it is not necessary to look at the shape or volume. It is necessary to understand its actual parameters. Otherwise, if there is a problem, such as standing wave height, bandwidth is too narrow, the magnetic ring is seriously heated or Burned out, etc., do not know where the reason comes from.

Measurement of magnetic permeability of ferrite beads:

1) Measure the outer diameter D of the magnetic beads, the inner diameter d, and the height H of the ring, in mm.

2) Use the enameled wire to wrap around 10~20 turns, tighten the point, don't be too loose, measure the inductance L, the unit is uH, the inductance is small, the measurement error is small, the inductance is small, the calculation error will be large, please according to actual needs Determine the number of turns N to wear.

3) Substituting the above data into the following equation to calculate the approximate permeability u0

U0=2500*L*(D+d)/((D-d)*H*N*N)

For example: the magnetic ring of 13X7X5, around 20 turns, the measured inductance is 23uH, which is substituted into the above formula.

U0=2500*23*(13+7)/((13-7)*5*20*20)=1150000/12000=95.8

The measurement result is closest to the specification of the magnetic permeability 100, and it is determined that the u0 of the magnetic ring is 100, and it is noted that the general u0 nominal error has +-10%.

For a magnetic bead without parameters, it is possible to first determine which material is based on the appearance characteristics, and then calculate the magnetic permeability, and then the main specifications of the magnetic bead can be determined.


Ⅴ Magnetic Bead Principle

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 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-impedance between the coil windings of the inductor. The most important performance parameters for ferrite for suppressing electromagnetic interference 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 impedance of the inductor 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.

Magnetic beads

Magnetic beads

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, the loss of the core increases and the resistance component increases. This 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 to dissipate.

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 component 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.

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 inductance are the same principle, but the frequency characteristics are different. The magnetic bead is composed of an oxygen magnet. Magnetic beads have a greater hindrance to high-frequency signals. The general specification is 100 ohms/100 mMHZ, which is much smaller than the inductor at low frequencies. Ferrite Bead is a kind of anti-interference component that has been developed rapidly. It is cheap, easy to use, and has a significant effect on filtering high frequency noise.


Ⅵ Magnetic Bead Difference

The inductor is an energy storage component, and the magnetic beads are energy conversion (consumption) devices [1]. 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.


6.1 Chip Inductor

Inductive components and EMI filter components are used extensively in the PCB circuit of electronic devices. 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.

Magnetic beads

Magnetic beads


6.2 Chip Beads

The function of the chip bead is mainly to eliminate the RF noise present in the transmission line structure (PCB circuit).

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 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.

The reason for using chip beads and chip inductors is whether the use of chip beads or chip inductors is mainly in application. 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.


Ⅶ High Current Patch

The closed magnetic circuit structure can be installed at high density and avoid interference, good solderability and heat resistance, and a large current of 6A. Widely used in notebook computers, disk drives, inkjet printers, hard disk drives, photocopiers, display monitors, game consoles, color TVs, video recorders, CD players, video cameras, digital cameras, automobiles Electronic products and anti-interference measures.


Ⅷ Magnetic Bead Selection

1. The unit of the magnetic beads is ohms, not Henry, so pay special attention to this. 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 DATASHEET of the magnetic bead generally provides a characteristic plot of frequency and impedance, generally 100MHz, such as 600R@100MHz, meaning that the impedance of the bead is equivalent to 600 ohms at 100MHz.

2. The ordinary filter is composed of lossless reactance components, and its role in the line is to reflect the stopband 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 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 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.


Ⅸ Magnetic Bead Application

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. At low frequencies, the resistance is much smaller than the inductance. The equivalent resistance of the inductor can be obtained by Z=2X3.14xf.

Ferrite Bead (FerriteBead) is an anti-interference component that has been developed rapidly. It is cheap and 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 die, 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.

Chip magnetic beads

Chip 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.

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.

Magnetic beads

Magnetic beads

Ferrite beads are also widely used for noise filtering of signal cables.

HH is a series of it, mainly used for power supply filtering, and the signal line is HB series;

1 means that one component encapsulates one magnetic bead, and if it is 4, it is packaged four by side;

H represents the constituent materials, and H, C, and M are intermediate frequency applications (50-200 MHz).

T low frequency application (<50MHz), S high frequency application (>200MHz);

3216 package size, 3.2mm long and 1.6mm wide, ie 1206 package;

500 impedance (typically at 100MHz), 50 ohms. There are three main product parameters:

Impedance [Z]@100MHz(ohm):Typical50Minimum37;

DC resistance DCResistance (mohm): Maximum20; rated current RatedCurrent (mA): 2500 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.

Magnetic beads are mainly used for high frequency isolation and suppression of differential mode noise.


Ⅹ Precautions

To properly select the beads, you must pay attention to the following points:

1. What is the frequency range of the unwanted signal;

2. Who is the noise source?

3. How much noise attenuation is needed;

4. What are the environmental conditions (temperature, DC voltage, structural strength);

5. What is the circuit and load impedance;

6. Is there room for placing magnetic beads on the pcb board;

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.


Ⅺ Common Type

Magnetic beads have excellent electromagnetic interference suppression properties and are widely used in computers, VCDs and other fields.

BGL (H) type magnetic beads, also known as high loss inductors, have excellent noise suppression performance over a wide frequency range.

BSZ type magnetic beads, also called high-Q magnetic beads, have a sharp rise in resistance in a certain frequency range, and can obtain high attenuation effect in a specific frequency range, mainly used in high-speed signal circuits, such as computers. Board.

The MBW type magnetic bead has changed the shortcoming of the rated current of the general type of magnetic beads, and has the characteristics of large-capacity flow, and its rated current can reach 4A. This product is mainly used for power supply filtering.


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