Magnetic beads have very high resistivity and permeability, which is equivalent to the series of resistance and inductance, but the value of resistance and inductance varies with frequency.It has better high-frequency filtering characteristics than ordinary inductors, showing resistance at high frequencies, so it can maintain high impedance over a wide range of frequencies, thus improving the effect of FM filtering.
As a power filter, inductance can be used.The circuit symbol of magnetic beads is the inductance. However, it can be seen from the model that magnetic beads are used. In terms of circuit functions, magnetic beads and inductors have the same principle, but different frequency characteristics
Magnetic beads are composed of oxygen magnets. Inductors are composed of magnetic cores and coils. Magnetic beads convert alternating signals into heat energy.
Magnetic beads have a great blocking effect on high-frequency signals. The general specification is 100 ohm /100 MMHZ, and the resistance is much smaller than the inductance at low frequency.
Ferrite Bead is a kind of anti-interference component which is developing rapidly at present, which is cheap, easy to use and has a significant effect in filtering high-frequency noise.
In a circuit, just run the wire through it (I use it in the form of ordinary resistors, the wire has been run through and glued, there are surface-mount forms, but they are rarely sold).Ferrite has little resistance to low frequency currents when current flows through a wire, and much attenuation to higher frequency currents.The equivalent circuit is an inductor and a resistor in series. The values of both components are proportional to the length of the magnetic beads.There are many kinds of magnetic beads. The manufacturer should provide technical specifications, especially the curves of impedance and frequency.
Some beads have multiple holes in them, and running wires through them increases the impedance of the assembly (the square of the number of passes through the beads).
Ferrite is a magnetic material, because through the current too much magnetic saturation, magnetic permeability decreases sharply.Large current filter should adopt magnetic beads specially designed in structure and pay attention to its heat dissipation measures.
Ferrite beads can be used not only to filter high frequency noise in power supply circuit (can be used for dc and ac output), but also widely used in other circuits, and their volume can be made very small.Especially in digital circuit, pulse signal contains high frequency high harmonic, which is the main source of high frequency radiation, so it can play the role of magnetic beads.
Ferrite beads are also widely used in noise filtering of signal cables.
Take hh-1h3216-500, which is often used for power filtering, for example, the meaning of each field of its model is in order:
HH is a series, mainly used for power filtering, used for signal line is HB series;
1 means that a component encapsulates a magnetic bead; if it is 4, it encapsulates four side by side;
H represents the constituent substance. H, C and M are medium frequency applications (50-200mhz).
T low frequency application (50MHz), S high frequency application (200MHz);
3216 package size, 3.2mm long, 1.6mm wide, namely 1206 package;
500 impedance (typically at 100MHz), 50 ohm.
Its product parameters are mainly three:
Impedance [Z] @100mhz (ohm) : Typical 50, Minimum 37;
DC Resistance (m ohm): Maximum 20;
Rated Current (mA): 2500.
The principle of magnetic beads
Ferrite is the main raw material of magnetic beads.Ferrite is a kind of ferromagnetic material with cubic lattice structure.Ferrite material is ferro – magnesium alloy or fe – ni alloy.Ferrite materials are often used in electromagnetic interference filters. Many manufacturers provide ferrite materials for electromagnetic interference suppression.This material is characterized by high frequency loss and high permeability, it can be inductively between the coil winding in the case of high frequency resistance to produce the smallest capacitance.For ferrites used to suppress electromagnetic interference, the most important performance parameters are permeability muon and saturation flux density Bs.Muon can be expressed as a complex number, real part of the inductance, imaginary part of the loss, with increasing frequency.Therefore, its equivalent circuit is a series circuit composed of inductor L and resistance R, both of which are functions of frequency.When a wire passes through the ferrite core, the inductance impedance is formally increased with increasing frequency, but the mechanism is completely different at different frequencies.
Made up of inductive impedance at low frequencies, impedance, low frequency when R is very small, the core permeability is higher, so the large inductance, L plays a main role, electromagnetic interference is reflected and suppressed, and then the core loss is small, the whole device is a characteristic of low loss, high Q inductance, the inductance is easy to cause resonance in low frequency band, therefore, may sometimes appear interference phenomenon of enhanced after using ferrite beads.
At high frequencies, impedance by resistance to composition, with increasing frequency, magnetic core’s permeability decreased, leading to the inductance of the inductance, inductance components decreases, however, at this time of the magnetic core loss increases, the resistance components increase, lead to the total impedance increases, when the high frequency signal through the ferrite, electromagnetic interference is absorbed into the form of heat energy wasted.
Ferrite suppression elements are widely used in printed circuit boards, power lines and data lines.High frequency interference can be filtered by adding ferrite suppression elements to the power line inlet end of the PCB.Ferrite magnetic rings or beads are specially 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 elements is proportional to the length of the magnetic bead, and the length of the magnetic bead has a significant effect on the inhibition effect, the longer the length of the magnetic bead, the better the inhibition effect.
The difference between magnetic beads and inductors
Inductors are energy storage elements, while magnetic beads are energy conversion (consumption) devices.Inductance is mostly used in power filter circuit, focusing on preventing conductive interference;Magnetic beads are mostly used in signal loops, mainly in EMI.Magnetic beads are used to absorb uhf signals, such as some RF circuits, PLL, oscillation circuits, including uhf memory circuits (DDR, RAMBUS, etc.) all need to add magnetic beads in the power input part, and inductance is an energy storage element, used in LC oscillation circuit, low and medium frequency filter circuit, etc., whose application frequency range rarely exceeds 50MHz.
1. Chip inductance: inductive elements and EMI filter elements are widely used in the board circuit of electronic equipment.These components include chip inductors and chip magnetic beads. The characteristics of these two devices are described below and their common and special applications are analyzed.The advantages of surface mount components are the small package size and the ability to meet the requirements of real space.In addition to different impedance values, current carrying capacity and other similar physical characteristics, other performance characteristics of the through-hole connector and the surface-mount device are basically the same.
When chip inductors are needed, they are required to realize the following two basic functions: circuit resonance and choke reactance.Resonance circuit includes resonance generation circuit, oscillation circuit, clock circuit, pulse circuit, waveform generation circuit and so on.The resonant circuit also includes a high Q bandpass filter circuit.To make the circuit resonant, both capacitance and inductance must be present in the circuit.There is parasitic capacitance at both ends of the inductor, which is caused by the ferrite body between the two electrodes of the device being equivalent to the capacitive medium.In resonant circuit, inductance must have high Q, narrow inductance deviation and stable temperature coefficient, so as to meet the requirements of narrow band and low frequency temperature drift of resonant circuit.High Q circuits have sharp resonance peaks.Narrow inductance bias ensures minimal resonance frequency deviation.The stable temperature coefficient guarantees the stable temperature variation characteristic of the resonant frequency.The difference between the standard radial inductance and axial inductance and chip inductance is only in packaging.
The inductance structure includes wound coil on dielectric material (usually alumina ceramic material), or hollow coil and wound coil on ferromagnetic material.In power applications, the main inductors used as choke are dc resistance (DCR), rated current, and low Q.When used as a filter, wide bandwidth characteristics are desired, so the high Q characteristics of the inductor are not required.A low DCR, defined as the dc resistance of a component in the absence of an ac signal, ensures a minimum voltage drop.
2. Chip magnetic beads: the function of chip magnetic beads is mainly to eliminate RF noise existing in the transmission line structure (). RF energy is the component of ac sine wave superimposed on the dc transmission level.To eliminate this unwanted signal energy, chip magnetic beads act as high-frequency resistors (attenuators), which allow dc signals to pass through and filter out ac signals.Normally, high frequency signals are above 30MHz, however, low frequency signals are also affected by chip magnetic beads.
The chip magnetic beads are composed of soft ferrite materials and form a single stone structure with high volume resistivity.The eddy current loss is inversely proportional to the resistivity of ferrite materials.The eddy current loss is proportional to the square of the signal frequency.Benefits of using a magnetic bead:
Miniaturization and lightweight.High impedance in the noise frequency range eliminates electromagnetic interference in the transmission line.Closed magnetic circuit structure can eliminate the serial winding of signal better.Excellent magnetic shielding structure.Reduce the dc resistance to avoid excessive attenuation of useful signals.
Significant high frequency and impedance characteristics (better elimination of energy).Eliminate parasitic oscillations in high frequency amplifier circuits.Effective operation in the frequency range of several MHz to several hundred MHz.To choose a magnetic bead correctly, you must pay attention to the following points: what is the frequency range of the unwanted signal?Who is the source of the noise?How much noise attenuation is required.What are the environmental conditions (temperature, dc voltage, structural strength)?What is the circuit and load impedance?Whether there is space to place magnetic beads on PCB board.The first three can be judged by looking at the impedance frequency curve provided by the manufacturer.All three curves are very important in the impedance curve, namely resistance, inductive reactance and total impedance.The total impedance is described by ZR22 PI fL()2+:=fL.For typical impedance curves see DATASHEET for magnetic beads.
Through this curve, the magnetic beads with the maximum impedance in the frequency range of the desired attenuation of noise and the minimum signal attenuation in the low frequency and dc are selected.In addition, if the working temperature rises too high or the external magnetic field is too large, the impedance of magnetic beads will be adversely affected.
The use of chip magnetic beads and chip inductance: is the use of chip magnetic beads or chip inductance mainly depends on the application.Chip inductors are used in resonant circuits.When it is necessary to eliminate unwanted EMI noise, using magnetic beads is the best choice.Application of chip magnetic beads and chip inductors: chip inductors: radio frequency (RF) and wireless communication, information technology equipment, radar detector, automotive electronics, cellular phone, pager, audio equipment, PDAs(personal digital assistant), wireless remote control system and low-voltage power supply module, etc.Chip magnetic beads: clock generating circuit, filter between analog circuits and digital circuits, internal I/O input/output connector (such as serial, parallel, keyboard, mouse, long distance and local LAN), radio frequency (RF) logic circuit and interference between devices, power supply circuit high frequency conducted interference filter, computer, printer, video recorders (VCRS), EMI noise dampening in the television system and mobile phone.
The selection of magnetic beads
1. The unit of magnetic bead is ohm, not hunter.Because the magnetic bead is labeled in terms of the impedance it produces at a certain frequency, the unit of impedance is also the ohm.
2. Ordinary filter is composed of loss-free reactance elements, whose function in the circuit is to reflect the stop-band frequency back to the signal source, so this kind of filter is also called reflection filter.When the reflection filter does not match the impedance of the signal source, some energy will be reflected back to the signal source, resulting in the enhancement of the interference level.In order to solve this problem, the ferrite magnetic ring or magnetic bead sleeve can be used in the input line of the filter.Thus, magnetic rings and beads actually absorb high-frequency components, and are sometimes called absorption filters.
Different ferrite suppression elements have different optimal suppression frequency ranges.In general, the higher the permeability, the lower the frequency of suppression.In addition, the larger the volume of ferrite, the better the inhibition effect.When the volume is fixed, the long and thin shape has better inhibition effect than the short and thick shape, and the smaller the inner diameter, the better the inhibition effect.However, in the case of dc or ac bias current, the problem of ferrite saturation still exists.
When EMI absorbs magnetic ring/magnetic bead to suppress differential mode interference, its current value is proportional to its volume. The imbalance between the two causes saturation and reduces the performance of components.When common mode interference is suppressed, two lines (positive and negative) of the power supply are passed through a magnetic ring at the same time. The effective signal is differential mode signal, and EMI absorbing magnetic ring/magnetic bead has no influence on it, while for common mode signal, a large inductance is displayed.A better way to use a magnetic ring is to make the wire through the ring repeat several times to increase the amount of inductance.According to its principle of suppressing electromagnetic interference, its inhibiting effect can be reasonably used.
Ferrite suppression elements should be installed close to the source of the disturbance.For input/output circuits, try to be close to the inlet and outlet of the shield.The absorption filter composed of ferrite magnetic ring and magnetic bead should not only choose the consumable material with high permeability, but also pay attention to its application.They presented to the high frequency component in a circuit resistance is about ten to several hundred Ω, so its role in the high impedance circuit is not obvious, on the contrary, in a low impedance circuit (such as power distribution, power supply or rf circuit) in use will be very effective.
Ferrite is widely used in EMI control because it attenuates higher frequencies and allows lower frequencies to pass almost unimpeded.Magnetic rings/beads used for EMI absorption can be made into various shapes and widely used in various occasions.If it is on PCB, it can be added to DC/DC module, data cable, power cord, etc.It absorbs high-frequency interference signals on the line where it is located, but does not create new zeros in the system and does not destroy the stability of the system.It can be used in conjunction with the power filter to supplement the high frequency performance of the filter and improve the filtering characteristics of the system.
GETWELL electronics co., LTD. Is a manufacturer of inductors and transformers in China. We offer high quality axial inductor, fixed inductor, ferrite beads, etc. Welcome to consult us!
Post time: Aug-12-2019