Electronic Design - The use of pcb design of magnetic beads

Whether to use chip beads or chip inductors in the design of PCB circuit boards mainly depends on the application scenario. For example, chip inductors are required in the resonant circuit; when eliminating unnecessary EMI noise, the use of chip beads is the best choice for PCB design.
1. In the PCB design, the unit of the magnetic beads is ohms, not Hunter, which should be paid special attention. Since the unit of the magnetic bead is nominally based on the impedance it generates at a certain frequency, the unit of impedance is also ohms. The DATASHEET of the magnetic bead will generally provide frequency and impedance characteristic curves. Generally, 100MHz is the standard. For example, 1000R 100MHz, the meaning is that the impedance of the magnetic bead is equivalent to 600 ohms at a frequency of 100MHz.
2. Ordinary filters are composed of lossless reactive components. Its function in the PCB design circuit 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 impedance of the signal source, a part of the energy will be reflected back to the signal source, forming an increase in the interference level. The PCB is designed to deal with this disadvantage. Ferrite magnetic rings or magnetic bead sleeves can be used on the incoming line of the filter, and the eddy current loss of high-frequency signals by the nuisance ring or magnetic beads can be used to convert high-frequency components into heat losses. Therefore, the magnetic ring and the magnetic beads absorb high-frequency components in practice, so they are sometimes called absorption filters.
Different ferrite suppression components have different optimal suppression frequency ranges. In the PCB design, the higher the permeability, the lower the frequency of suppression. In addition, the larger the volume of ferrite, the better the suppression effect. Some online researches found that when the volume is constant, the long and thin shape has better restraining effect than the short and thick one, and the smaller the inner diameter, the better the restraining effect. However, in the presence of DC or AC bias current, there is still the problem of ferrite saturation. The larger the cross-section of the suppression element, the less likely to be saturated, and the greater the acceptable bias current. When the EMI absorption magnetic ring/magnetic bead suppresses the differential mode interference, the current value passing through it is proportional to its volume, and the two are out of adjustment to form saturation, which reduces the component performance; when suppressing common mode interference, connect the two wires of the power supply (positive and negative) Passing through a magnetic ring at the same time, the effective signal is a differential mode signal, and the EMI absorption magnetic ring/magnetic bead has no effect on it, while the common mode signal will show a larger inductance. Another better way to use the magnetic ring is to repeat the winding of the wire of the magnetic ring that passes through it several times to increase the inductance. Based on its suppression principle of electromagnetic interference, its suppression effect can be used reasonably.
The ferrite suppression element should be installed near the center of the interference source. Regarding the input/output circuit of PCB design, it should be as close as possible to the entrance and exit of the shielding case. For the absorption filter composed of ferrite bead and magnetic beads, in addition to using high-permeability lossy materials, it is also necessary to pay attention to its application sites. Their resistance to high-frequency components in PCB design circuits is about ten to several hundred ω, so its role in high-impedance circuits is not obvious. On the contrary, in low-impedance circuits (such as power distribution, power supply or radio frequency circuits) ) Will be very effective.
Since ferrite can attenuate higher frequencies while allowing lower frequencies to pass through without obstacles, it has been widely used in EMI control. The magnetic ring/magnetic beads used for EMI absorption can be made into various shapes, and are generally used in various places. If it is on the PCB board, it can be added to the DC/DC module, data line, power line, etc. It absorbs high-frequency interference signals on the line where it is located, but it will not generate new poles and zeros in the PCB design system, and will not destroy the stability of the system. It is used in conjunction with the power supply filter, which can complement the lack of high-frequency performance of the filter and improve the filtering characteristics in the system.
Magnetic beads are specially used to suppress high-frequency noise and spike interference on signal lines and power lines, and also have the ability to absorb electrostatic pulses.
Magnetic beads are used to absorb ultra-high frequency signals. For example, some RF circuits, PLLs, oscillator circuits, and ultra-high frequency memory circuits (DDR SDRAM, RAMBUS, etc.) need to add magnetic beads to the power input part, and inductance is a kind of storage Energy components, used in LC oscillator circuits, medium and low frequency filter circuits, etc., and their application frequency range rarely exceeds 50MHZ.

The function of the magnetic beads is mainly to eliminate the RF noise existing in the transmission line structure (circuit). The RF energy is the AC sine wave component superimposed on the direct transmission output level. The DC component is the required useful signal, while the radio frequency RF energy is useless. The electromagnetic interference is transmitted and radiated (EMI) along the line. To eliminate these unnecessary signal energy, use chip beads to play the role of high-frequency resistance (attenuator), the device allows DC signals to pass through, while filtering AC signals. Usually the high frequency signal is above 30MHz, but the low frequency signal will also be affected by the chip beads.
The chip beads are composed of soft ferrite materials, forming a monolithic structure with high volume resistivity. Eddy current loss is inversely proportional to the resistivity of ferrite material. Eddy current loss is proportional to the square of the signal frequency.
The benefits of using chip beads: miniaturization and light weight have high impedance in the frequency range of radio frequency noise, eliminating electromagnetic interference in the transmission line. Closed magnetic circuit structure can better eliminate signal crosstalk. Excellent magnetic shielding structure to reduce DC resistance to avoid excessive attenuation of useful signals. Significant high frequency characteristics and impedance characteristics (better elimination of RF energy). Eliminate parasitic oscillations in high-frequency amplifier circuits. Effectively work in the frequency range of several MHz to several hundred MHz.
Several suggestions to correctly choose the magnetic bead comparison center in PCB design:
1. What is the frequency range of unneeded signals?
2. Who is the noise source
Third, can there be room to place the magnetic beads on the PCB board
4. How much noise attenuation is needed
5. What are the environmental conditions (temperature, DC voltage, structural strength)
6. What is the circuit and load impedance
The first three can be distinguished by looking at the impedance frequency curve provided by the manufacturer. The three curves in the impedance curve of PCB design are very important, namely resistance, inductance and total impedance. The total impedance is described by ZR22πfL()2+:=fL. After this curve, select the magnetic bead model that has the largest impedance in the frequency range where the noise is to be attenuated and the signal attenuation is as small as possible at low frequency and DC. The impedance characteristics of the chip magnetic beads will be affected under excessive DC voltage. In addition, if the operating temperature rises too high or the external magnetic field is too large, the impedance of the magnetic beads will be adversely affected.
Application places of chip beads and chip inductors in PCB design:
Chip inductors: radio frequency (RF) PCB boards and wireless communications, information technology equipment, radar detectors, automobiles, cellular phones, pagers, audio equipment, PDAs (personal digital assistants), wireless remote control systems, and low-voltage power supply modules.
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 telecommunications, local area network), radio frequency (RF) circuit Between the board and the logic device that is susceptible to interference, the power supply circuit filters out high-frequency conduction interference, and suppresses EMI noise in computers, cameras, video recorders (VCRS), TV systems and mobile phones.