Selection of Magnetic Beads in PCB Circuit Design

Selection of Magnetic Beads in PCB Circuit Design
The choice of magnetic bead PCB circuit design, correct choice of magnetic bead, we must pay attention to these points
1. The structure of the magnetic bead is ohm rather than Hunt, so special attention should be paid to it. Because according to the impedance generated at a specific frequency, the organization of the magnetic bead is nominal, and the organization of the impedance is also ohm. The characteristic curves of frequency and impedance are usually provided in the magnetic bead database, usually based on 100MHz, such as 1000R@100MHz , which means that the impedance of the magnetic bead is equal to 6.00 ohms at 100MHz
2. Common filter is composed of lossless reactance elements. Its function in the line is to reflect the stopband frequency back to the signal source. This filter is also called reflection filter. When the impedance of the reflection filter does not match the signal source, some energy will be reflected back to the signal source, resulting in enhanced interference level. To solve this problem, ferrite magnetic ring or magnetic bead sleeve can be used on the inlet line of the filter, and the eddy current loss of the magnetic ring or magnetic bead on the high-frequency signal can be used to convert the high-frequency component into heat loss. In this case, the magnetic ring and magnetic bead will actually absorb high-frequency components, which is sometimes called absorption filter
Different ferrite suppression elements have different optimal suppression frequency ranges. Generally, the higher the permeability, the lower the rejection frequency. In addition, the larger the ferrite volume, the better the inhibition effect. When the volume is constant, the slender shape has better inhibition effect than the short thick shape. The smaller the inner diameter, the better the inhibition effect. However, in the case of DC or AC bias current, when EMI absorbs magnetic ring/bead to suppress differential mode interference, its current value is proportional to its volume. The imbalance between the two will lead to saturation and reduce the efficiency of the components; When suppressing common mode interference, two wires of the power supply (positive and negative) pass through the magnetic ring at the same time. The effective signal is differential mode signal. The electromagnetic interference absorbing magnetic ring/magnetic bead has no effect on it, but it will show a large amount of inductance on the common mode signal. Another better way to use the magnetic ring is to repeatedly wind the wire passing through the magnetic ring for several times. In order to add inductance, its suppression effect can be reasonably used according to the principle of electromagnetic interference suppression
Printed circuit board

Ferrite suppression elements shall be installed close to interference sources. For input/output circuits, it shall be as close as possible to the inlet and outlet of the mask enclosure. For the absorption filter composed of ferrite magnetic rings and magnetic beads, in addition to the consumables with high permeability, attention should also be paid to its application. Their resistance to the medium and high frequency components of the line is about 10 to hundreds of Ω). In conclusion, their role in high impedance circuits is not obvious. On the contrary, it is very effective in low impedance circuits such as power distribution, power supply or RF circuits. 4、 Conclusion
Ferrite is widely used for electromagnetic interference control because it can attenuate higher frequencies and let lower frequencies pass through almost unimpeded Magnetic ring/magnetic bead used for electromagnetic interference absorption can be made into various shapes and widely used in various occasions For example, on PCB, they can be added to DC/DC modules, data lines, power lines, etc It absorbs high-frequency interference signals on the line, however, it will not generate new poles and zeros in the system, nor will it destroy the stability of the system When used together with the power filter, it can make up for the lack of high frequency efficiency of the filter and improve the filtering characteristics of the system
Magnetic beads are used to suppress high-frequency noise and peak interference on signal lines and power lines, and also have the ability to absorb electrostatic pulses
Magnetic beads are used to absorb UHF signals. For example, some RF circuits, phase-locked loops, oscillation circuits and ultra-high frequency storage circuits (DDR, SDRAM, Rambus, etc.) need to add magnetic beads to the power input part, while the inductor is an energy storage element, which is used for LC oscillation circuits and medium and low frequency filtering circuits. Its application frequency range rarely exceeds 50MHz
The function of the magnetic bead is mainly to eliminate the RF noise existing in the transmission line structure (circuit). RF energy is the AC sine wave component superimposed on the DC transmission level. DC component is a useful signal needed, while RF energy is useless. Electromagnetic interference transmission and radiation (EMI) along the line. In order to eliminate the unnecessary signal energy, the chip magnetic bead acts as a high-frequency resistor (attenuator). The device allows DC signals to pass through and filter AC signals. Generally, the high frequency signal is greater than 30MHz. However, the low-frequency signal will also be affected by the chip magnetic bead
The wafer magnetic bead is composed of soft ferrite data, forming an integral structure with high volume resistivity. The eddy current loss is inversely proportional to the resistivity of ferrite data. The eddy current loss is proportional to the square of the signal frequency. The advantages of using wafer magnetic beads are miniaturization and lightweight. It has high impedance in the frequency range of RF noise and eliminates electromagnetic interference in transmission lines. The closed magnetic circuit structure can better eliminate the signal of the series winding. Excellent magnetic mask structure. Reduce DC resistance to avoid excessive attenuation of useful signals. Significant high frequency characteristics and impedance characteristics (better elimination of RF energy). Eliminate parasitic oscillation in high-frequency amplifier circuit. Effective operation in the frequency range of several MHz to hundreds of MHz
To correctly select the magnetic bead, the following points must be noted:
1. What is the frequency range of useless signals;
2. Who is the noise source;
3. How much noise attenuation is required;
4. What are the environmental conditions (temperature, DC voltage, structural strength);
5. Circuit and load impedance;
6. Whether there is space to place the magnetic bead printed circuit board;
The first three can be judged by observing the impedance frequency curve provided by the manufacturer. Among the impedance curves, three curves are very important, namely resistance, inductive reactance and total impedance. The total impedance is described by zr22 Ï FL() 2+:=FL. Through this curve, the magnetic bead model with the maximum impedance in the frequency range of noise attenuation and the signal attenuation as small as possible under low frequency and DC is selected. Excessive DC voltage will affect the impedance characteristics of the magnetic beads. In addition, if the operating temperature rise is too high or the external magnetic field is too large, the impedance of the magnetic bead will be adversely affected. Reasons for using chip magnetic beads and chip inductors: Whether to use chip magnetic beads or chip inductors depends mainly on the application. Chip inductors are required in resonant circuits. When it is necessary to eliminate unnecessary EMI noise, the chip magnetic bead is the best choice. Application of wafer magnetic bead and wafer inductor:
Chip inductor: RF and wireless communication, information technology equipment, radar detector, automobile, cellular telephone, pager, audio equipment, PDA (personal digital assistant), wireless remote control system, low-voltage power module, etc
Chip magnetic bead: filter between clock generation circuit, analog circuit and digital circuit, input/output input/output internal connector (such as serial port, parallel port, keyboard, mouse, remote communication, LAN), filter between RF circuit and logic device susceptible to interference, filter high-frequency conducted interference in power circuit, computer, video recorder (VCR) EMI noise suppression in TV systems and mobile phones

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