Electronic Design - How to Choose Magnetic Beads in PCB Circuit Design

The selection techniques of magnetic beads in PCB circuit design are as follows:

The reason for using chip beads and chip inductors: whether to use chip beads or chip inductors mainly depends on the application. SMD inductors are required in the resonant circuit. When it is necessary to eliminate unnecessary EMI noise, the use of chip beads is the best choice.

1. The unit of the magnetic bead is ohms, not Hunter. Pay special attention to this point. Because 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 generally provides frequency and impedance characteristic curves. Generally, 100MHz is the standard, such as 1000R 100MHz, which means 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, and their 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 impedance of the signal source, a part of the energy will be reflected back to the signal source, resulting in an increase in the interference level. In order to solve this problem, a ferrite magnetic ring or a magnetic bead sleeve can be used on the incoming line of the filter to use the eddy current loss of the ring or magnetic bead on the high-frequency signal to convert the high-frequency component into heat loss. Therefore, the magnetic ring and the magnetic beads actually absorb high-frequency components, so they are sometimes called absorption filters.

Different ferrite suppression components have different optimal suppression frequency ranges. Generally, the higher the permeability, the lower the frequency of suppression. In addition, the larger the volume of ferrite, the better the suppression effect. Some big cow research on the website of My Love Program found that when the volume is constant, the long and thin shape has better suppression effect than the short and thick one, and the smaller the inner diameter, the better the suppression 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 bias current that can be tolerated. When the EMI absorption magnetic ring/magnetic bead suppresses the differential mode interference, the current value passing through it is proportional to its volume. The imbalance of the two causes saturation and reduces the performance of the component; when suppressing the 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, but for the common mode signal, it will show a larger inductance. Another better way to use the magnetic ring is to make the wires of the magnetic ring that pass through it repeatedly to increase the inductance. According to its suppression principle of electromagnetic interference, its suppression effect can be used reasonably.

Ferrite suppression components should be installed near the interference source. For the input/output circuit, 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 the use of lossy materials with high permeability, attention should also be paid to its application. Their resistance to high-frequency components in the circuit 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) Use will be very effective.

Because ferrite can attenuate higher frequencies while allowing lower frequencies to pass almost unimpeded, it has been widely used in EMI control. The magnetic ring/magnetic beads used for EMI absorption can be made into various shapes and widely used in various occasions. 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 system, and will not destroy the stability of the system. It is used in conjunction with the power supply filter, which can well supplement the lack of performance of the high-frequency end of the filter and improve the filtering characteristics in the system.

Magnetic beads are specially designed 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, oscillation 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). RF energy is an AC sine wave component superimposed on the DC transmission 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 unwanted signal energy, chip beads are used to play the role of high-frequency resistance (attenuator). The device allows DC signals to pass, while filtering out AC signals. Usually the high frequency signal is above 30MHz, however, the low frequency signal will also be affected by the chip beads.

SMD magnetic beads are composed of soft ferrite materials, which form a monolithic structure with high volume resistivity. Eddy current loss is inversely proportional to the resistivity of ferrite materials. Eddy current loss is proportional to the square of the signal frequency. The advantages 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 cross-winding. Excellent magnetic shielding structure. Reduce the 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.

To choose the magnetic beads correctly, the author gives some core suggestions:

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

2. Who is the source of the noise;

3. Is there room to place 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 judged by observing the impedance frequency curve provided by the PCB manufacturer. Three curves are very important in the impedance curve, namely resistance, inductance and total impedance. The total impedance is described by ZR22πfL()2+:=fL. Through 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 under 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. You can also go to the Shenzhen Electronics Show to choose. The reason for using chip beads and chip inductors: whether to use chip beads or chip inductors mainly depends on the application. SMD inductors are required in the resonant circuit. When it is necessary to eliminate unnecessary EMI noise, the use of chip beads is the best choice.