Selection of magnetic beads in PCB circuit design

1. The unit of magnetic beads is ohm, not hunter, so you should pay special attention to this. Because the unit of magnetic beads is nominal according to the impedance it produces at a certain frequency, and the unit of impedance is also ohm. The datasheet of magnetic beads generally provides a characteristic curve of frequency and impedance, which is generally based on 100MHz. For example, 1000R@100MHz means that the impedance of the magnetic beads is equivalent to 600 ohms at a frequency of 100MHz.

2. Ordinary filters are composed of lossless reactive elements. Their function in the 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 signal source impedance, part of the energy will be reflected back to the signal source, causing an increase in the interference level. To solve this problem, ferrite rings or magnetic beads can be used on the incoming line of the filter, and the eddy current loss of the high-frequency signal by the ferrite rings or magnetic beads can be used to convert the high-frequency component into heat loss. Therefore, the magnetic rings and magnetic beads actually absorb the high-frequency components, so they are sometimes called absorption filters.

Different ferrite suppression components have different optimal suppression frequency ranges. Generally, the higher the magnetic permeability, the lower the suppression frequency. In addition, the larger the volume of the ferrite, the better the suppression effect. When the volume is constant, the long and thin shape has a better suppression effect 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 still the problem of ferrite saturation. The larger the cross-section of the suppression component, the less likely it is to saturate, and the larger the bias current it can withstand. When the EMI absorption magnetic ring/bead suppresses differential mode interference, the current value passing through it is proportional to its volume. The imbalance between the two causes saturation, which reduces the performance of the component; when suppressing common mode interference, the two wires (positive and negative) of the power supply are passed through a magnetic ring at the same time. The effective signal is a differential mode signal, and the EMI absorption magnetic ring/bead has no effect on it, but it will show a large inductance for the common mode signal. Another good way to use the magnetic ring is to let the wire passing through the magnetic ring be repeatedly wound several times 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 close to the interference source. For input/output circuits, they should be as close to the entrance and exit of the shielding shell as possible. For absorption filters composed of ferrite rings and beads, in addition to selecting lossy materials with high magnetic permeability, attention should also be paid to its application. The resistance they present to high-frequency components in the circuit is about ten to several hundred Ω, so its effect in high-impedance circuits is not obvious. On the contrary, it will be very effective when used in low-impedance circuits (such as power distribution, power supply or radio frequency circuits). IV. Conclusion

Since ferrite can attenuate higher frequencies while allowing lower frequencies to pass almost unimpeded, it has been widely used in EMI control. Magnetic rings/beads used for EMI absorption can be made into various shapes and are widely used in various occasions. For example, on PCB boards, they can be added to DC/DC modules, data lines, power lines, etc. It absorbs high-frequency interference signals on the line, but will not generate new zeros and poles in the system, and will not destroy the stability of the system. It can be used in conjunction with the power supply filter to well supplement the shortcomings of the filter's high-frequency performance and improve the filtering characteristics of 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. Some RF circuits, PLL, oscillation circuits, and ultra-high frequency memory circuits (DDR SDRAM, RAMBUS, etc.) all require magnetic beads to be added to the power input part. Inductors are energy storage elements used in LC oscillation circuits, medium and low frequency filtering circuits, etc., and their application frequency range rarely exceeds 50MHZ.

The function of 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 useful signal needed, while the RF energy is useless electromagnetic interference transmitted and radiated along the line (EMI). To eliminate these unwanted signal energies, chip magnetic beads are used to play the role of high-frequency resistors (attenuators). The device allows DC signals to pass through and filters out AC signals. Usually high-frequency signals are above 30MHz, however, low-frequency signals will also be affected by chip magnetic beads.

Chip beads are made of soft ferrite materials, forming 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. Benefits of using chip beads: Miniaturization and light weight. High impedance in the RF noise frequency range, eliminating electromagnetic interference in transmission lines. Closed magnetic circuit structure, better elimination of signal crosstalk. Excellent magnetic shielding 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 oscillations in high-frequency amplifier circuits. Effectively work in the frequency range of several MHz to hundreds of MHz.

To choose magnetic beads correctly, you must pay attention to the following points:

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

2. Who is the source of the noise?

3. How much noise attenuation is needed?

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

5. What are the circuit and load impedances?

6. Is there space to place magnetic beads on the PCB board?

The first three can be judged by observing the impedance frequency curve provided by the manufacturer. In the impedance curve, all three curves are very important, namely resistance, inductive reactance and total impedance. The total impedance is described by ZR22πfL()2+:=fL. Through this curve, select the ferrite bead model with the maximum impedance in the frequency range where the noise is expected to be attenuated and the signal attenuation is as small as possible at low frequency and DC. The impedance characteristics of chip ferrite beads will be affected under excessive DC voltage. In addition, if the operating temperature rise is too high or the external magnetic field is too large, the impedance of the ferrite beads will be adversely affected. Reasons for using chip ferrite beads and chip inductors: Whether to use chip ferrite beads or chip inductors mainly depends on the application. Chip inductors are required in resonant circuits. When it is necessary to eliminate unwanted EMI noise, using chip ferrite beads is the best choice. Application occasions for chip ferrite beads and chip inductors:

Chip Inductors: Radio Frequency (RF) 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, etc.

Chip beads: Clock generation circuits, filtering between analog circuits and digital circuits, I/O input/output internal connectors (such as serial ports, parallel ports, keyboards, mice, long-distance telecommunications, local area networks), between radio frequency (RF) circuits and susceptible logic devices, filtering out high-frequency conducted interference in power supply circuits, EMI noise suppression in computers, video recorders (VCRS), TV systems and mobile phones.