PCB Tech - Selection of magnetic beads in PCB circuit design

Selection of magnetic beads in PCB circuit design, To correctly select magnetic beads, we must pay attention to these points.

1. The unit of magnetic bead is Ohm instead of hunter, which should be paid special attention to. Because the unit of magnetic bead is nominal according to the impedance generated at a certain frequency, and the unit of impedance is also ohm. The characteristic curve of frequency and impedance is generally provided on the database of magnetic bead, generally based on 100MHz, such as 1000R@100MHz, It means that the impedance of the magnetic bead is equivalent to 600 ohms at 100MHz

2. Ordinary filter is composed of lossless reactance elements. Its function in the line is to reflect the stopband frequency back to the signal source, so this kind of filter is also called reflection filter. When the impedance of the reflection filter and the signal source does not match, some energy will be reflected back to the signal source, resulting in the enhancement of interference level. In order to solve this disadvantage, Ferrite magnetic ring or bead sleeve can be used on the inlet line of the filter to convert the high-frequency components into heat loss by using the eddy current loss of the high-frequency signal by the magnetic ring or bead. Therefore, the magnetic ring and bead actually absorb the high-frequency components, so it is sometimes called absorption filter

Different ferrite suppression elements have different optimal suppression frequency ranges. Generally, the higher the permeability, the lower the suppression frequency. In addition, the larger the volume of ferrite, the better the suppression effect. When the volume is certain, the long and thin shape has better suppression effect than the short and thick shape, and the smaller the inner diameter, the better the suppression effect. However, in the case of DC or AC bias current, When EMI absorbing magnetic ring / bead suppresses differential mode interference, its current value is directly proportional to its volume. The imbalance between the two causes saturation and reduces the performance of the element; When suppressing common mode interference, the two wires (positive and negative) of the power supply pass through a magnetic ring at the same time. The effective signal is the differential mode signal. The EMI absorbing magnetic ring / bead has no effect on it, but it will show a large amount of inductance for the common mode signal. Another better method in the use of magnetic ring is to make the wires of the passing magnetic ring wind several times repeatedly, In order to increase the inductance, its suppression effect can be used reasonably according to its suppression principle of electromagnetic interference

The ferrite suppression element shall be installed close to the interference source. For the input / output circuit, it shall be as close as possible to the inlet and outlet of the shielding shell. For the absorption filter composed of ferrite magnetic ring and magnetic beads, in addition to the consumable materials with high permeability, it shall also pay attention to its application. Their resistance to high-frequency components in the line is about 10 to hundreds of Ω, Therefore, its role in high impedance circuits is not obvious. On the contrary, it will be very effective in low impedance circuits (such as power distribution, power supply or RF circuits). IV. conclusion

Ferrite is widely used in EMI control because it can attenuate higher frequencies and let lower frequencies pass almost unimpeded. Magnetic rings / beads used for EMI absorption can be made into various shapes and are 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 produce new zeros and poles in the system and will not destroy the stability of the system. When it is used together with the power filter, it can well supplement the lack of high-frequency terminal performance of the filter and improve the filtering characteristics of the system

Magnetic beads are specially 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, PLL, oscillation circuits and UHF memory circuits (DDR, SDRAM, Rambus, etc.), magnetic beads need to be added to the power input part, while inductance is an energy storage element, which is used in LC oscillation circuit, medium and low frequency filter circuit, and its 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 the AC sine wave component superimposed on the DC transmission level. The DC component is the required useful signal, while RF RF energy is useless. Electromagnetic interference is transmitted and radiated along the line (EMI). To eliminate these unnecessary signal energy, Chip magnetic beads play the role of high-frequency resistance (attenuator). This device allows DC signals to pass through and filters AC signals. Usually, the high-frequency signal is more than 30MHz. However, the low-frequency signal will also be affected by chip magnetic beads

Chip magnetic beads are composed of soft ferrite materials, forming a monolith structure with high volume resistivity. The eddy current loss is inversely proportional to the resistivity of ferrite materials. The eddy current loss is directly proportional to the square of signal frequency. The advantages of using chip magnetic beads are miniaturization and lightweight. It has high impedance in the frequency range of RF noise and eliminates electromagnetic interference in transmission lines. Closed magnetic circuit structure, Better eliminate series winding of signals. Excellent magnetic shielding structure. Reduce DC resistance to avoid excessive attenuation of useful signals. Significant high-frequency characteristics and impedance characteristics (better eliminate RF energy). Eliminate parasitic oscillation in high-frequency amplification circuit. Effectively work in the frequency range of several MHz to hundreds of MHz

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

1. What is the frequency range of unwanted 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 magnetic beads on PCB;

The first three can be judged by observing the impedance frequency curve provided by the manufacturer. In the impedance curve, the three curves are very important, namely resistance, inductive reactance and total impedance. The total impedance is described by zr22 π FL () 2 +: = fl. through this curve, Select the magnetic bead model that has the maximum impedance within the frequency range where you want to attenuate the noise and the signal attenuation is as small as possible under low frequency and DC. The impedance characteristics of chip magnetic beads will be affected under excessive DC voltage. In addition, if the working temperature rise is too high or the external magnetic field is too large, The impedance of magnetic beads will be adversely affected. The reason for using chip magnetic beads and chip inductors: whether to use chip magnetic beads or chip inductors mainly depends on the application. Chip inductors need to be used in resonant circuits. When it is necessary to eliminate unwanted EMI noise, chip magnetic beads are the best choice. Applications of chip magnetic beads and chip inductors:

Chip inductors: radio frequency (RF) and wireless communication, information technology equipment, radar detector, automobile, cellular telephone, pager, audio equipment, PDAs (personal digital assistant), wireless remote control system and low-voltage power supply module, etc

Chip magnetic beads: filtering between clock generation circuit, analog circuit and digital circuit, I / O input / output internal connectors (such as serial port, parallel port, keyboard, mouse, long-distance telecommunications, local area network), between radio frequency (RF) circuit and easily disturbed logic equipment, filtering high-frequency conducted interference in power supply circuit, computer, computer, video recorder (VCRs), EMI noise suppression in TV systems and mobile phones