Analysis of the principles of use and selection of SMD magnetic beads

Whether to use SMD beadsor SMD inductors mainly depends on the application. SMD inductors are needed in the resonant circuit, and when unnecessary EMI noise needs to be eliminated, the use of SMD beads is the better choice.

1. The unit of the magnetic bead is ohm, not Hunter. This point must be paid special attention to. 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 a characteristic curve of frequency and impedance, which is generally based on 100MHz, 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, causing the interference level to increase. In order to solve this problem, a ferrite magnetic ring or magnetic bead sleeve can be used on the inlet 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 better 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 online researches found that when the volume is constant, a long and thin shape has a better suppression effect than a 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 component, the less saturated it is, 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 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 close to the source of interference. 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 ferrite bead, in addition to the use of lossy materials with high permeability, attention should 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.

Since 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 damage 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 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 is mainly to eliminate the RF noise that exists 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 the radio frequency RF energy is useless electromagnetic interference. Transmission and radiation (EMI) along the line. To eliminate these unwanted signal energy, chip beads are used to play the role of high-frequency resistance (attenuator). This 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.

Some suggestions for choosing the core of magnetic beads correctly:

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

2. Who is the source of the noise;

3. Is there space to place magnetic beads on the PCB board;

Fourth, how much noise attenuation is needed;

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