PCB Blog - Selection skills of magnetic beads in PCB circuit design

In PCB board design the reason for using SMD magnetic beads and SMD inductors: whether to use SMD magnetic beads or SMD inductors mainly depends on the application. Chip inductors are required in resonant circuits. When it is necessary to eliminate unwanted EMI noise, the use of chip beads is the choice.

1. The unit of magnetic beads is ohms, not hunters. Special attention should be paid to this point. Because the unit of the magnetic bead is nominal according to the impedance it produces at a certain frequency, the unit of impedance is also ohm. The DATASHEET of the magnetic bead generally provides the characteristic curve of frequency and impedance. Generally, 100MHz is used as 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. Its function 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 portion of the energy is reflected back to the signal source, resulting in an increase in the interference level. In order to solve this drawback, a ferrite magnetic ring or magnetic bead sleeve can be used on the incoming line of the filter, and the eddy current loss of the high-frequency signal by the ferrite ring or magnetic bead can be used 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 suppression frequency ranges. Generally, the higher the permeability, the lower the frequency of suppression. In addition, the larger the volume of the ferrite, the better the inhibitory effect. When the volume is constant, the long and thin shape has better inhibitory effect than the short and thick shape, and the smaller the inner diameter, the better the inhibitory 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 element, the less likely it is to be saturated, and the greater the bias current that can be tolerated. When the EMI absorbing magnetic ring/magnetic bead suppresses differential mode interference, the current value passing through it is proportional to its volume, and the imbalance of the two causes saturation, which reduces the performance of the component; when suppressing common mode interference, connect the two wires (positive and negative) of the power supply to the power supply. 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, while for the common mode signal, it will show a large inductance. There is also a better method in the use of the magnetic ring, which is to make the wire passing through the magnetic ring coil several times to increase the inductance. According to its principle of suppressing electromagnetic interference, its suppressing effect can be used reasonably. Ferrite suppression components should be installed close to the source of interference. For input/output circuits, it should be as close as possible to the inlet and outlet of the shielding case. For the absorption filter composed of ferrite magnetic ring and magnetic beads, in addition to the use of consumable materials with high magnetic permeability, attention should also be paid to its application. The resistance they present to high-frequency components in the line is about ten to several hundred Ω, so its effect is not obvious in high-impedance circuits, on the contrary, in low-impedance circuits (such as power distribution, power supply or RF circuits) Use will be very effective.

Ferrites are widely used in EMI control because they can attenuate higher frequencies while allowing lower frequencies to pass almost unimpeded. Magnetic rings/beads for EMI absorption can be made into various shapes and are widely used in various applications. For example, on the PCB board, it can be added to DC/DC modules, data lines, power lines, etc. It absorbs high-frequency interference signals on the line where it is located, but it will not generate new zeros and poles 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 complement the lack of high-frequency performance 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, such as some RF circuits, PLL, oscillator circuits, including ultra-high frequency memory circuits (DDR SDRAM, RAMBUS, etc.) need to add magnetic beads to the power input part, and the inductance is a kind of storage. Energy components, used in LC oscillator circuits, filter circuits in medium and low frequencies, etc., the 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. The DC component is the required useful signal, while the RF RF energy is useless. of electromagnetic interference (EMI) transmitted and radiated along the line. To remove these unwanted signal energy, a chip bead is used to act as a high frequency resistor (attenuator), which allows DC signals to pass and filters out AC signals. Usually high-frequency signals are above 30MHz, however, low-frequency signals are also affected by the chip beads. The chip magnetic bead is composed of soft ferrite material, which constitutes a monolithic structure with high volume resistivity. Eddy current losses are inversely proportional to the resistivity of the ferrite material. Eddy current losses are proportional to the square of the signal frequency. Benefits of using SMD beads: Miniaturization and Lightweight High impedance in the RF noise frequency range, eliminating EMI in transmission lines. Closed magnetic circuit structure to better eliminate signal cross-winding. Excellent magnetic shielding structure. Reduce the DC resistance to avoid excessive attenuation of the useful signal. Significant high frequency characteristics and impedance characteristics (better removal of RF energy). Eliminate parasitic oscillations in high frequency amplifier circuits. Effective operation is in the frequency range of a few MHz to several hundred MHz.

To select the magnetic beads correctly, there are the following suggestions:
1. What is the frequency range of the unwanted signal?
2. Who is the noise source?
3. Whether there is space to place magnetic beads on the PCB;
4. How much noise attenuation is required;
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. In the impedance curve three curves are very important, namely resistance, inductive reactance and total impedance. The total impedance is described by ZR22πfL()2+:=fL. From this curve, choose a bead model that has impedance in the frequency range where you want to attenuate noise and minimize signal attenuation at low frequencies and DC. The impedance characteristics of the chip magnetic beads will be affected when the DC voltage is too large. In addition, if the working temperature is too high, or the external magnetic field is too large, the impedance of the magnetic beads will be adversely affected. The reason for using chip beads and chip inductors: whether to use chip beads or chip inductors mainly depends on the application. Chip inductors are required in resonant circuits. need to eliminate unwanted

When EMI noise, the use of chip beads is the choice.
SMD magnetic beads and chip inductors applications: SMD 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 system and low voltage power supply module, etc.
SMD magnetic 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), radio frequency (RF) circuits It can filter out high-frequency conducted interference in the power supply circuit, and suppress the EMI noise in the computer, computer, video recorder (VCRS), TV system and mobile phone PCB board between the logic equipment that is susceptible to interference.