PCB Tech - Did you choose the right PCB design magnetic beads？

Whether to use chip beads or chip inductors in PCB design mainly depends on the application scenario. For example, it is necessary to use chip inductors in the resonant circuit; when eliminating unwanted EMI noise, the use of chip beads is the best 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 beads generally provides frequency and impedance characteristic curves. Generally, 100MHz is the standard, such as 1000R 100MHz, which 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 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 online researches have 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 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 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 wind 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 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 using 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.

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. 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 present in the transmission line structure (circuit). RF energy is an AC sine wave component superimposed on the DC transmission level. The DC component is a useful signal, but the 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). 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.

The chip magnetic beads are composed of soft ferrite material, 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.

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, reducing DC resistance, so as to avoid excessive attenuation of useful signals. Significant high-frequency characteristics and impedance characteristics (better eliminate RF energy). Eliminate parasitic oscillations in high-frequency amplifier circuits. Effectively work in the frequency range of several MHz to several hundred MHz.

Some suggestions for choosing the core of magnetic beads correctly in PCB layout and design:

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

2. Who is the noise source

3. Is there space 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 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.

Application occasions of chip 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.