Selection of magnetic beads in PCB circuit design

- Dec 27, 2019-

Selection of magnetic beads in PCB circuit design

The reason for using chip beads and chip inductors: whether to use chip beads or chip inductors is mainly in applications. Need to use the chip inductor in the resonance circuit. When you need to eliminate unwanted EMI noise, the use of chip beads is the best choice.

 

  1. The unit of the magnetic beads is ohms, not Hunters, so pay special attention. 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 beads generally provides frequency and impedance characteristic curve diagrams, generally based on 100MHz, such as 1000R 100MHz, which means that the impedance of the magnetic beads is equivalent to 600 ohms at 100MHz frequency.

 

  2. Ordinary filters are composed of lossless reactance 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 signal source impedance, a part of the energy is reflected back to the signal source, causing an increase in the interference level. In order to solve this problem, a ferrite ring or bead sleeve can be used on the input line of the filter. The eddy current loss of the high-frequency signal by the nourishing ring or bead is used to convert the high-frequency component into heat loss. Therefore, magnetic rings and beads actually absorb high-frequency components, so they are sometimes called absorption filters.

 

Different ferrite suppression elements have different optimal suppression frequency ranges. Generally, the higher the magnetic permeability, the lower the frequency of suppression. In addition, the larger the volume of the ferrite, the better the suppression effect. I love the research of some big cattle on the Internet and found that: when the volume is constant, the long and thin shape is better than the short and thick suppression effect, and the smaller the inner diameter, the better the suppression effect. However, in the case of DC or AC bias current, there is also the problem of ferrite saturation. The larger the cross-section of the suppression element, the less saturated it is, and the larger the bias current can be tolerated. When EMI absorbs the magnetic ring / bead to suppress differential mode interference, the current value passing through it is proportional to its volume, and the imbalance between the two causes saturation, which reduces the performance of the component; when suppressing common mode interference, 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 absorbing magnetic ring / bead has no effect on it, and it will show a large inductance for common mode signals. Another good method in the use of the magnetic ring is to make the wire of the magnetic ring passing through it 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 near the source of interference. For the input / output circuit, it should be as close as possible to the entrance and exit of the shield case. For the absorption filter composed of ferrite ring and magnetic beads, in addition to selecting a high-permeability lossy material, attention should also be paid to its application. Their resistance to high frequency components in the line is about ten to several hundred ohms, so its role in high impedance circuits is not obvious. On the contrary, in low impedance circuits (such as power distribution, power supply or RF circuits) Use will be very effective.

 

Ferrite is widely used in EMI control because it can attenuate higher frequencies and let lower frequencies pass almost unhindered. Magnetic rings / beads used for EMI absorption can be made into various shapes and widely used in various occasions. If it is on a PCB board, it can be added to DC / DC modules, data lines, power lines, etc. It absorbs high-frequency interference signals on the line, 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 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 used to suppress high-frequency noise and spike interference on signal lines and power lines, and also have the ability to absorb static pulses.

 

Magnetic beads are used to absorb ultra-high frequency signals. Like some RF circuits, PLLs, oscillating circuits, including ultra-high frequency memory circuits (DDR SDRAM, RAMBUS, etc.), magnetic beads need to be added to the power input part, and the inductance is a storage Energy components, used in LC oscillator circuits, low-frequency filter circuits, etc., its application frequency range rarely exceeds 50MHZ.

 

The function of the 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 a useful signal that is needed, but the RF RF energy is useless Electromagnetic interference is transmitted and radiated (EMI) along the lines. To eliminate these unwanted signal energies, use chip beads to act as high-frequency resistors (attenuators). This device allows DC signals to pass and filters out AC signals. High-frequency signals are usually above 30MHz, however, low-frequency signals are also affected by chip beads.

 

SMD 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 the ferrite material. Eddy current loss is proportional to the square of the signal frequency. The advantages of using chip beads: miniaturization and light weight High impedance in the frequency range of RF noise, eliminating electromagnetic interference in transmission lines. Closed magnetic circuit structure to better eliminate signal winding. Excellent magnetic shielding structure. Reduce the DC resistance to avoid excessive attenuation of the wanted signal. Significant high frequency and impedance characteristics (better RF energy elimination). Eliminates parasitic oscillations in high-frequency amplifier circuits. Effective operation in the frequency range of several MHz to several hundred MHz.

 

To choose the correct magnetic beads, the author gives some core suggestions:

 

First, what is the frequency range of the unwanted signals;

2. Who is the source of the noise?

Whether there is 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);

26. What is the circuit and load impedance?

 

The first three can be judged by observing the impedance frequency curve provided by the manufacturer. All three curves are very important in the impedance curve, namely resistance, inductive reactance and total impedance. The total impedance is described by ZR22πfL () 2 +: = fL. Through this curve, choose a magnetic bead model that has maximum impedance in the frequency range where you want to attenuate noise and minimize signal attenuation at low frequencies and DC. The resistance of chip beads is affected by excessive DC voltage. In addition, if the operating temperature is too high or the external magnetic field is too large, the resistance of the beads will be adversely affected. Can also go to the Shenzhen Electronics Show to select. Reasons for using chip beads and chip inductors: Whether to use chip beads or chip inductors mainly lies in the application. Need to use the chip inductor in the resonance circuit. When you need to eliminate unwanted EMI noise, the use of chip beads is the best choice.

 

Applications of chip beads and chip inductors:

 

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 systems, and low-voltage power supply modules.

 

SMD beads: clock generation circuit, filtering between analog and digital circuits, I / O input / output internal connectors (such as serial port, parallel port, keyboard, mouse, long distance telecommunications, local area network), radio frequency (RF) circuits Between high-frequency and interference-resistant logic devices, high-frequency conducted interference is filtered in the power supply circuit, and EMI noise suppression in computers, computers, video recorders (VCRS), television systems, and mobile phones is suppressed.

Previous:12 issues you should pay attention to in analog circuit design Next:Talking about the Development and Application Prospect of Flexible Circuit Board