Do you really know how to choose magnetic beads for PCB design?
Whether to use SMD magnetic beads or SMD inductors in PCB design depends mainly on the application scenario. For example, SMD inductors are needed in resonant circuits; when eliminating unwanted EMI noise, using SMD magnetic beads is the best choice.
1.The unit of magnetic beads is ohms, not hunters, so pay special attention to this.
Because the unit of magnetic beads is nominal according to the impedance it produces at a certain frequency, the unit of impedance is also ohms. The DATASHEET of magnetic beads generally provides a characteristic curve of frequency and impedance, generally based on 100MHz, such as 1000R 100MHz, which means that at a frequency of 100MHz, the impedance of the magnetic beads is equivalent to 600 ohms.
2.Ordinary filters are composed of lossless reactive elements.
Their 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 signal source impedance, part of the energy will be reflected back to the signal source, causing an increase in the interference level. To solve this problem, ferrite magnetic rings or magnetic beads can be used on the incoming line of the filter, and the eddy current loss of the high-frequency signal by the ferrite rings or magnetic beads can be used to convert the high-frequency component into heat loss. Therefore, the magnetic rings and magnetic beads actually absorb the 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 suppression frequency. In addition, the larger the volume of the ferrite, the better the suppression effect. Some experts on the Internet have found that: when the volume is constant, the long and thin shape has a better suppression effect than the short and thick one, and the smaller the inner diameter, the better the suppression 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 saturate, and the greater the bias current it can withstand. When the EMI absorption magnetic ring/bead suppresses differential mode interference, the current value passing through it is proportional to its volume.
The imbalance between the two causes saturation, which reduces the performance of the component. When suppressing common mode interference, the two wires (positive and negative) of the power supply are passed through a magnetic ring at the same time.
The effective signal is a differential mode signal, and the EMI absorption magnetic ring/bead has no effect on it, but it will show a large inductance for the common mode signal. There is also a good way to use the magnetic ring. A good way is to let the wire passing through the magnetic ring be repeatedly wound several times to increase the inductance. According to its principle of suppressing electromagnetic interference, its suppression effect can be used reasonably.
Ferrite suppression components should be installed close to the interference source.
For input/output circuits, they should be as close as possible to the entrance and exit of the shielding shell. For the absorption filter composed of ferrite magnetic rings and magnetic beads, in addition to selecting lossy materials with high magnetic permeability, it is also necessary to pay attention to its application occasions. The resistance they present to high-frequency components in the circuit is about ten to several hundred Ω, so it is not obvious in high-impedance circuits. On the contrary, it will be very effective in low-impedance circuits (such as power distribution, power supply or radio frequency circuits).
Since ferrite can attenuate higher frequencies while allowing lower frequencies to pass almost unimpeded,
it has been widely used in EMI control. Magnetic rings/magnetic beads used for EMI absorption can be made into various shapes and are widely used in various occasions. For example, on PCB boards, they can be added to DC/DC modules, data lines, power lines, etc. It absorbs high-frequency interference signals on the line, but 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 power supply filters to well supplement the shortcomings of the high-frequency performance of the filter and improve the filtering characteristics of the system.
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. Some RF circuits, PLL, oscillation circuits, and ultra-high frequency memory circuits (DDR SDRAM, RAMBUS, etc.) all require magnetic beads to be added to the power input part. Inductors are energy storage components used in LC oscillation circuits, medium and low frequency filter circuits, etc., and their application frequency range rarely exceeds 50MHZ.
The function of magnetic beads is mainly to eliminate 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 the required useful signal, while the RF energy is useless electromagnetic interference transmitted and radiated along the line (EMI). To eliminate these unwanted signal energies, chip magnetic beads are used to play the role of high-frequency resistors (attenuators). The device allows DC signals to pass through and filters out AC signals. Usually the high-frequency signal is above 30MHz, however, low-frequency signals will also be affected by chip magnetic beads.
Chip magnetic beads are composed of soft magnetic ferrite materials, 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.
Benefits of using chip beads: miniaturization and light weight with high impedance in the RF noise frequency range, eliminating electromagnetic interference in the transmission line. Closed magnetic circuit structure, better eliminate signal crosstalk. Excellent magnetic shielding structure, reduce DC resistance to avoid excessive attenuation of useful signals. Significant high-frequency characteristics and impedance characteristics (better elimination of RF energy). Eliminate parasitic oscillations in high-frequency amplifier circuits. Effectively work in the frequency range of several MHz to hundreds of MHz.
Several core suggestions for correctly selecting magnetic beads in PCB design:
- What is the frequency range of the unwanted signal
- Who is the noise source
- Is there space to place magnetic beads on the PCB board
- How much noise attenuation is required
- What are the environmental conditions (temperature, DC voltage, structural strength)
- What is the circuit and load impedance
The first three can be judged by observing the impedance frequency curve provided by the manufacturer. In the impedance curve, all three curves are very important, namely resistance, inductance and total impedance. The total impedance is described by ZR22πfL()2+:=fL. Through this curve, select the bead model with the maximum impedance in the frequency range where you want to attenuate noise and the smallest signal attenuation at low frequency and DC. The impedance characteristics of the chip bead will be affected under excessive DC voltage. In addition, if the operating temperature rise is too high or the external magnetic field is too large, the impedance of the bead will be adversely affected.
Applications of chip beads and chip inductors:
Chip inductors: 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.
Chip 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), between RF circuits and susceptible logic devices, filtering high-frequency conducted interference in power supply circuits, computers, machines, video recorders (VCRS), TV systems, and EMI noise suppression in mobile phones.
