In the EMC design process of product digital circuits , we often use magnetic beads, so what is the principle of magnetic bead filtering and how to use it? The ferrite material is an iron-magnesium alloy or an iron-nickel alloy. This material has a high magnetic permeability, and it can produce the smallest capacitance between the coil windings of the inductor under the condition of high frequency and high resistance . Ferrite materials are usually used at high frequencies because they are mainly inductive at low frequencies, making the loss in the line very small. At high frequencies, they are predominantly reactive in ratio and vary with frequency. In practical applications, ferrite materials are used as high-frequency attenuators for radio frequency circuits. In fact, ferrite is better equivalent to the parallel connection of resistance and inductance. At low frequency, the resistance is short-circuited by the inductance, and at high frequency, the impedance of the inductance becomes quite high, so that all the current passes through the resistance. Ferrite is a consumer device on which high-frequency energy is converted into heat energy, which is determined by its resistance characteristics. Ferrite beads have better high-frequency filtering characteristics than ordinary inductors. Ferrite exhibits resistance at high frequencies, which is equivalent to an inductor with a very low quality factor , so it can maintain high impedance in a wide frequency range, thereby improving high-frequency filtering performance. The impedance in the low frequency band is composed of the inductive reactance of the inductance. At low frequencies, R is very small, and the magnetic permeability of the magnetic core is high, so the inductance is large, and L plays a major role, and the electromagnetic interference is reflected and suppressed; and at this time, the magnetic core The loss is small, and the whole device is an inductor with low loss and high Q characteristics. This inductor is easy to cause resonance. Therefore, in the low frequency band, sometimes there may be a phenomenon of enhanced interference after using ferrite beads. The impedance in the high frequency band is composed of resistive components. As the frequency increases, the magnetic permeability of the magnetic core decreases, resulting in a decrease in the inductance of the inductor and a decrease in the inductive reactance component. However, at this time, the loss of the magnetic core increases and the resistive component increases. cause the total impedance to increase when high frequency signals
When passing through ferrite, electromagnetic interference is absorbed and dissipated in the form of heat energy.
Ferrite suppression components are widely used on printed circuit boards, power lines and data lines. If a ferrite suppression element is added to the inlet end of the power line of the printed board, high-frequency interference can be filtered out. Ferrite magnetic rings or magnetic beads are specially used to suppress high-frequency interference and spike interference on signal lines and power lines. It also has the ability to absorb electrostatic discharge pulse interference.
Whether to use chip beads or chip inductors mainly depends on the actual application. Chip inductors are required in resonant circuits. When it is necessary to eliminate unwanted EMI noise, chip beads are the best choice.
Applications Chip inductors: radio frequency ( RF ) and wireless communications, information technology equipment, radar detectors, automotive electronics , cellular phones, pagers, audio equipment, PDAs (personal digital assistants), wireless remote control systems, and low-voltage power supply modules, etc.
Chip beads: clock generation circuit, filter between analog circuit and digital circuit, I/O input/output internal connector (such as serial port, parallel port, keyboard, mouse, long-distance telecommunications, local area network), radio frequency (RF) circuit Between and susceptible logic devices, filter out high-frequency conduction interference in power supply circuits, EMI noise suppression in computers, printers, video recorders (VC RS ), TV systems and mobile phones.
The unit of the magnetic bead is ohm, because the unit of the magnetic bead is nominal according to the impedance it produces at a certain frequency, and the unit of impedance is also the ohm. The DATASHEET of the magnetic bead generally provides a characteristic curve of frequency and impedance, which is generally based on 100MHz. For example, at a frequency of 100MHz, the impedance of the magnetic bead is equivalent to 1000 ohms. For the frequency band we want to filter, we need to choose the larger the impedance of the magnetic bead, the better, usually choose the impedance above 600 ohms.
In addition, when selecting magnetic beads, it is necessary to pay attention to the flow rate of the magnetic beads. Generally, it needs to be derated by 80%. When it is used in a power circuit , the influence of DC impedance on the voltage drop should be considered.
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