What is magnetic bead filtering? What is its working principle? In the process of product digital circuit EMC design, we frequently use magnetic beads, so what is the meaning and working principle of magnetic bead filtering?
The ferrite material is an iron-magnesium alloy or an iron-nickel alloy. This material has a high magnetic permeability, and it can be the smallest capacitance between the coil windings of the inductance under the condition of high frequency and high resistance. Ferrite materials are typically used at high frequencies because at low frequencies they are predominantly inductive, resulting in very little loss on the wire. At high frequencies, they are predominantly reactive 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 resistor and Inductor. At low frequency, the resistor is short-circuited by the inductor, and at high frequency, the impedance of the inductor becomes quite high, so that all the current flows through the resistor. Ferrite is a dissipating 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 common inductors. Ferrite is resistive at high frequencies, equivalent to an inductor with a low quality factor, so it can maintain a high impedance over a fairly wide frequency range, thereby improving high-frequency filtering.
in low frequency
The impedance 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, L plays a major role, and the electromagnetic interference is reflected and suppressed; and the loss of the magnetic core is relatively high at this time. Small, the whole device is a low-loss, high-Q inductor, which is easy to cause resonance. Therefore, in the low frequency band, sometimes the phenomenon of increased interference after the use of ferrite beads may occur.
in high frequency
The impedance is composed of resistance components. As the frequency increases, the magnetic permeability of the magnetic core decreases, resulting in a decrease in the inductance of the inductance and a decrease in the inductive reactance component. However, at this time, the loss of the magnetic core increases, and the resistance component increases, resulting in a total of The impedance increases, and when the high frequency signal passes through the ferrite, the electromagnetic interference is absorbed and dissipated in the form of heat energy. Ferrite suppression components are widely used on printed circuit boards, power 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 rings or beads are specially designed to suppress high-frequency interference and spike interference on signal lines and power lines, and 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, the use of chip beads is the best choice.
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.
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), radio frequency (RF) circuits Between interference-prone logic devices, filter high-frequency conducted interference in power supply circuits, EMI noise suppression in computers, printers, video recorders (VCRS), 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 ohm. The characteristic curve of frequency and impedance is generally provided on the DATASHEET of the magnetic bead. Generally, 100MHz is used as the standard. For example, when the frequency of 100MHz is used, the impedance of the magnetic bead is equivalent to 1000 ohms. For the frequency band we want to filter, it is necessary to select the larger the impedance of the magnetic beads, the better, usually the impedance above 600 ohms is selected.
In addition, when selecting magnetic beads, you need to pay attention to the flux of the magnetic beads. Generally, it needs to be derated by 80%. When using it in the power circuit, the influence of DC impedance on the voltage drop should be considered. The above is the working principle of magnetic bead filtering, I hope it can help you.