Filter capacitors, common-mode inductors, and magnetic beads are common in EMC design circuits and are also the three most powerful tools for eliminating electromagnetic interference. For the role of these three in the circuit, I believe there are many engineers who are not clear. This article from the design and design, detailed analysis of the principle of eliminating EMC's three major weapon.
Three large filter capacitor
Although the resonance of the capacitor is undesirable from the viewpoint of filtering out high-frequency noise, the resonance of the capacitor is not always harmful. When the frequency of the noise to be filtered is determined, the capacity of the capacitor can be adjusted so that the resonance point just falls on the disturbance frequency.
In practical projects, the frequency of the electromagnetic noise to be filtered out is often as high as several hundred MHz or even more than 1 GHz. For such high-frequency electromagnetic noise, a penetrating capacitor must be used to effectively filter out. The reason why ordinary capacitors can not effectively filter out high-frequency noise is due to two reasons. One reason is that capacitor lead inductance causes capacitance resonance, which presents a large impedance to high-frequency signals and weakens the bypass effect on high-frequency signals. Another reason is that the parasitic capacitance between the wires couples high-frequency signals and reduces the filtering effect.
The reason why the lead-through capacitor can effectively filter out the high-frequency noise is that the lead-through capacitor has not only the problem that the inductance of the capacitor is too low due to the lead inductance, but also the lead-through capacitor can be directly mounted on the metal panel and use the metal panel to achieve high performance. Frequency isolation effect. However, when using lead-through capacitors, the problem to be noted is the installation problem. The biggest weakness of the penetrating capacitor is the fear of high temperature and temperature shocks, which causes great difficulties in soldering the feedthrough capacitor to the metal panel. Many capacitors are damaged during soldering. In particular, when a large number of feedthrough capacitors need to be mounted on the panel, it is difficult to repair them as long as one of them is damaged, because when the damaged capacitor is removed, it may cause damages adjacent to other capacitors.
Three major weapon of common mode inductance
Because common EMC problems are common mode interferences, common mode inductors are also one of our common and powerful components. A common mode inductor is a common mode interference suppression device with a ferrite core. It consists of two sizes. A coil with the same number of turns is wound symmetrically on the same ferrite toroidal core to form a four-terminal device, which has a suppressing effect on the large-amplitude inductance of the common-mode signal and exhibits a small signal for the differential-mode signal. Leakage inductance hardly works. The principle is that the magnetic flux in the magnetic ring is superimposed on each other when flowing through the common-mode current, so that it has a considerable inductance and suppresses the common-mode current, and when the two coils flow through the differential-mode current, the magnetic flux in the magnetic ring The mutual cancellation, almost no inductance, so the differential mode current can pass without attenuation. Therefore, common-mode inductors can effectively suppress common-mode interference signals in balanced circuits, but have no effect on differential-mode signals that are normally transmitted on the line.
Common mode inductors should meet the following requirements when they are manufactured:
1) The wires wound on the core of the coil should be insulated from each other to ensure that no short circuit occurs between the turns of the coil under transient overvoltage. 2) The core should not saturate when a large current flows through the coil. 3) The magnetic core in the coil should be insulated from the coil to prevent breakdown between the two under transient overvoltage. 4) The coil should be wound as a single layer as much as possible. This will reduce the parasitic capacitance of the coil and enhance the ability of the coil to transient overvoltage.
Under normal circumstances, at the same time pay attention to select the desired filter frequency band, the greater the common-mode impedance, the better, so we need to look at the device data when selecting the common-mode inductance, mainly based on impedance frequency curve selection. In addition, attention should be paid to the influence of differential mode impedance on the signal when selecting, and the main focus is on differential impedance, with particular attention to high-speed ports.
Three magnetic weapon beads
In the EMC design process of product digital circuits, we often use magnetic beads. The ferrite material is iron-magnesium alloy or iron-nickel alloy. This kind of material has high magnetic permeability. It can be the inductance between the coil windings. High-frequency high resistance produces the smallest capacitance. Ferrite materials are often used at high frequencies, because their main inductance characteristics at low frequencies make the losses on the wire small. At high frequencies, they are mainly exhibited as a reactance characteristic and change with frequency. In practical applications, ferrite materials are used as high-frequency attenuators for RF circuits. Actually, the ferrite is preferably equivalent to a parallel connection of a resistor and an inductor. At a low frequency, the resistor is short-circuited by the inductor. At high frequencies, the impedance of the inductor becomes so high that the current passes through the resistor. Ferrite is a consumable device where 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. Ferrites exhibit resistance at high frequencies and correspond to inductors with very low quality factors, so they can maintain high impedance over a wide range of frequencies, thereby improving high-frequency filtering performance. In the low frequency range, the impedance is formed by the inductive reactance of the inductor. At low frequencies, R is small and the magnetic permeability of the magnetic core is high. Therefore, the inductance is large, L plays a major role, and the electromagnetic interference is reflected and suppressed; and The loss of the core is small, and the whole device is a low-loss, high-Q characteristic inductor. This kind of inductance is likely to cause resonance. Therefore, in the low frequency range, the phenomenon that interference is enhanced after using ferrite beads may sometimes occur. In the high frequency range, 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 inductor 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 an increase in the total impedance. When the high-frequency signal passes through the ferrite, electromagnetic interference is absorbed and converted into heat energy in the form of dissipation.
Ferrite suppression components are widely used in printed circuit boards, power lines, and data lines. If a ferrite suppression element is added to the inlet end of the printed board's power supply line, high-frequency interference can be filtered out. The ferrite beads or beads are designed to suppress high-frequency interference and spike interference on signal lines and power lines. It also has the ability to absorb ESD pulse interference.
The use of chip beads or chip inductors is also mainly in practical applications. Chip inductors are needed in the resonant circuit. When it is necessary to eliminate unwanted EMI noise, the use of chip beads is the best choice. Chip Beads and Chip Inductors 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 system and low voltage power supply module. Chip beads: clock generation circuit, filtering between analog and digital circuits, I/O input/output internal connectors (such as serial, parallel, keyboard, mouse, long-distance telecommunications, local area network), radio frequency (RF) circuits Between EMI-prone and disturbance-prone logic devices, high-frequency conducted noise is filtered out of the power supply circuit, and EMI noise is suppressed in computers, printers, video recorders (VCRS), television systems, and mobile phones.
The unit of magnetic beads is ohms, because the unit of magnetic beads is nominal according to its impedance at a certain frequency, and the unit of impedance is also ohms. Magnetic beads DATASHEET will generally provide frequency and impedance characteristics of the graph, generally 100MHz as a standard, such as the frequency of 100MHz frequency when the magnetic impedance is equivalent to 1000 ohms. For the frequency band that we want to filter, we need to select the impedance of the bead as large as possible. Usually, the impedance above 600 ohms is selected.
In addition, when selecting the magnetic beads, it is necessary to pay attention to the flux of the magnetic beads, and it is generally required to derate 80% of the treatment. When the power supply circuit is used, it is necessary to consider the influence of the DC impedance on the voltage drop.
Bee Eyes Beam Light ,Mini Bee Eye Moving Head,Bee Eye Led Moving Head,Bee Eyes Beam Lights
Guangzhou Cheng Wen Photoelectric Technology Co., Ltd. , https://www.cwledwall.com