EMC common components introduction
Common Mode Inductance
Since most of the problems faced by EMC are common mode interference, common mode inductors are also one of our commonly used powerful components! Here is a brief introduction to the principle and usage of common mode inductors.
The common mode inductor is a common mode interference suppression device with ferrite as the core. It consists of two coils with the same size and the same number of turns symmetrically wound on the same ferrite ring core to form a four-terminal Devices that exhibit large inductance for common-mode signals have a suppressive effect, but exhibit small leakage inductance for differential-mode signals that have little effect. The principle is that the magnetic flux in the magnetic ring is superimposed on each other when the common mode current flows, so that it has a considerable inductance, which inhibits the common mode current, and when the two coils flow through the differential mode current, the magnetic flux in the magnetic ring Through canceling each other, there is almost no inductance, so the differential mode current can pass without attenuation. Therefore, common-mode inductors can effectively suppress common-mode interference signals in a balanced line, but have no effect on differential-mode signals normally transmitted by the line. Common mode inductors should meet the following requirements during production:
1) The wires wound on the coil core should be insulated from each other to ensure that no breakdown and short circuit occurs between the turns of the coil under the action of instantaneous overvoltage.
2) When the coil flows through the instantaneous large current, the magnetic core should not be saturated.
3) The magnetic core in the coil should be insulated from the coil to prevent breakdown between the two under the action of instantaneous overvoltage.
4) The coil should be wound in a single layer as much as possible, which can reduce the parasitic capacitance of the coil and enhance the ability of the coil to transmit instantaneous overvoltage.
Usually, pay attention to the selection of the frequency band to be filtered at the same time. The larger the common-mode impedance, the better. Therefore, we need to look at the device data when selecting a common-mode inductor, mainly based on the impedance-frequency curve. In addition, when choosing, pay attention to the influence of differential mode impedance on the signal, mainly focus on differential mode impedance, and pay special attention to high-speed ports.
magnetic beads
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 them? 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 applied at high frequencies, because at low frequencies they are mainly inductive, making the loss on 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. In the low frequency band, the impedance is composed of the inductance 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 electromagnetic interference is reflected and suppressed; and at this time The loss of the core is small, and the whole device is a low-loss, high-Q characteristic inductance. This inductance is easy to cause resonance. Therefore, in the low frequency band, sometimes the phenomenon of interference enhancement after using ferrite beads may appear. In the high-frequency band, 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. , leading to an increase in the total impedance, 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 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. And when it is necessary to eliminate unwanted EMI noise, chip beads are the best choice. Applications of chip beads and chip inductors: 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, 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, filtering 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 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 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.
filter capacitor
Although undesirable from the standpoint of filtering out high-frequency noise, capacitor resonance is not always detrimental. When the noise frequency to be filtered out is determined, the capacity of the capacitor can be adjusted so that the resonance point just falls on the disturbance frequency.
In actual engineering, the frequency of electromagnetic noise to be filtered is often as high as hundreds of MHz, or even exceeds 1GHz. Feedthrough capacitors must be used to effectively filter out such high-frequency electromagnetic noise. The reason why ordinary capacitors cannot effectively filter out high-frequency noise is because of two reasons. One reason is that the capacitor lead inductance causes capacitor 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, reducing the filtering effect.
The reason why feedthrough capacitors can effectively filter out high-frequency noise is because feedthrough capacitors not only do not have the problem of low resonance frequency caused by lead inductance, but also can be directly installed on metal panels, using metal panels to play a high The role of frequency isolation. However, when using feedthrough capacitors, the problem to be paid attention to is the installation problem. The biggest weakness of the feedthrough capacitor is the fear of high temperature and temperature shock, which causes great difficulties when soldering the feedthrough capacitor to the metal panel. Many capacitors are damaged during soldering. Especially when a large number of feedthrough capacitors need to be installed on the panel, as long as one is damaged, it is difficult to repair, because when the damaged capacitor is removed, it will cause damage to other adjacent capacitors. With the increasing complexity of electronic equipment, there are more and more installations of mixed strong and weak currents and digital logic circuits inside the equipment, and the mutual disturbance between circuit modules has become a serious problem. One of the methods to solve the mutual disturbance of such circuit modules is to isolate circuits of different natures with metal isolation compartments. However, all wires passing through the isolation compartment must pass through the feedthrough capacitor, otherwise it will cause isolation failure. When there are a large number of connections between different circuit modules, it is very difficult to install a large number of feedthrough capacitors on the isolation compartment. In order to solve this problem, many foreign manufacturers have developed a "filter array board", which is a device composed of a feedthrough capacitor welded to a metal plate in advance by a special process. Using a filter array board can easily solve the problem of a large number of wires passing through the metal panel. question. However, the price of this filter array board is often higher, and the price per pin is about 30 yuan.
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