Electronic circuit designers often only consider the function of the product, and do not have functional and electromagnetic compatibility (ie, EMC) means that the device or system is operating in its electromagnetic environment and does not tolerate any electromagnetic interference in any device in its environment. The ability to comprehensively consider, so the product also produces a large number of functional harassment and other harassment while completing its functions. Moreover, the sensitivity requirements cannot be met. The electromagnetic compatibility design of electronic circuits should be considered from several aspects. Here we mainly study the selection of components.
1, common mode inductance
Since the problems faced by EMC are mostly common mode interference, common mode inductors are also one of the most powerful components we use. Here is a brief introduction to the principle and use of common mode inductors.
The common mode inductor is a common mode interference suppression device with ferrite as the core. It is symmetrically wound on the same ferrite toroidal core by two coils of the same size and the same number of turns to form a four-terminal. The device, which exhibits a large inductance for the common mode signal, has little effect on the differential mode signal exhibiting a small leakage inductance. The principle is that the magnetic fluxes in the magnetic ring are superimposed on each other when the common mode current flows, so that the inductance is relatively large, and the common mode current is suppressed; and when the two coils flow through the differential mode current, the magnetic in the magnetic ring By canceling each other, there is almost no inductance, so the differential mode current can pass without attenuation. Therefore, the common mode inductor can effectively suppress the common mode interference signal in the balanced line, and has no effect on the differential mode signal that the line normally transmits.
The common mode inductor should meet the following requirements when it is manufactured:
(1) The wires wound on the coil core should be insulated from each other to ensure that no breakdown short circuit occurs between the turns of the coil under the action of instantaneous overvoltage;
(2) When the coil flows through a large instantaneous current, the core does not appear saturated;
(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 much as possible, which can reduce the parasitic capacitance of the coil and enhance the coil's ability to withstand transient overvoltage.
Under normal circumstances, at the same time pay attention to select the frequency band required for filtering, the larger the common mode impedance, the better. Therefore, we need to look at the device data when selecting the common mode inductor, mainly based on the impedance frequency curve. In addition, pay attention to the influence of differential mode impedance on the signal, mainly focusing on differential mode impedance, paying special attention to high-speed ports.
2, magnetic beads
In the product digital circuit EMC design process, we often use magnetic beads, then the principle of magnetic bead filtering and how to use it?
The ferrite material is a ferro-magnesium alloy or an iron-nickel alloy. This material has a high magnetic permeability, which minimizes the capacitance generated between the coil windings of the inductor at high frequency and high resistance.
Ferrite materials are typically used at high frequencies because they are primarily inductive at low frequencies, making the losses on the line small. At high frequencies, they are primarily reactive and change with frequency. In practical applications, ferrite materials are used as high frequency attenuators for RF circuits. In fact, the ferrite is preferably equivalent to the parallel connection of the resistor and the inductor. The resistor at the low frequency is short-circuited by the inductor, and the impedance of the inductor at a high frequency becomes so high that the current passes through the resistor. Ferrite is a consumer device in which high-frequency energy is converted into thermal energy, which is determined by its resistance characteristics.
Ferrite beads have better high frequency filtering characteristics than ordinary inductors. Ferrite exhibits electrical resistance at high frequencies, which is equivalent to an inductor with a low quality factor, so it can maintain a high impedance over a relatively wide frequency range, thereby improving high-frequency filtering performance. In the low frequency band, the impedance is composed of the inductive reactance of the inductor. When the low frequency is small, 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 then the magnetic The loss of the core is small, and the whole device is a low-loss, high-Q inductor. This kind of inductance is easy to cause resonance. Therefore, in the low frequency band, the interference enhancement after the use of the ferrite bead may sometimes occur. In the high frequency band, the impedance is composed of a resistance component. 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 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, the electromagnetic interference is absorbed and converted into heat energy to be dissipated.
Ferrite suppression components are widely used in printed circuit boards, power lines, and data lines. High frequency interference can be filtered out by adding a ferrite suppression element to the input end of the power line of the printed board. Ferrite magnetic rings or magnetic beads are designed to suppress high-frequency interference and spike interference on signal lines and power lines. They also have the ability to absorb electrostatic discharge pulse interference.
The use of chip beads or chip inductors is mainly in practical applications. A chip inductor is required in the resonant circuit. The use of chip beads is the best choice when eliminating unwanted EMI noise. Application 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. Chip bead: clock generation circuit, filtering between analog circuit and digital circuit, I/O input/output internal connector (such as serial port, parallel port, keyboard, mouse, long distance telecommunication, local area network), radio frequency (RF) circuit Between high-frequency conducted interference in the power supply circuit and EMI noise suppression in computers, printers, video recorders (VCRS), television systems, and mobile phones, and interference-prone logic devices.
3, 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 noise frequency to be filtered is determined, the capacitance can be adjusted to make the resonance point just fall on the disturbance frequency.
In actual engineering, the frequency of electromagnetic noise to be filtered is often as high as several hundred MHz or even more than 1 GHz. For such high frequency electromagnetic noise, a feedthrough capacitor must be used to effectively filter out.
The reason why ordinary capacitors cannot effectively filter out high-frequency noise is because of two reasons: one is that the capacitor lead inductance causes capacitance resonance, and the high-frequency signal exhibits a large impedance, which weakens the bypass effect on the high-frequency signal; Another reason is that the parasitic capacitance between the wires couples the high frequency signals, reducing the filtering effect.
The reason why the core capacitor can effectively filter out the high frequency noise is because the core capacitor not only has no problem that the capacitor resonance frequency is too low, but the core capacitor can be directly mounted on the metal panel, and the metal panel is used to be high. The role of frequency isolation. However, when using a feedthrough capacitor, the problem to be aware of is the installation problem. The biggest weakness of the feedthrough capacitor is the fear of high temperature and temperature shock, which causes great difficulty in soldering the feedthrough capacitor to the metal panel. Many capacitors are damaged during the soldering process. Especially when a large number of feedthrough capacitors need to be mounted on the panel, as long as there is a damage, it is difficult to repair, because when the damaged capacitor is removed, damage to other capacitors is caused.
With the increase of the complexity of electronic equipment, the internal and external hybrids of the equipment are mixed and installed, and digital logic circuits are mixed and installed. The mutual disturbance between circuit modules becomes a serious problem. One of the ways to solve the mutual disturbance of such circuit modules is to isolate circuits of different nature by using metal isolation compartments. However, all wires that pass through the cofferdam pass through the feedthrough capacitor, otherwise isolation failure will occur. When there are a large number of wires connected 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 that uses a special process to solder the feedthrough capacitor to a metal plate in advance. The filter array board can easily solve a large number of wires passing through the metal panel. problem. However, the price of such filter array boards tends to be high.
There are several types of EMC devices, the strength and type of noise are different, and the applicable devices are different. For example, for low-frequency noise, such as noise with a frequency of tens of KHz, it is necessary to select a capacitor or an inductor instead of a magnetic bead; the magnetic bead is mainly used to filter out noise on a general power line or signal line, and is suitable for high application. Frequency noise environment, such as the frequency is tens of MHz to several GHz. However, the magnetic beads have a poor effect on filtering out the noise of the differential signal lines. At this time, the common mode choke coil should be selected. The common mode choke coil is specially used to filter out the noise on the differential signal line. Ordinary EMC devices are not capable of filtering common mode noise on differential signal lines. Therefore, it is important to select the appropriate EMC device based on the specific noise type and frequency range.
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