A comprehensive overview of electromagnetic interference suppression technology

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A comprehensive overview of electromagnetic interference suppression technology [Copy link]

People may still feel unfamiliar with the term electromagnetic interference (EMI), but the impact of EMI is something that almost everyone has experienced. For example, when watching TV, if someone nearby uses an electric drill, hair dryer or other electrical appliances, snowflakes will appear on the TV screen, and harsh noises will be emitted from the speaker... People have long been accustomed to such phenomena, but the harm of electromagnetic interference is far more than that. In fact, electromagnetic interference has caused the failure of civil aviation systems, poor communications, computer operation errors, malfunctions of automatic control equipment, and even endangered personal safety. Therefore, it has become a top priority to strengthen the popularization of electromagnetic compatibility (EMC) knowledge and improve EMI suppression technology. The
　　so-called electromagnetic compatibility refers to the state in which electronic circuits and systems do not affect each other and are compatible with each other in terms of electromagnetics. The research on EMC technology began in foreign countries in the 1930s. Some countries and international organizations such as the Federal Communications Commission (FCC) of the United States, the German Association of Electrical and Electronics Engineers (VDE), and the International Special Committee on Radio Interference (CISPR) have successively formulated some guiding documents and regulations. At present, a relatively complete system has been formed and strictly followed. The American computer industry has fully implemented the FCC regulations. China's electromagnetic compatibility work started late, and relevant standards have only been issued since the 1980s. The application has been difficult due to lack of experience and technology. How to catch up with the international advanced level as soon as possible so that China's electronic products can meet the increasingly urgent domestic demand and occupy a place in the international market has become a major issue of concern to everyone. This article would like to talk about some superficial views on electromagnetic interference suppression technology, throw out a brick to attract jade, and discuss with you together.
　　The definition of electromagnetic interference refers to the disturbance caused by external noise and useless electromagnetic waves in reception. The degree of electromagnetic interference of a system or a certain line in the system can be expressed as the following relationship:
　　N=G&TImes;C/I
　　G: noise source intensity;
　　C: coupling factor of noise transmitted to the interfered part through a certain path;
　　I: sensitivity of the interfered circuit.
　　G, C, and I constitute the three elements of electromagnetic interference. Electromagnetic interference suppression technology is a variety of measures taken around these three elements, which can be summarized into three points: 1. Suppress the source of electromagnetic interference; 2. Cut off the electromagnetic interference coupling path; 3. Reduce the sensitivity of electromagnetic sensitive devices. The following is an introduction to these three aspects.
　　1. Suppress interference sources
　　To remove interference sources, we must first determine where the interference sources are. The closer to the interference sources, the better the suppression effect. Generally speaking, the places where the current and voltage change dramatically, that is, the places where di/dt or du/dt is large, are interference sources; specifically, relay opening and closing, capacitor charging, motor operation, integrated circuit switch operation, etc. may become interference sources. In addition, the mains power supply is not an ideal 50Hz sine wave, but is full of various frequency noises, which is an interference source that cannot be ignored. The suppression method can use low-noise circuits, transient suppression circuits, rotating device suppression circuits, voltage stabilization circuits, etc.; the selection of devices should use low-noise, high-frequency characteristics, and high-stability electronic components as much as possible. It should be noted that inappropriate device selection in the suppression circuit may generate new interference sources.
　　2. Cut off the electromagnetic interference coupling path
　　The electromagnetic interference coupling path is mainly conducted and radiated. The most common noise is directly coupled to the circuit through the wire. The main measure to suppress conducted interference is to connect filters in series. There are four types of filters: low pass (LPF), high pass (HPE), band pass (BPF), and band stop (BEF). Different types of filters are selected according to the difference between the signal and noise frequencies. If the noise frequency is much higher than the signal frequency, an LC low pass filter is often used. This filter has a simple structure and a good noise filtering effect. However, for military or TEMPEST technology and civilian products with higher requirements, a feed-thru filter must be used.
　　Feed-thru filters are also called through filters. The circuit structures include C-type, T-type and LC-type. They are characterized by excellent high-frequency characteristics and can work above 1GHz. This is determined by its "coaxial" nature. Since it has no parasitic inductance, the self-resonant frequency is increased. The feed-thru filter is small in size, light in weight, and allows a large current. It can be widely used in various occasions.
　　The noise conducted through the power supply line can be filtered out with a power supply filter. The power supply filter that only meets the VDE0871 standard has an insertion loss of 20-100dB in the range of 30K-30MHZ. The power filter can be connected not only to the input of the power grid, but also to the output of the noise source circuit to suppress the noise output, and it can be used for both AC and DC. The power filter port is divided into high-resistance and low-resistance ends. The correct connection method should be selected according to the different input and load impedances. The principle of connection is based on the most mismatched impedance, that is, the high-resistance input end is connected to the filter impedance end, and the low-resistance load end is connected to the filter high-resistance end; vice versa.
　　For the wiring design of transmission lines and printed circuit boards, attention should be paid to separating the incoming and outgoing lines, signal lines and power lines as much as possible. For key lines, lossy line filters, three-terminal capacitors, magnetic rings and other devices can be used for interference suppression. For the interface end, there are D-type, circular, and square connector products with filtering abroad. This type of connector is to add capacitors or inductors to ordinary connectors to form a filtering circuit. Its characteristics are that it does not occupy PCB space and does not increase the volume, which is extremely important for the high-density design of modern components. Recently, domestic manufacturers have also produced it, and the quality is not lower than that of foreign countries, which can replace imports.
　　For radiated interference, the main measures are to use shielding technology and layering technology. Shielding technology is a science. Choosing appropriate shielding materials and shielding in appropriate locations are crucial to the shielding effect. Especially the design of the shielding room. There are many types of shielding materials to choose from, including various metal plates, finger-shaped beryllium copper alloy reeds, copper wire mesh, braided copper tape, conductive rubber, conductive glue, conductive glass, etc. They should be selected according to needs. The design of the shielding room should fully consider the shielding and overlap of doors, windows, vents, and inlet and outlet ports. In addition to electrostatic shielding, magnetic shielding and grounding and earthing technology should also be considered.
　　3. Reduce the sensitivity of electromagnetic sensitive devices
　　The sensitivity of electromagnetic sensitive devices is a double-edged sword; on the one hand, people hope that the receiving device has high sensitivity to improve the ability to receive signals; on the other hand, the higher the sensitivity, the greater the possibility of being affected by noise. Therefore, according to the specific situation, the use of derating design, avoidance design, network passivation, function passivation and other methods is the solution to the problem.
　　In summary, there are many methods for suppressing electromagnetic interference, and one or more comprehensive methods can be selected. But no matter what method is chosen, the electromagnetic compatibility of the system should be considered from the beginning of the design, rather than making up for it after the sheep have gone. It is reported that if electromagnetic compatibility design is carried out at the beginning of product development, about 90% of conducted and radiated interference can be controlled. According to reliability, safety, quality requirements, environmental control, and cost-effectiveness, appropriate electromagnetic interference suppression technology is selected, which is the research content of electromagnetic compatibility.