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Modern electronic devices are operating in complex electromagnetic environments. In view of the electromagnetic interference often lead to the electronic equipment failure or even the safety accident, discusses the electromagnetic compatibility design of the electronic system. In this paper, the electromagnetic interference source is analyzed, the design concept of electromagnetic compatibility is discussed, the design mechanism of anti-electromagnetic interference is studied, and the technical measures against electromagnetic interference are put forward in view of the frequent failures of electronic equipment. Taking the specific technical measures adopted by a certain control device for EMC design as an example, the good effect of anti-electromagnetic interference is verified, and the safety and reliability of the control device are remarkably improved. Engineering practice shows that the most important technical measures against electromagnetic interference is the system of good grounding and shielding, and reasonable wiring.
With the rapid development of microelectronic technology, the application of electronic equipment is more and more widespread, and the integration of electronic systems is getting higher and higher. However, under complicated electromagnetic environment, electronic systems have obvious sensitivity and vulnerability to electromagnetic interference. In order to reduce the breakdown and eliminate the accident, must design the electromagnetic compatibility of the electronic equipment. As long as the electrical and electronic equipment will produce electromagnetic fields, bio-magnetic, magnetoelectric, so the electromagnetic environment is very complex, on the one hand requires the use of electronic equipment on the surrounding electromagnetic environment does not cause pollution, on the other hand also requires the electronic equipment In the real environment of the electromagnetic environment does not decline performance or failure resulting in serious accidents. Therefore, we must study the electromagnetic compatibility of electronic equipment, electromagnetic interference caused by the control and protection. Based on the importance of EMC design, the following issues related to some discussion.
A common phenomenon of electromagnetic interference and its analysis
Electromagnetic and its induction phenomenon is ubiquitous, so the electromagnetic working environment of the electronic system is very complicated. From engineering application point of view, electromagnetic interference can be classified according to different working frequency. For example, there are common low-frequency conducted disturbances caused by harmonic signal voltage fluctuations, grid frequency changes and low-frequency induced voltages, grid voltage imbalances, short-term fluctuations in power supply fluctuations and short-term interruptions, and low frequency radiation interference between magnetic fields and electric fields. Due to the induced continuous wave voltage and current oscillation and unidirectional transient caused by high-frequency conducted interference, electromagnetic fields (continuous wave, transient) and magnetic field, electric field caused by high-frequency radiation interference; due to the insulation properties of materials caused by electrostatic discharge Interference and so on. The above mentioned interferences include the vast majority of electromagnetic interference phenomena in engineering applications.
In the anti-interference performance analysis of electronic systems, we must make a comprehensive analysis of the inherent characteristics of the system and its application environment. Possible types of electromagnetic interference in electronic circuit systems are, for example, circuit-based mutual interference due to the existence of common impedance coupling of the circuit loops; due to the presence of a varying electric field between the source of interference and the interfering object, capacitive coupling may occur Capacitive interference, as it will produce interference voltage; Spatial electromagnetic waves of electricity, magnetic field strength changes may lead to induced potential induced conductive current and conducted voltage interference; alternating magnetic field interference source, the current may cause changes in inductive components Induced by the voltage, resulting in inductive interference and so on.
2 electromagnetic compatibility and design mechanism
2.1 Electromagnetic compatibility
With the widespread application of electrical and electronic equipment in modern production, the equipment connection is more and more complex, the power is more and more large, the number is rapidly increasing, the equipment requirements are also getting higher and higher, the frequency band is widening, the equipment sensitivity is higher, As a result, the issue of electromagnetic compatibility becomes even more important. Electromagnetic Compatibility (EMC) implies that an electronic system in an electromagnetic environment is such that it is impossible for any other things to constitute an unacceptable electromagnetic interference capability and that the equipment or system can function normally. Electromagnetic compatibility technology is involved in all aspects of communications, computing, electronics, manufacturing, military and life. It is a rapidly growing interdisciplinary discipline. Electromagnetic compatibility is a science that studies how different devices can co-exist without affecting each other under limited space, limited time and limited spectrum resources. It can be known from the above definition of electromagnetic compatibility that the meanings of electromagnetic compatibility include: the device does not produce unacceptable interference to other devices around itself, and is not affected by the interference of other devices. Electromagnetic compatibility research involves many aspects, first of all is the study of the characteristics of electromagnetic interference sources; Secondly, electromagnetic emission intensity, interference mechanism and electromagnetic interference suppression methods and electromagnetic interference in the time-frequency domain characteristics of research, third, especially It is noteworthy that the device itself anti-electromagnetic interference performance; Finally, how to evaluate electromagnetic radiation and conduction characteristics of electromagnetic compatibility, what equipment and measurement methods used to measure electromagnetic interference, how to deal with the measurement data and measurement results. From a wider range of considerations, it also covers the electromagnetic compatibility within the system and between systems. Electromagnetic compatibility studies include natural and man-made sources of electromagnetic interference, such as lightning and electrostatic discharge, which are sources of natural electromagnetic interference. Measurements of interference sources include measurements of open space, radiation, conduction and pulsed interference (electrical surges, fast transient pulses Group and Electrostatic Discharge), shielding, grounding, bonding and filtering, etc., in technologies that achieve electromagnetic compatibility, but also include the use of special design techniques to suppress electromagnetic interference.
2.2 anti-electromagnetic interference design mechanism
To constitute electromagnetic interference must have three conditions at the same time: First, there must be sources of interference exists, there is no source of interference, it is clearly impossible to produce electromagnetic interference to the equipment; Second, the existence of electromagnetic interference transmission channels exist, or impossible to form pairs Equipment, electromagnetic interference; Third, the equipment to be able to accept the interference signal and directly affect the normal operation of the equipment, so even if the equipment is subjected to interference, if you take technical measures to eliminate its interference on the equipment. For example, a robust device can not be affected by electromagnetic interference. Anti-electromagnetic interference design mechanism is to take the electromagnetic compatibility design, so that the above three conditions are not available at the same time, in order to achieve the purpose of improving equipment against electromagnetic interference.
Taking the anti-jamming design of electronic equipment as an example, the high-frequency interference in the equipment is particularly prominent. The first is to control the radio frequency energy emitted by the equipment so as to be as small as possible so as not to interfere with other equipment. Secondly, in order to prevent the equipment from external interference, Reduce the RF energy entering the device. Electromagnetic interference can be achieved by means of radiation or conduction transmission. If the interference source energy is directly radiated to the control line, the signal line and the power supply line can directly interfere with the equipment through the coupling path of the common signal, the control cable or the common power line after entering the equipment normal work. Therefore, the device port or sensitive loop, using common mode or differential mode anti-interference measures to minimize its impact on reducing the radiation and conduction energy, improve the anti-jamming performance of the device, the anti-electromagnetic interference design mechanism is to eliminate At the same time to meet the above three prerequisites [3-4]. Based on this, its anti-electromagnetic interference technical measures can be designed in various ways. As the technology advances, the technical measures that can be taken will be more and more abundant to ensure the electromagnetic compatibility of equipment design.
3 anti-jamming design strategy
Anti-electromagnetic interference design is in complex electromagnetic spectrum environment, the use of integrated technical measures to ensure the correct performance of electronic equipment. In accordance with the anti-electromagnetic interference design mechanism, the first is to suppress the source of interference to prevent electromagnetic interference; followed by anti-electromagnetic interference measures to block the transmission of interference; Finally, to reduce the sensitivity of electronic devices to interference, or improve the electronic device Excellent performance, to prevent and suppress electromagnetic interference. For the electronic system cable connector, PCB layout, signal layout, interference suppression wiring, components, filters, grounding and bypass and other aspects of electromagnetic interference signals may be introduced, the use of isolation, circuit impedance control, filtering, solution Coupling, sealing, grounding, shielding, wiring and other anti-electromagnetic interference measures.
3.1 PCB version of the rational layout and wiring design
In the circuit layout, power supply, analog and digital circuit components layout and wiring are different, the layout of the components should be placed separately, should be high and low frequency circuits separate as much as possible their respective isolation, pay attention to the direction of signal transmission, Ways and strong and weak signal device distribution do not interfere with each other. For circuits that easily cause noise interference, such as clock generators, input terminals such as crystal oscillator and CPU clock, etc., should be as close as possible to each other in order to reasonably layout the entire PCB and reduce the sources of interference. Strong current and weak current circuits and noise-prone devices should be as far as possible from the logic circuit. Minimize the mutual coupling of unwanted signals in signal path and circuit element layout. To avoid simulations, digital circuits generate common impedance coupling, separating low-level analog and digital circuits and away from unfiltered power supplies and high-level signal lines. On PCB layout, different high, medium and low speed Logic circuits are laid out in different areas, to ensure that the same layer adjacent to the wiring, the same board adjacent layers, parallel lines between adjacent boards as long as possible the minimum length; EMI filter placed on the same circuit board and as close to the EMI source ; Rectifier, DC / DC converter and switching elements and transformers should be placed as close as possible to shorten the length of the wire. Filter capacitors, voltage regulator components and rectifier diodes should also be placed as close as possible to reduce the external interference; noise and non-noise components as far as possible, the printed circuit board according to the current switching characteristics and frequency partition to eliminate large current, High-speed switching lines and noise-sensitive wiring parallel to each other.
PCB layout in the circuit, in order to improve electromagnetic compatibility, the following wiring strategies can be taken: In order to avoid the concentration of electric field coupled to the stronger noisy adjacent path, the path at the corner 45 ° to avoid the use of straight-line routing; And the sensitive signal path does not use the stub to avoid oscillation in the stub; keep the path from drive to load the same width, in order to avoid reflections lead to unbalanced line impedance; in multiple PCB ground connection , In order to avoid the stub signal path, it is necessary to eliminate the use of tree-type high-speed and sensitive signal lines, also eliminate the radial arrangement of high-speed and sensitive signal lines to avoid reflection and radiation interference; dense power and ground Through-hole will lead to increased power supply impedance, the power supply at this point to form a high impedance, affecting the RF current transmission, and therefore should avoid vias density is too large; all copper directly connected to the ground to prevent the copper area into a radiating antenna; Commonly used routing strategy, the other routing strategy is not discussed here.
3.2 grounding system design
Grounding system design is complex, many factors to consider. Electromagnetic shielding facilitates the mutual isolation of electromagnetic interference. In electronic devices, such as the use of shielding and grounding, the vast majority of electromagnetic interference problems in electronic equipment can be solved. In order to minimize the grounding impedance of the grounding system, grounding system design can use the following technical measures.
① ground point selection. Low-frequency circuit inductance less affected, in order to avoid multi-point ground loop interference, in the work below 1MHz frequency, should be used to single-point ground. Inductance in high-frequency circuit greater impact on work in the 10MHz frequency above, you can use more near the ground, the ground should be short and thick, in order to reduce the ground impedance.
② Digital circuits and analog circuits must be strictly separate ground, and were connected to the power supply ground, both ground can not be mixed, in addition, we should also try to increase the analog circuit ground area to reduce the ground impedance.
③ Since the conductor inductance is proportional to the conductor length and inversely proportional to the diameter, the ground wire should be as short and thick as possible to allow three times the current allowed on the printed wiring board to improve noise immunity.
④ digital circuit grounding wire should form a closed loop, to avoid large power consumption increase the potential difference, in order to improve PCB noise immunity.
⑤ In order to reduce the grounding resistance, one of the multi-layer circuit board as a ground plane and play a shielding effect, generally around the printed circuit board for the ground.
⑥ There is capacitance between the power board plane and the insulating thin layer on the ground plane, placing it in adjacent layers to form a decoupling capacitor improves the high-frequency response.
⑦ low-speed circuits and components should be placed and placed as close as possible to the power plane, and the distribution and placement of high-speed circuits and components should be as close as possible to the ground plane.
⑧ more power supply, the power should be separately grounded.
Electronic equipment grounding system structure is complex, there are a variety of grounding methods, such as digital systems (logic) and analog system ground, the chassis ground (shielding ground) and the system ground, grounding technology in both multi-layer and single-layer PCB board Widely used, the goal is to minimize the impedance of the ground, reducing the adverse effects of the ground loop potential.
With the rapid development of microelectronic technology, electronic equipment replacement faster and faster, electromagnetic compatibility design becomes more important. However, the successful experience in the design of electronic equipment shows that if the shielding and grounding measures are used in combination, external electromagnetic interference can be suppressed and the vast majority of electromagnetic interference problems in electronic equipment can be solved.