By the way, after the CCC certification mark in China, the EMC letter is used to indicate the EMC certification.
When we watch TV, if someone is using a hair dryer or an electric shaver, there will be annoying snowflake stripes on the screen. The rice cooker cooks the uncooked rice, and the air conditioner that has been turned off is turned off. After a while, it starts up again... These are common electromagnetic interference phenomena. More seriously, if the electromagnetic interference signal interferes with the medical electronic device that is monitoring the condition or the aircraft that is flying, it will have unimaginable consequences.
Electromagnetic compatibility (EMC) refers to the ability of a device or system to operate in its electromagnetic environment and to produce unacceptable electromagnetic interference to any device in its environment. Therefore, EMC includes two requirements: on the one hand, the electromagnetic interference generated by the equipment in the normal operation process cannot exceed a certain limit; on the other hand, the appliance has certain electromagnetic interference in the environment. The degree of immunity, ie electromagnetic sensitivity.
Electromagnetic interference is any electromagnetic phenomenon that degrades the performance of a device or system. The so-called electromagnetic interference refers to the performance degradation of equipment or systems caused by electromagnetic interference.
EMC includes EMI (electromagnetic interference) and EMS (electromagnetic tolerance). The so-called EMI electromagnetic interference is electromagnetic noise generated by the machine itself in the process of performing its functions, which is not conducive to other systems; and EMS refers to The ability of the machine to be unaffected by the surrounding electromagnetic environment while performing its intended function.
1. Basic concepts, types and test standards for electromagnetic compatibility1.1 basic concepts
Electromagnetic Compatibility Testing EMC refers to the ability of a device or system to operate in its electromagnetic environment and to produce unacceptable electromagnetic interference to any device in its environment. Since Maxwell established electromagnetic theory, Heizhi has discovered electromagnetic waves for more than 100 years, and electromagnetic energy has been fully utilized. Especially in today's scientifically developed world, the fields of broadcasting, television, communication, navigation, radar, telemetry and remote control, and computers have developed rapidly, creating enormous material wealth for human beings, especially the explosive development of information and network technology. The dialogue is getting closer. EMC testing includes test methods, measuring instruments and test sites. The test methods are based on various standards. The measuring instruments are based on the frequency domain. The test site is a prerequisite for EMC testing and an important factor in measuring EMC's working level. EMC testing is greatly affected by the site, especially for electromagnetic radiation emission, radiation receiving and radiation sensitivity testing.
1.2 Electromagnetic compatibility type
There are two general types of laboratories for EMC testing:
One is a comprehensive design and testing laboratory, or testing center, that has been certified by the EMC authority and certified by the quality system and has statutory testing qualifications.
The other type is an EMC laboratory with certain test functions based on the actual needs and funding of the unit. Primarily for pre-compliance testing and EMC evaluation, in order to have a self-test and evaluation tool before the final EMC certification.
1.3 test standard
The International Organization for Standardization has and is developing EMC standards and specifications. Although China started late in this respect, it has developed rapidly. With the development of the market economy, China must participate in the competition of the world technology market, and electronic products for import and export must pass the EMC test. In 1998, China has enacted legislation to enforce EMC testing on six types of imported electronic products and communication terminal products.
At the same time, the official EMC certification bodies and laboratories are also being validated and accepted in China. EMC testing of products is an indispensable technical means to achieve electromagnetic compatibility. Enforcing the implementation of electromagnetic compatibility standards is a prerequisite for ensuring product quality and improving market competitiveness.
2. Method of electromagnetic compatibility test2.1 Static immunity test
Static electricity is divided into contact discharge and air discharge. Static electricity is the accumulated high voltage. When it comes into contact with the metal casing of the equipment, it will discharge instantaneously, which will affect the normal operation of the electronic equipment, may cause equipment failure or restart, and have better safety requirements. This is not allowed on occasions. Static electricity can affect the display, and flashing or black screens may appear, affecting normal display and operation.
2.2 Electrical fast transient burst immunity detection
An electrical fast transient burst is a series of high frequency, high voltage transient pulses applied to the device to see if the device is affected. The main method of protection group pulse is “sparging†and “blockingâ€. The effect of closing the magnetic ring is better than that of the magnetic ring. It can also be added to the board level and fixed in the printed board, which makes the equipment more reliable. . Adding a magnetic ring to both ends of the power line, signal line, and communication line can protect the group pulse interference.
2.3 lightning surge detection
Lightning surges mainly include two aspects, one is power supply lightning protection, and the other is signal lightning protection. The lightning protection of the power supply is mainly for the system level. The system-level design should be designed according to the three-level lightning protection. The power supply is installed at the main power supply terminal to protect the power supply of the system. After the power supply is protected against lightning, the power supply enters. The isolation transformer and the isolation transformer can better protect the electromagnetic interference signal and suppress its influence on the system. Signal lightning protection is to protect the signal path of the system. It mainly involves the board level design. In the board level design, lightning protection devices, such as gas discharge tubes, are added to increase the TVS bleeder circuit. When there is a large current, the matching resistor and The TVS and gas discharge tubes are vented to protect the rear stage circuit. Then the signal is photoelectrically isolated and then enters the system. The system can collect a stable signal, so that the system can analyze and judge normally, and the command is issued normally and works normally. On the other hand, the signal range is designed to be wide, and the system works normally when the signal fluctuates normally.
2.4 Anti-interference detection of RF field induced conduction
The sensation test may affect the display signal, acquisition drive, etc., which may cause the display to flicker or black screen, affecting the operation of the device, may cause the acquisition drive to work abnormally, and may not collect the required signal, and cannot drive the field device. The RF test is to interfere with the signal line and power supply in the frequency range of 0.15k~80M, and the intensity of the third level is 10V/m. The principle of radiation protection is to shield the power supply and signal lines, and the shielding layer is well grounded. Select the appropriate frequency for filtering and filter the interference.
2.5 Radiation emission detection, RF field radiation immunity detection
This test is mainly to test the system's anti-RF signal and overall shielding performance. As long as the system is well shielded, the system ground is well grounded and the system can pass the test. Through the relevant electromagnetic compatibility test, the product can be put on the market, the test run is carried out, and the problems occurring in the trial run are summarized to prepare for the improvement of the product.
3. Electromagnetic compatibility control methodEMC technology has adopted a proactive prevention, overall planning and a combination of "confrontation" and "grooming" in the strategy of controlling interference. Among the various catastrophic hazards of conquering nature, human beings have summarized a series of strategies such as prevention and treatment, confrontation and grooming, which are also extremely effective ways of thinking in controlling electromagnetic hazards.
The well-known techniques for suppressing interference propagation can also be treated with avoidance and grooming techniques. Sometimes these avoidance and grooming techniques are simple and ingenious, and can replace the costly and mass-sized hardware measures, and get twice the result with half the effort. In the timing of solving the electromagnetic interference problem, the passive control method of salvaging and repairing measures should be adopted by the equipment development in the late stage of exposing the incompatibility problem.
Turn into predictive analysis and design at the initial stage of equipment design, pre-test calculations, and comprehensively plan implementation rules and procedures to prevent problems before they occur. The EMC design and reliability design, maintenance and maintainability design and the basic functional structure design of the product are carried out simultaneously and in parallel. EMC control technology is one of the components of modern parallel engineering. EMC control strategy and control technology solutions can be divided into the following categories:
(1) Transmission channel suppression: The specific methods include filtering, shielding, lapping, grounding, and wiring.
(2) Spatial separation: location location control, natural terrain isolation, azimuth control, electric field vector direction control.
(3) Time separation: time sharing criteria, radar pulse synchronization, active time separation, passive time
Separated.
(4) Frequency management: frequency regulation, filtering, frequency modulation, digital transmission, photoelectric conversion.
(5) Electrical isolation: transformer isolation, optical isolation, relay isolation, DC/DC conversion
4. Electromagnetic compatibility design methodThe basic principles and methods of EMC design are firstly based on the requirements and corresponding indicators of EMC for product design. Then, according to relevant EMC standards and specifications, the EMC requirements of design products are decomposed into component level, circuit level and module level. And the product-level index requirements, in accordance with the functional requirements to be achieved at all levels, design hierarchically. EMC design should consider a lot of problems, but fundamentally, how to improve the immunity of the equipment and prevent electromagnetic leakage. The usual measures, on the one hand, the equipment or the system itself should use the equipment, circuits and components with the least interference with each other and make a reasonable layout. Then through the grounding, shielding and filtering technology, the electromagnetic disturbance is suppressed and isolated. There are different design methods and measures for different equipment or systems.
(1) Component selection and circuit analysis are the basis of EMC design.
(2) EMC design of the power system.
(3) Anti-interference design of the grounding system.
(4) EMC design of printed circuit boards. In addition, it is worth noting that the protection against electrostatic discharge must not be ignored when designing EMC. The key to ESD protection is to prevent the generation and accumulation of electrostatic nuclei, and then to block the occurrence of ESD effects.
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