How to carry out EMC design of AC regulated power supply
EMC performance is an important index requirement of AC regulated power supply. Based on the requirements for the use value of the AC regulated power supply, its EMC performance should not only meet the immunity index of a higher severity level and the qualified electromagnetic interference limit, but more importantly, it should be for its load (sensitive to EMI). electronic equipment) to provide sufficient EMC safety margin. In this paper, combined with the EMC performance requirements of the product, the relevant requirements and test methods are explained in detail, and personal opinions are put forward.
1 Basic concepts
Electromagnetic compatibility (ElectromagneTIcCompaTIbility, referred to as EMC) is an important quality index of electrical and electronic products. It can be considered that product quality is mainly composed of two major contents: quality norms and technical indicators. The former involves general norms, that is, the international IEC, and the basic standards formulated by the country in China; the latter is the regulation of product functions and its technical requirements. Electromagnetic compatibility and safety requirements are basic standards. Now EMC has formed a complete system from basic standards, common standards, family standards, and product standards. In addition, there is special legislation for this purpose internationally. For example, the European Union has formulated regulations, stipulating that from January 1, 1996, electrical and electronic products must obtain the qualification certification of low-voltage management (LV Directive) and electromagnetic compatibility management (EMC Directive) before they can be sold in the market. Over the years, new EMC standards have been officially released in China. However, it should be pointed out that the relevant EMC standards of IEC will continue to be upgraded from draft or old versions to official versions, and relevant national EMC standards will also be continuously updated and released, and the latest version shall prevail for relevant EMC tests.
The so-called EMC is defined in GB/T4365-1996 "Electromagnetic Compatibility Terminology" as: the ability of a device or system to work normally in its electromagnetic environment without causing unacceptable electromagnetic disturbance to anything in the environment. This definition summarizes three aspects. First, the limitability of electromagnetic disturbance. Electromagnetic disturbance is ubiquitous, but it can be constrained by quality standards and its harmfulness can be limited by technical means. This means that the limit value of the intensity of electromagnetic disturbance sent out should be stipulated for the product to ensure that the electromagnetic environment is qualified. Second, the immunity of electromagnetic disturbance. This means that the product should be able to work normally in the electromagnetic environment with specified electromagnetic disturbance intensity without reducing its performance index. Third, the standardization and compatibility of the electromagnetic environment. That is to say, taking any measures against electromagnetic disturbance cannot degrade the performance of itself or other products or systems in the same electromagnetic environment, and can only coexist in a friendly "peaceful" manner. For example, in order to reduce conduction interference, a capacitor is connected in parallel between the equipment power phase line and the ground line. For the equipment, the capacity of the capacitor must meet the limit value requirements of the leakage current in the safety standard; for the system, it must be prevented from becoming a system interference coupling source and affecting the system work. Therefore, the EMC test of the product should include two aspects: (1) Test the electromagnetic disturbance intensity it sends to the outside world to confirm whether it meets the limit value requirements stipulated in the relevant standards.
EMC test items and requirements
EMC test requirements are divided into 3 categories according to product use: namely, military use, industrial and commercial environment use, and civilian and residential environment use. The test items, requirements, and methods of the latter two are relatively consistent, and the difference lies in the requirements for indicators. The military category is quite different from the latter two categories because of its special use. In addition, due to the particularity of use, aviation and marine equipment have the same high requirements as military equipment, and there are international general standards and specifications. Based on the conditions of use of AC regulated power supplies sold on the market, this article focuses on the latter two categories.
In view of the increasing attention to EMC issues in the society, involving many professions and products, IEC has treated EMC requirements as the basic standard of IEC. This is the famous IEC61000 series standard. This standard has been regarded internationally as a common standard with the same importance as the safety standard. One of them, IEC61000-4 "Testing Technology", is the basic standard for guiding EMC testing. Since EMC technology is a complex, multi-disciplinary, and constantly evolving new technology, the relevant EMC test items, requirements, and methods are also constantly being revised and improved. Therefore, many items in IEC61000-4 have not yet been released officially and are still in draft form. In order to make it easier for readers to understand this knowledge, we will introduce the projects involving AC regulated power supplies, and focus on the IEC projects adopted by relevant national standards.
Conditions and methods of EMC testing
Testing depends on three factors: methods, techniques, and equipment. The method is determined by both the measurement principle and the use of the test equipment. The technology is all the test methods adopted to obtain the correct test results (higher accuracy), and the equipment is everything that reflects the above two factors to serve the test. technical device. These must all be standardized to guarantee reproducibility and authenticity of the tests.
EMC test conditions are determined by the test method. The specific test methods are divided into the test bench method carried out under laboratory conditions and the field method carried out under actual use conditions. It is impossible to simulate all the interference phenomena that may be encountered in the field, especially the field method has insurmountable limitations. However, through standardized testing, information on the EMC performance of the device under test can be obtained more comprehensively. For this reason, the international recommendation is to adopt the test bench method first, unless it cannot be carried out in the laboratory, the field method is generally not used.
The main method of immunity test is to select the appropriate severity level according to the electromagnetic environment conditions of the equipment, combined with the measures taken by the user for the equipment, to test according to the relevant test methods, and finally to evaluate the test results according to the qualified judgment conditions proposed by the product standards Eligibility. This is the main difference between immunity test and other tests.
The electromagnetic disturbance source in the electromagnetic environment, the coupling method of the electromagnetic disturbance source to the equipment, the sensitivity of the equipment to electromagnetic disturbance, and the protection measures of the user on the work site are directly related to the severity level. That is, the use environment determines the form of interference, and the installation protection conditions determine the severity level of interference. GB/T13926.4 specifically stipulates the electrical environment conditions under the operation of equipment corresponding to the severity level in the electromagnetic environment:
Level 1, with a well-protected environment, such as a computer room;
Level 2, protected environments such as control rooms or terminal rooms of factories and power plants;
Level 3, typical industrial environment, such as industrial process devices, relay rooms of power plants and open-air high-voltage substations;
Level 4, harsh industrial environments, such as power stations, industrial process equipment without special installation measures, outdoor areas, etc.
In IEC801-5, the source of the surge is the power switching transient or the lightning transient of the indirect lightning strike, and the installation conditions and protective facilities of the equipment are classified as follows (applicable to the surge):
Class 0: Well-protected electrical environment with primary and secondary overvoltage protection, usually in a special room, and the surge voltage will not exceed 25V;
Category 1: electrical environment with local protection and primary overvoltage protection, and the surge voltage does not exceed 500V;
Type 2: The power line is separated from other lines, the electrical environment with good cable isolation, and the surge voltage does not exceed 1kV;
Category 3: The electrical environment where power cables and signal cables are laid in parallel, and the surge voltage does not exceed 2kV;
Category 4: The interconnection line is laid along the power cable as it is outdoors, and the electrical environment where the electronic circuit and the electrical circuit use cables, the surge voltage does not exceed 4kV;
Category 5: The electrical environment where electronic devices are connected to telecommunication cables and overhead power lines in non-populated areas.
There is no surge test for Category 0. General power supply products are in Class 1 or Class 2 electrical environment, and the severity level can be selected as Class 1 or Class 2.
