Electromagnetic Compatibility Requirements and Test Methods for AC Stabilized Power Supply
1 Basic concepts
Electromagnetic Compatibility is an important quality index for 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 IEC EMC standards 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 phase line of the equipment power supply and the ground line. For the equipment, the capacity of the capacitor must meet the limit value requirements of the national standard for leakage current; 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;
(2) Conduct a sensitivity test under the electromagnetic environment conditions of the specified electromagnetic disturbance intensity to confirm whether it meets the immunity requirements stipulated in the relevant standards.
These two aspects are respectively Electromagnetic Interference or Electromagnetic Disturbance and Immunity to a Disturbance in the EMC test project; the term Susceptibili-ty was commonly used in the latter. The occurrence of electromagnetic interference depends on the strength of the interference source, the coupling method of the interference, and the sensitivity of the equipment to the interference. Therefore, the performance requirements and test methods for EMC in the relevant standards are not only divided into different items according to different properties and types, but also divided into two categories according to the different transmission methods of electromagnetic disturbances: namely, conducted interference and radiated interference. The former mainly detects the intensity and frequency range of power frequency power harmonics and high-frequency noise that the equipment under test conducts outward through the power line or signal line, which belongs to the near-field and induction field effects of electromagnetic disturbance. The latter detects the intensity and frequency range of radio frequency noise directly radiated by the device under test, which is mainly aimed at the far-field effect of electromagnetic disturbance. It is worth noting that in recent years, the international community has paid special attention to the two aspects of power supply harmonic interference and equipment immunity requirements. The former involves the environmental protection requirements of public power grids. The latter is to ensure the reliability of the equipment or system. For this reason, many standards also specifically separate the power supply harmonic current content requirements and immunity as two major technical requirements from the EMC project, and separate them as separate items. It must be pointed out that for the information society, the loss caused by the abnormal operation of information technology equipment is often difficult to measure in currency. Under the realistic conditions that it is impossible to completely avoid electromagnetic disturbances, it is particularly important to improve the immunity of products under the specified electromagnetic environment conditions.
Specifically, in addition to confirming whether the EMC performance of the product meets the requirements of the national and industry standards in the routine testing of specific EMC items by the manufacturer (according to domestic and foreign literature reports, no serious EMC design and thorough testing has been carried out and sufficient It is difficult to pass the EMC test), and it can also evaluate the degree of influence of external electromagnetic disturbances on the product and the effectiveness of related suppression measures, and find out the specific reasons (sources and action pathways) of the product being damaged by the EMC test in order to adopt corresponding measures. measure. Therefore, conducting EMC testing at the initial stage of product design and finalization is an indispensable task for products to enter commercialization. From another perspective, understanding the EMC performance of products is an important factor for commodity inspection and user approval of products. All of these require an understanding of EMC performance requirements and testing knowledge.
The EMC test of power supply products has its specific requirements, which are determined by the functions of this type of product. First of all, the power supply product is used as the power interface between the power supply (generally the mains) and the load it serves (typically information technology equipment that is sensitive to electromagnetic disturbance). Its basic function is to ensure that the connected load is not affected by the power factor. malfunction or damage due to influence. In this way, the EMC requirements for power supply products are naturally higher than other products. The most typical example here is that the EMC standard of power supply products requires that the input source terminal and output load terminal of the power supply products should be tested for conducted interference respectively. In addition, if the power supply product is an indispensable part of the system work (such as UPS), and the product is sold as a general product, then the product may be subjected to a second EMC test. The first time is to test the EMC performance characteristics specified in the product's own standards; the second time is to conduct a systematic EMC test based on user opinions and the system to which it belongs.
A large number of studies have shown that the electromagnetic disturbance from the mains is the most important and the worst type of electromagnetic disturbance. As long as this type of interference can be resolved, the immunity problem will be basically solved. Therefore, some people say that the technical characteristics of today's information society are "one machine, three pieces", that is, computer and hardware, software, and electrical components. In this way, the AC stabilized power supply as the power interface between the mains and electronic equipment, especially information technology equipment, should have an effective power filter function, at least it should have a significant attenuation and suppression effect on electromagnetic disturbance. This should be regarded as an essential function of AC power supply. Naturally, for an AC stabilized power supply with anti-interference function, it should not only improve its anti-interference performance, but also have the EMC performance of electronic products that are connected to its output and are sensitive to EMI, so as to obtain greater EMC* * Margin, this is a major functional requirement of the anti-interference AC regulated power supply with the function of purifying the interference noise. This is one of the basis for compiling SJ/T10541-94 "General Technical Conditions for Anti-interference AC Stabilized Power Supply".
On the other hand, some requirements similar to EMC have been reflected in the performance indicators of power supply products. For example, the source voltage effect of the AC regulated power supply and the total relative harmonic content requirements of the output voltage. In addition, some EMC items that are only sensitive to weak current electronic equipment, such as anti-power frequency magnetic field interference, electrostatic discharge, radiated electromagnetic field interference, etc., may have little impact on high-power electrical equipment, so they are not listed as necessary in SJ/T10541-94 Test items. In this way, the EMC requirements for AC regulated power supplies are different from those of other electrical and electronic products.
2 EMC test items and requirements
EMC test requirements are divided into 3 categories according to product use: namely, ** category, industrial and commercial environment use category, and civil and residential environment use category. The test items, requirements, and methods of the latter two are relatively consistent, and the difference lies in the requirements for indicators. The ** 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 latest IEC61000 series standards. This standard has been regarded internationally as a general standard with the same importance as the national 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 officially released and are still in draft form. In order to make it easier for readers to understand this aspect of knowledge, we will introduce the projects involving AC regulated power supplies, and focus on the IEC projects adopted by relevant national standards.
