EMC performance is an important indicator requirement for AC regulated power supplies. Based on the requirements for the use value of AC regulated power supply, its EMC performance should be in addition to its own immunity level that can achieve higher severity levels and qualified electromagnetic interference limits, and more importantly, its load (sensitive to EMI) Electronic equipment) provides sufficient EMC safety margins. In this paper, combined with the EMC performance requirements of the product, the relevant requirements and test methods are described in more detail, and personal opinions are put forward.
1 basic conceptElectromagnetic compatibility (ElectromagneTIcCompaTIbility, referred to as EMC) is an important quality indicator for electricians and electronic products. It can be considered that the product quality is mainly composed of two major contents: quality specifications and technical indicators. The former involves general norms, that is, the international IEC, the domestic basic standards set by the state; the latter is the product function regulations and technical requirements. Electromagnetic compatibility and safety requirements are fundamental standards. Now EMC has formed a complete system from basic standards, general standards, family standards, and product standards. In addition, there is special legislation in the international arena for this purpose. For example, the European Union has enacted regulations that, from January 1, 1996, electricians and electronic products must be qualified for low-voltage management (LVDirecTIve) and electromagnetic compatibility management (EMCDirecTIve) 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 IEC related EMC standards will continue to be upgraded from the draft or the old version to the official version. The relevant national EMC standards will also be continuously updated and released. The latest EMC version should be subject to the latest version.
The so-called EMC, defined in GB/T4365-1996 "Electromagnetic Compatibility Terminology" is: the ability of a device or system to function properly in its electromagnetic environment without posing an unacceptable electromagnetic disturbance to anything in the environment. This definition summarizes the three aspects. First, electromagnetic disturbance can be restricted. Electromagnetic disturbance is ubiquitous, but it can be constrained by quality specifications and technically limited. This means that the product should be limited to the limit of the electromagnetic interference intensity sent out to ensure that the electromagnetic environment is qualified. Second, the immunity of electromagnetic harassment. This means that the product should work properly in an electromagnetic environment that specifies the intensity of electromagnetic disturbances and should not degrade its performance. Third, the normative and compatibility of the electromagnetic environment. That is, any measures taken against electromagnetic disturbances can not degrade themselves or other products or systems in the same electromagnetic environment, and can only be friendly "peaceful" coexistence. For example, in order to reduce conducted interference, a capacitor is connected between the phase line of the device power supply and the ground. For the equipment, the capacity of the capacitor must meet the limit value of the leakage current in the safety standard; for the system, to prevent it from becoming a system interference coupling source affects the system operation. Therefore, the EMC test of the product should include two aspects: (1) testing the electromagnetic disturbance intensity sent to the outside world to confirm whether it meets the limit value requirements stipulated by the relevant standards;
(2) Perform a sensitivity test on the electromagnetic environment under the specified electromagnetic disturbance intensity to confirm compliance with the immunity requirements specified in the relevant standards.
These two aspects are electromagnetic interference (Electromagnetic Interference) or electromagnetic disturbance in the EMC test project.
Chinese map method classification number: TM937 document identification code: A article number: 0219? 2713 (2000) 09? 454? 05
(Electromagnetic Disturbance) and Immunity Toa Disturbance; the latter used to be the term Susceptibility. The occurrence of electromagnetic interference depends on the strength of the interference source, the coupling mode of the interference, and the sensitivity of the instrument to the interference. Therefore, the performance requirements and test methods of the relevant standards for EMC are divided into different items according to different natures and different types. They are also divided into two categories according to different modes of electromagnetic disturbance propagation: conductive interference and radiation interference. The former mainly detects the strength and frequency range of harmonics and high-frequency noise of the power frequency power supply 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 the RF noise radiated directly from the device under test, which is mainly for the far-field effect of electromagnetic disturbance. It is worth noting that in recent years, the international attention has been paid to the harmonic interference of power supplies and the immunity requirements of equipment. The former involves the environmental requirements of the utility grid. The latter is to ensure the reliability of the equipment or system. To this end, many standards also specifically separate the power supply harmonic current content requirements and immunity as two technical requirements from the EMC project, a single independent item. It must be pointed out that for the information society, the losses caused by the malfunction of information technology equipment are often difficult to measure by currency. Under the realistic conditions that it is impossible to completely avoid electromagnetic disturbance, it is particularly important to improve the product immunity under the specified electromagnetic environment.
Specifically, the manufacturer conducts routine testing of the EMC specific project on the product, in addition to determining whether the EMC performance of the product meets the requirements of the national and industry standards (according to domestic and foreign literature reports, no serious EMC design and testing is performed and taken Sufficient technical measures, it is difficult to pass the EMC test), and can also evaluate the degree of influence of external electromagnetic disturbance on the product and the effectiveness of the relevant suppression measures, and identify the specific reasons (source and action path) of the product being damaged by the EMC test. Corresponding measures. Therefore, EMC testing at the beginning of product design is an indispensable part of the product's commercialization. On the other hand, understanding the EMC performance of a product is an important factor in commodity inspection and user-approved products. All of this requires knowledge of EMC's performance requirements and testing.
EMC testing of power products has its own special requirements, and its particularity is determined by the function of such products. First of all, the power supply is used as the power interface between the power supply (generally the mains) and the load (typically the information technology equipment sensitive to electromagnetic disturbance). The basic function is to ensure that the connected load is not affected by the power factor. Work and malfunction or damage. This makes the EMC requirements for power supply products higher than other products. The most typical example here is the conducted interference test for the input source and output load of the power supply product. 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-purpose product, then the product may have to undergo secondary EMC testing. One is to test the EMC performance characteristics specified by the product itself; the second time is based on the user's opinion to perform the EMC test of the system together with the system to which it belongs.
Numerous studies have shown that electromagnetic disturbance from the mains is one of the most important and worst electromagnetic disturbances. As long as this type of interference can be solved, the problem of immunity is basically solved. Therefore, some people say that the technical characteristics of today's information society are "one machine, three pieces", that is, computers and hardware, software, and power parts. In this way, the AC regulated power supply, which is the power interface between the commercial power and the electronic equipment, especially the information technology equipment, should have an effective power filter function, and at least should have a significant attenuation suppression effect on the electromagnetic disturbance. This should be a necessary function of the AC regulated power supply. Naturally, for an AC regulated power supply with anti-interference function, it should not only improve its own immunity performance, but also have the EMC performance of the electronic products that are sensitive to EMI connected to its output, and obtain greater EMC safety. Yudu, this is a veritable functional requirement for anti-interference AC power supply with interference function for interference noise. This is also one of the basis for the preparation of SJ/T10541-94 "General technical conditions for anti-interference AC power supply".
On the other hand, some EMC-like requirements have been reflected in the performance specifications of power products. For example, the source voltage effect of the AC regulated power supply and the total relative harmonic content of the output voltage are required. In addition, some EMC projects that are only sensitive to weak electronic equipment, such as resistance to power frequency magnetic field interference, electrostatic discharge, and radiated electromagnetic field interference, may have little impact on high-power electrical equipment, so it is not listed as necessary in SJ/T10541-94. Test items. In this way, the EMC requirements for the AC regulated power supply have special places that are different from other electricians and electronic products.
2 EMC test items and requirementsEMC test requirements are divided into three categories according to product use: military, industrial and commercial use, and residential and residential use. The latter two test items, requirements, methods, etc. are more consistent, the difference lies in the requirements of the indicators. The military category is quite different from the latter two because of its special use. In addition, due to the special use of aviation and marine equipment, in addition to the higher requirements of military equipment, there are international standard specifications. Based on the conditions of use of AC regulated power supplies sold in the market, this article focuses on the latter two categories.
In view of the increasing concern about EMC issues in the society and the many professions and products involved, IEC has treated EMC requirements as the basic standard of IEC. This is the famous IEC61000 series of standards. This standard has been regarded internationally as a common standard as important as safety standards. One of the IEC61000-4 "Testing Techniques" is the basic standard for guiding EMC testing. Since EMC technology is a complex and constantly evolving new technology involving multiple disciplines, EMC test projects, requirements, and methods are constantly being revised and improved. Therefore, many of the projects in IEC61000-4 have not yet been officially released and are still in the draft. In order to facilitate the reader to understand this knowledge, we will introduce the project involving AC regulated power supply, and focus on the IEC project adopted by the relevant national standards.
IEC61000-4, the full name is "Electromagnetic Compatibility for Electric Estate Electronic Equipment, Part 4: Testing and Measurement Technology", "Electromagnetic Compatibility of Electrical and Electronic Equipment Part 4: Testing Technology". Among them are 11 test items. The items and requirements related to the AC regulated power supply standard SJ/T10541-94 are as follows:
IEC61000-4-4: Electrical FastTransients (Burst) ImmunityTest (Electrical Fast Transient Burst), SJ/T10541-94 and SJ/T10542-94 adopt this standard. The purpose of this project is to verify the immunity of the device to various transient disturbances caused by switching transients (inductive load interruptions, relay contact trips, etc.). The test severity level (interference generator (50Ω internal resistance) open circuit output test voltage at the power input end) is 1 level, 0.5kV; 2 levels, 1kV; 3 levels, 2kV; 4 levels 4kV.
IEC61000-4-5SurgeTransientsImmunityTest (surge or surge). The content is temporarily in the draft. Related to this can be referred to the well-known IEC801-5 and the US standard IEEEstd587? 1980 "IEEE Guide for Surge Voltages in Low-Voltage AC Power Circuits" (IEEE Guide: Surge in a low voltage AC power circuit). The electronic industry standards SJ/T10541-94 and SJ/T10542-94 cite some of this aspect. The purpose of the project is to verify the immunity of the equipment to high-energy surges caused by power switching and lightning. The severity level is divided as above, but the output impedance of the interference generator is 2Ω, while the former is 50Ω. The interference generator output separates the voltage voltage of 1.2×50μs (for high-resistance load) and short-circuit (discharge) current wave for 8×20μs (for low-resistance load). Similar test requirements are listed in many electrical and electronic product standards.
In addition, high-frequency spike noise sensitivity tests are used internationally. Especially in Japan, this test is very common. The US military standard MILSTD461, 462 uses similar projects, but the noise simulation generator required to use much more power than the former. There are similar standards in China, such as the 50 kHz to 100 MHz power line conduction sensitivity test in GB4859-84; the CS06 item in GJB151-86 and GJB152-86, the noise is rectangular pulse wave. In addition, GB6162-85 (refer to IEC255-4) uses attenuated oscillating waves. SJ/T10541-94 and SJ/T10542-94 suggest that one of these two methods can be arbitrarily selected. The purpose of the project is the same as IEC61000-4-4. The rectangular pulse wave is characterized by fast rise and low repetition rate, and the characteristic of the damped oscillation wave is high amplitude and low energy. For products containing digital circuits, rectangular pulse waves are more sensitive to interference.
Projects related to electromagnetic interference include power frequency harmonic limit requirements, conducted interference and radiation interference limit requirements. SJ/Z9029.2-87 (equivalent to IEC555-2-1982) specifies the harmonic current limit values ​​required for equipment in low-voltage power supply systems. For high-power semiconductor converters, GB / T3859.2-93 limits the maximum power capacity (the ratio of the short-circuit capacity of the power system to the apparent power of the converter's fundamental wave) for different converters (by pulse number). Current harmonics. It must be pointed out that electricians and electronic products have been identified as power pollution through the power line to the mains, and the requirements for this are to be strengthened as the “environmental protection†of the mains system.
The judgment of the EMC test results, the anti-interference test and the electromagnetic interference test are respectively used in completely different ways. The latter uses the quantitative limit value as the qualified threshold decision point; the former is generally determined by qualitative methods, that is, according to the performance classification of the product in the test (take GB/T13926-92 as an example):
Class a: In the product performance index specification (within tolerance), the performance is normal;
Class b: Temporary reduction or loss of function or performance, but self-recovery;
Class c: Temporary reduction or loss of function or performance, but requiring operator intervention or system reset;
Class d: Reduced or lost functionality that is unrecoverable due to damage.
Among the four categories, class a is qualified, and class d is unqualified. For class b and class c, the qualification is determined by the manufacturer and the user according to the specific circumstances. Naturally, the technical measures taken for these two types are different. It seems that the judgment of the immunity test is too loose, which is actually the principle of the maximum degree of freedom of the standard. Because of the wide variety of devices under test and the large differences, it is difficult to make universal quantitative regulations for the qualification. Of course, a certain evaluation standard should be given for a specific type of product. SJ/T10541?94 embodies this requirement. The converter class of the converter is similarly specified in GB/T3859.1-93, as the basis for the qualification judgment. The standard defines level 3, namely:
Class F: Performance level, which refers to a combination of all kinds of electrical disturbance limits that the converter can withstand without degrading performance;
Class T: Trip level refers to the combination of all kinds of electrical disturbance limit values ​​that the converter can withstand without interrupting the operation of the protection device; it can be further divided into two cases: automatic reclosing after interference and cannot Automatic reclosing (to be manually used, etc.);
Class D: Damage level is a combination of all electrical disturbance limits that the converter can withstand without causing permanent damage.
Obviously, the F level here is equivalent to the class A, the class D is equivalent to the class d, and the class T is the class b and class c.
For the AC regulated power supply with anti-interference function requirements, it is stipulated in SJ/T10541 that in addition to ensuring normal operation, it should also be lost.
The output provides a suitable sensitivity threshold for the load; the peak value of the residual voltage of the interference superimposed on the output voltage should not be greater than 20% of the nominal value of the output voltage. This is the purpose principle of the standard compliance standard, fully considering the essential difference between the AC stabilized power supply and other electrical and electronic products in terms of functional requirements. That is, the former should serve the latter, and the former also serves as the power filter for the latter power EMI. The purpose is to provide sufficient EMI safety margin for EMI-sensitive equipment and improve the immunity level of the equipment. According to the provisions of GB6833.4, the requirements of the electronic instrument for the transient sensitivity of the power supply are: it should be able to withstand 20% of the transient voltage shock of the nominal source voltage change without causing malfunction. For this reason, the maximum allowable transient voltage value (sensitivity threshold) is specified in SJ/T10541 as the output of the AC regulated power supply when receiving the immunity test. In addition, considering that the AC regulated power supply is to provide suitable AC voltage conditions for electronic equipment, SJ/T10541 also stipulates that the relative deviation (ie, output effect) of the output voltage of the AC regulated power supply should be in its reference during the immunity test. Within the condition (tolerance G), this is used as a basis for determining whether or not performance is degraded. The combination of the two, in a scientific, reasonable, practical, easy-to-operate way, solves the problem of no quantitative indicators based on the specific assessment and conformity assessment of the anti-interference performance of the AC stabilized power supply. However, the general standard only uses the general error, performance degradation or degradation as the basis for assessment and conformity assessment. Obviously, this qualitative method is not easy to operate.
3 EMC test conditions and methodsTesting relies on three factors: methods, techniques, and equipment. The method is determined by both the measurement principle and the method of use of the test equipment. The technique is all the test means taken to obtain the correct test result (higher accuracy), and the equipment is the one that reflects the above two factors for the test service. Technical device. These must be standardized to ensure that the test is reproducible and authentic.
EMC test conditions are determined by the test method. The specific test methods are divided into a test bench method under laboratory conditions and a field method 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, you can get more comprehensive information on the EMC performance of the device under test. To this end, it is recommended internationally to use the test bench method first, unless it is not possible in the laboratory, generally without the on-site method.
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, and test according to the relevant test methods. Finally, the test results are evaluated according to the qualified judgment conditions proposed by the product standard. Eligibility. This is the main difference between the immunity test and other tests.
The electromagnetic disturbance source in the electromagnetic environment, the coupling mode of the electromagnetic disturbance source to the equipment, the sensitivity of the equipment to the electromagnetic disturbance and the user's protective measures 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 the interference. GB/T13926.4 specifies the electrical environmental conditions under the operation of equipment corresponding to the severity level in the electromagnetic environment:
Level 1, a well-protected environment, such as a computer room;
Level 2, protected environment, such as control rooms or terminal rooms of factories and power plants;
Level 3, typical industrial environment, such as industrial process equipment, power plants and relay rooms in open-air high-voltage substations;
Level 4, harsh industrial environment, such as power stations, industrial process equipment that does not take special installation measures, outdoor areas, etc.
The source of the surge in IEC801-5 is the lightning transient of power switching transient or indirect lightning strike. The installation conditions and protective facilities of the equipment are classified as follows (for surge):
Class 0: A well-protected electrical environment with primary and secondary overvoltage protection, usually in a special room, the surge voltage does not exceed 25V;
Class 1: Locally protected electrical environment with one overvoltage protection, the surge voltage does not exceed 500V;
Class 2: The power line is separated from other lines, the cable is well insulated, and the surge voltage does not exceed 1kV;
Category 3: The electrical environment in which the power cable and the signal cable are laid in parallel, the surge voltage does not exceed 2kV;
Category 4: The interconnection line is laid along the power cable as if it were outdoors, and the electrical environment of the cable is used for both the electronic circuit and the electrical circuit, and the surge voltage does not exceed 4 kV;
Category 5: Electrical environment in which electronic devices are connected to telecommunications cables and overhead power lines in non-populated areas.
No surge test is performed for class 0. General power products are in Class 1 or Class 2 electrical environments, with a severity level of 1 or 2.
It must be pointed out that the use of the environment as a relevant condition for the immunity test is an important feature of the immunity test. Because if you ignore these correlations, regardless of the application working environment conditions of the device, and think that the device should be "independent", it should be suitable for insertion into any combination device (or system), which will result in all devices under test must accept Interference tests for all projects and erroneous conclusions to the highest severity level. This not only imposes excessively unreasonable and severe restrictions on the device to be used, but also has to bear a large economic burden due to the large number of tests required.
In addition, the immunity test involves high-voltage signals. In addition to strict compliance with relevant safety regulations, it is necessary to perform safety tests on the equipment after the immunity test.
For high-power electric products such as AC stabilized power supply, it is necessary to select an anti-interference project characterized by high frequency and high energy introduced from the mains, and to select a higher severity level than other electricians and electronic products.
Another important feature of the immunity test is the strict and unambiguous regulation of the test generator's technical parameters. In order to compare the immunity performance of the device, there is a test device that produces a relatively uniform and repeatable reproduction, which is the interference simulation generator. Obviously, the output internal resistance, output waveform requirements, open circuit voltage amplitude and error of the generator must be specified to ensure consistent test results and good repeatability. Otherwise, because the impedance of the source of the different devices under test is different, the impedance matching of the generator is different and the generator cannot output the same waveform or amplitude under load. In fact, impedance mismatch is an effective means of suppressing electromagnetic disturbances.
The electromagnetic disturbance test items of the AC stabilized power supply to the outside world (through the mains network) are: harmonic conduction interference test and high frequency conducted interference test.
The harmonic conduction interference test is to test the power frequency current harmonics of the power inlet and the input of the equipment; to measure the maximum value of the current harmonics under 40 times, and to test the current harmonics of the neutral line for the three-phase power supply. In the AC stabilized power supply performance project, the project is evaluated based on the source current relative harmonic content.
Conducted interference test of AC stabilized power supply is the same as other electronic products. GB6833-86 electronic measuring instrument electromagnetic compatibility test specification can be used (refer to the radio interference limit value and measurement method using HP company standard or GB9254-88 information technology equipment (etc. Efficient use of CISPR? 22? 1985). An important test device in the high-frequency conducted interference test is to use artificial power network (ArtificialMainNetwork), in the US standard, it is called Line Impedance Stability Network (LISN). This is due to different Under the power condition, the high-frequency impedance of the mains at the power input end of different equipment is also different. In order to make the test result reflect the real situation, the network must be connected to the required equipment and its power terminal. The radio frequency isolation between the grid and the grid can provide stable high-frequency impedance for the equipment. The number of branches of the artificial power network is the same as the number of lines of the power supply system, and the connection between the network and the interference measuring instrument should ensure impedance matching (50Ω/50μH). ), test each power cord separately, the measurement is dry Voltage value. GJB152-86 recommends the current probe method to measure the conducted interference current; the 10μF feedthrough capacitor is connected between the power line and the ground, which is the same as the LISN. The current probe method is easy to use, quick to measure, and convenient for field test. Closer to the actual situation, it is likely to be measured in the future. In addition, the military standard uses a peak detector, and the GB9254 uses a quasi-peak detector.
The RF radiation interference test is more complicated and involves testing problems such as test sites, antennas, and test line connections. The test site is open in the field and the background electromagnetic noise level is at least 6 dB less than the allowable limit. This requirement is difficult to achieve, and the standard also recommends that an electromagnetic shielding room (as well as an anechoic chamber, etc.) be replaced. When testing the radiation field strength, the equipment to be tested should be strictly wired according to the actual working mode. The power lines and signal lines are not allowed to be specially curled or shrunk to reflect the authenticity.
In short, based on the use value of AC stabilized power supply, its EMC performance should be: in addition to its own high-severity level of immunity indicators, qualified electromagnetic interference limits and the provision of appropriate AC voltage conditions, more importantly To work for its load (electronic equipment sensitive to EMI, especially information technology equipment), work under harsh electromagnetic environment conditions to provide sufficient EMC safety margin. This is not only the basic function of the AC regulated power supply, but also the basis for its EMC requirements and EMC testing.
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