Electromagnetic disturbance source The electromagnetic energy emitted by any form of natural or electric energy device can cause harm to people or other living things sharing the same environment, or cause electromagnetic hazards to other equipment, subsystems or systems, resulting in performance degradation or failure, which is called As the source of electromagnetic disturbance, the conditions, methods and elements of the electromagnetic compatibility test are listed below.
1. Characteristics of Electromagnetic Disturbance Sources
1. Emission levels under specified bandwidth conditions
2. Spectrum width
According to the frequency distribution characteristics of electromagnetic disturbance energy, its spectrum width can be determined. Among continuous wave disturbances, hum disturbance has the narrowest spectrum width, while among pulse disturbances, the unit pulse function has the widest spectrum width.
3. Waveform
Electromagnetic disturbances come in various waveforms. Waveform is an important factor in determining the frequency width of electromagnetic disturbance.
4. Occurrence rate
The distribution of electromagnetic disturbance field strength or power over time is related to the occurrence rate of electromagnetic disturbance. According to the occurrence rate of electromagnetic disturbance, it can be divided into three types: periodic disturbance, non-periodical disturbance and random disturbance.
5. Polarization characteristics of radiated disturbance
The polarization characteristic refers to the characteristic of the direction of the disturbance field strength vector changing with time at a given point in space, which depends on the polarization characteristics of the antenna. When the polarization characteristics of the disturbance source antenna and the antenna of the sensitive equipment are the same, the induced voltage generated by the radiated disturbance at the input end of the sensitive equipment is the strongest.
6. Directional characteristics of radiated disturbance
The disturbance source radiates electromagnetic disturbance in all directions in space, or the ability of sensitive equipment to receive electromagnetic disturbance from various directions is different. The parameters describing this radiation capability or receiving capability are called directional characteristics.
7. Antenna effective area
This is a parameter that characterizes the ability of sensitive equipment to receive disturbance field strength. Obviously, the larger the effective area of the antenna, the stronger the ability of sensitive equipment to receive electromagnetic disturbance.
2. Classification of Electromagnetic Disturbance Sources
According to the classification of electromagnetic disturbance sources, it can be divided into three categories: natural disturbance sources, man-made disturbance sources and transient disturbance sources.
1. Natural sources of harassment
Natural disturbance sources are characterized by their uncontrollability. According to their different causes and physical properties, natural disturbance sources can be divided into four categories: electronic noise, sky electric noise, extraterrestrial noise, and other natural noise such as sedimentary static electricity.
The statistical characteristics of the electromagnetic disturbances produced by them vary greatly, sometimes showing a flat Gaussian distribution of the spectrum, and sometimes showing occasional pulse disturbances.
This kind of harassment is an objective existence, and only by knowing its distribution and changing rules can the electromagnetic environment level be provided.
(1). electronic noise source
Electronic noise mainly comes from the components inside the device, and is an important factor determining the noise figure of a receiver. Common sources of electronic noise include thermal noise, shot noise, distribution noise, 1/f noise, and antenna noise.
(2). electrical noise
Sky electrical noise is a variety of natural phenomena that occur in the atmosphere, including lightning, etc. The radio noise generated can cause electromagnetic disturbance, and it is the dominant natural disturbance source below 30mhz, which has a great impact on radio communication.
(3. Extraterrestrial noise
That is, the noise from the outer space of the earth, the main noise sources include the sun, the background radiation of the sky and the cosmic sources distributed in the Milky Way.
(4). Deposition of static electricity and other natural noise _
The electromagnetic disturbance caused by the static electricity emitted by the aircraft is called deposited static electricity, which is an important natural disturbance source, and the electromagnetic disturbance caused by it will directly affect the performance and safety of the entire aircraft. The disturbance caused by the accumulation of electrostatic charges on the surface of the aircraft and the resulting corona discharge and streamer discharge, whose frequency spectrum is distributed in the range of several hertz to gigahertz, seriously affects the radio communication and navigation. There are other natural noises in nature that should also be noticed.
2. Human harassment source
Man-made disturbance sources are characterized by being known and controllable. Man-made disturbance can be divided into two categories: radio disturbance and non-radio disturbance.
(1) Continuous wave disturbance source
The electromagnetic disturbance generated by the continuous wave disturbance source is mainly pure or narrowband signal
Modulated sine waves, and periodic signals with high repetition rates.
Sources of such harassment include the following:
a. Transmitter: The electromagnetic disturbance generated includes intentional emission signal, harmonic emission signal and spurious emission signal.
b. Local oscillator: The fundamental wave and harmonics generated by the local oscillator of the receiver can be conducted through the power line, and then directly radiated from the case or antenna.
c. Hum: A connecting wave disturbance caused by periodic low frequency signals entering the system.
3. Transient disturbance source
Industrial, scientific and medical equipment (ism), vehicles, motor boats and spark ignition engine installations, household appliances, portable power tools and similar appliances, fluorescent lamps and lighting fixtures, and information technology equipment are major sources of transient disturbances.
(1) Switching action
When the switchgear with contacts is disconnected, at the moment when the distance between the two contacts of the switch transitions from zero to disconnection, spark discharge will be generated to form disturbance. Due to the rapid reduction of the current from a certain value to zero, di/dt is very large, and instantaneous voltage pulses with high amplitudes are generated in switching devices with inductive coils.
(2) Among all kinds of motors, the disturbance generated by the rotating motor containing commutators and brushes is the largest.
(3) Ignition device
Internal combustion engine drive equipment used in vehicles, ships, etc. is equipped with a spark ignition device. When the stored charge is spark-discharged through the spark plug, the peak value of the discharge current is about 200 amps, the discharge time is within microseconds, and the peak voltage is as high as 10 kV or more.
(4) High-voltage transmission lines
There are two types of radiated disturbances generated by transmission lines; gap breakdown and corona discharge.
4. Nonlinear phenomena
Almost all electromagnetic disturbances are related to nonlinearities.
Conditions and Methods of Electromagnetic Compatibility Test
Measurement depends on three factors: method, technology, and equipment. The method is determined by both the measurement principle and the method of use of the measurement equipment. The technology is all the measurement methods adopted to obtain the correct measurement results (higher accuracy), and the equipment is everything that reflects the above two factors to serve the measurement. technical device. These must all be standardized to guarantee reproducible and authentic measurements.
EMC measurement conditions are determined by the measurement method.
It must be pointed out that taking the environment as a relevant condition for immunity measurement is an important feature of immunity measurement. Because if these correlations are ignored, regardless of the application and working environment conditions of the device, it is considered that the device should be "independent", and it should be suitable for insertion into any combination device (or system), which will result in all devices under test having to accept The interference experiment of all items, and the wrong conclusion to reach the highest severity level. This not only imposes an unreasonably high and unreasonably severe restriction on the apparatus to be used, but also entails a great economic burden due to the large number of experiments required.
In addition, the immunity measurement involves high-voltage signals. In addition to strictly complying with the relevant safety regulations, it is also necessary to perform safety measurements on the equipment after the immunity test.
For high-power electrical products such as AC regulated power supplies, it is necessary to select immunity items characterized by high frequency and high energy imported from the mains, and to choose a higher severity level than other electrical and electronic products. .
Another important feature of the immunity measurement is to make strict and clear regulations on the technical parameters of the experimental generator. In order to compare the immunity performance of the equipment, it is necessary to have an experimental device that can produce more consistent and repeatable reproduction, which is the interference simulation generator. Obviously, the generator's output internal resistance, output waveform rules, open circuit voltage amplitude and error must be specified; in order to ensure the consistency of the experimental results and good repeatability. Otherwise, due to the different source impedances of different devices under test, the impedance matching to the generator is different, so the generator cannot output the same waveform or amplitude under load. In fact, impedance mismatch is an effective means to suppress electromagnetic disturbance.
The electromagnetic disturbance measurement items of the AC regulated power supply to the outside world (through the mains network) include: harmonic conduction interference measurement, high frequency conduction interference measurement.
Harmonic conduction interference measurement is to measure the power frequency current harmonics at the input end of the power supply of the equipment; the maximum value of the current harmonics below the 40th order is measured, and the current harmonics of the neutral line should also be measured for the three-phase power supply. In the AC stabilized power supply performance item, this item is assessed by the relative harmonic content of the source current.
The measurement of radio frequency radiation interference is more complicated, involving measurement techniques such as measurement sites, antennas, and measurement line connections. The measurement site is open in the field, and the background electromagnetic noise level is at least 6dB lower than the allowable limit value. This kind of rule is difficult to implement, and the standard also recommends that electromagnetic shielding rooms (and anechoic rooms, etc.) can be used as a replacement. When measuring the radiation field strength, the equipment under test should be wired strictly according to the actual working mode. The power line and signal line are not allowed to be deliberately curled or shrunk to reflect the authenticity.
Measurement depends on three factors: method, technology, and equipment. The method is determined by both the measurement principle and the method of use of the measurement equipment. The technology is all the measurement methods adopted to obtain the correct measurement results (higher accuracy), and the equipment is everything that reflects the above two factors to serve the measurement. technical device. These must all be standardized to guarantee reproducible and authentic measurements.
EMC measurement conditions are determined by the measurement method. The specific measurement methods are divided into the 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 measurement, information on the EMC performance of the device under test can be obtained more comprehensively. For this reason, it is recommended to use the bench method first in the world. Unless it cannot be carried out in the laboratory, the on-site method is generally not used.
The main method of immunity measurement is to select the appropriate severity level according to the electromagnetic environment conditions of the equipment, combined with the method adopted by the user for the equipment, measure according to the relevant measurement methods, and finally evaluate the measurement according to the qualified judgment conditions proposed in the product standard Whether the result is qualified. This is the main difference between immunity measurements and other measurements.
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 method of the user to 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 specifies the electrical environment conditions for equipment working in the electromagnetic environment corresponding to the severity level:
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 environments, such as industrial process devices, relay rooms in power plants and open-air high-voltage substations;
Level 4, harsh industrial environments, such as power stations, industrial process equipment without special installation methods, 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 electronic circuit and electrical circuit use the electrical environment of the cable, and 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.
Surge measurement is not performed for category 0. Generally, the power supply products are in Class 1 or Class 2 electrical environment, and the optional severity level is Class 1 or Class 2.
It must be pointed out that taking the environment as a relevant condition for immunity measurement is an important feature of immunity measurement. Because if these correlations are ignored, regardless of the application and working environment conditions of the device, it is considered that the device should be "independent", and it should be suitable for insertion into any combination device (or system), which will result in all devices under test having to accept The interference experiment of all items, and the wrong conclusion to reach the highest severity level. This not only imposes an unreasonably high and unreasonably severe restriction on the apparatus to be used, but also entails a great economic burden due to the large number of experiments required.
In addition, the immunity measurement involves high-voltage signals. In addition to strictly complying with the relevant safety regulations, it is also necessary to perform safety measurements on the equipment after the immunity test. ]
For high-power electrical products such as AC regulated power supplies, it is necessary to select immunity items characterized by high frequency and high energy imported from the mains, and to select a higher severity level than other electrical and electronic products.
Another important feature of the immunity measurement is to make strict and clear regulations on the technical parameters of the experimental generator. In order to compare the immunity performance of the equipment, it is necessary to have an experimental device that can produce more consistent and repeatable reproduction, which is the interference simulation generator. Obviously, the generator's output internal resistance, output waveform rules, open circuit voltage amplitude and error must be specified; in order to ensure the consistency of the experimental results and good repeatability. Otherwise, due to the different source impedances of different devices under test, the impedance matching to the generator is different, so the generator cannot output the same waveform or amplitude under load. In fact, impedance mismatch is an effective means to suppress electromagnetic disturbance.
The electromagnetic disturbance measurement items of the AC regulated power supply to the outside world (through the mains network) include: harmonic conduction interference measurement, high frequency conduction interference measurement.
Harmonic conduction interference measurement is to measure the power frequency current harmonics at the input end of the power supply of the equipment; the maximum value of the current harmonics below the 40th order is measured, and the current harmonics of the neutral line should also be measured for the three-phase power supply. In the AC stabilized power supply performance item, this item is assessed by the relative harmonic content of the source current.
The conduction interference experiment of the AC regulated power supply is the same as other electronic products, and the GB6833-86 electronic measuring instrument EMC test specification can be used (refer to the radio interference limit value and measurement method of the HP company standard or GB9254-88 information technology equipment (equivalent Using CISPR? 22? 1985). An important measurement device in the measurement of high-frequency conducted interference is to use artificial power network (Artificial Main Network), which is called Line Impedance Stabilization Network (LISN) in American standards. This is due to different power Under certain conditions, the high-frequency impedance presented by the mains at the power input terminals of different devices is also different. In order to make the measurement results reflect the real situation, a regular network must be connected between the device under test and its power terminals. This network can make the device Realize radio frequency isolation from the power grid, and 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 ), each power line is measured separately, and the measurement is the interference voltage value. GJB152-86 recommends the current probe method to measure the conduction interference current; where a 10μF feedthrough capacitor is connected between the power line and the ground, and the effect is the same as LISN The same. The current probe method is easy to use, quick to measure, convenient for on-site measurement, and is closer to the actual situation. It may be the main measurement in the future. In addition, the military standard uses a peak detector, and GB9254 uses a quasi-peak detector.
The measurement of radio frequency radiation interference is more complicated, involving measurement techniques such as measurement sites, antennas, and measurement line connections. The measurement site is open in the field, and the background electromagnetic noise level is at least 6dB lower than the allowable limit value. This kind of rule is difficult to implement, and the standard also recommends that electromagnetic shielding rooms (and anechoic rooms, etc.) can be used as a replacement. When measuring the radiation field strength, the equipment under test should be wired strictly according to the actual working mode. The power line and signal line are not allowed to be deliberately curled or shrunk to reflect the authenticity.
In short, using the value rule of AC regulated power supply, its EMC performance should be: in addition to the immunity index that can reach a higher severity level, qualified electromagnetic interference limits, and the provision of suitable AC voltage conditions, the more important thing is It is necessary to provide sufficient EMC safety margin for its load (electronic instruments and equipment sensitive to electromagnetic interference, especially information technology equipment) to work under harsh electromagnetic environment conditions. This is not only the basic function of the AC regulated power supply, but also the basis for its EMC rules and its EMC measurement.
As an EMC designer, it is very important to understand the conditions, methods and elements of EMC testing. Only by mastering the conditions, methods and elements of EMC testing can you be handy in design.
[The above information is compiled and released by Aibo Testing. If there is any discrepancy, please correct it in time. If there is any quotation, please indicate the source. Welcome to discuss together. We have been paying attention to its development! Focus: CCC/SRRC/CTA/operator warehousing]