Electrical Fast Pulse Tolerance Test (EFT)

2018-07-23 12:09:12 SUSHI GUANGBO TEST TECHNOLOGY CO.,LTD 已读
Electrical Fast Pulse Tolerance Test (EFT)
 
Overview of Electrical Fast Transient Bursts
 
This standard mainly introduces the test method of national standard GB/T17626.4:1998 "Electromagnetic compatibility test and measurement technology electric fast transient pulse group immunity test", corresponding to the international standard IEC61000-4-4:1995 "electromagnetic compatibility 4th Part: Test and Measurement Techniques, Part 4: Electrical Fast Transient Burst Immunity Test.
 
This standard is a basic standard that specifies the test level and measurement method for the oscillating wave immunity test of electrical and electronic equipment.
 
Electrical fast transient burst EFT test standard type analysis
 
The electrical fast transient burst immunity test mainly simulates the switching transient process, such as the type of interference caused by transient disturbances such as disconnecting inductive loads and relay contact bounce.
 
Its main features are: short rise time, high frequency content, can reach three or four hundred megabytes; high repetition rate and low energy.
 
Test judgment
 
A. Performance is normal within the technical requirements limits.
 
B, function or performance temporarily reduced or lost, but can recover on its own.
 
C. Function or performance is temporarily reduced or lost, but requires operator intervention or system reset.
 
D. The function of the device (component) or software is damaged, or the data is lost, resulting in the loss or loss of the function of self-recovery or normal state.
 
Comparison of new and old standards for electric fast transient burst EFT test
 
The electrical fast transient burst immunity test has been an important part of the equipment immunity test for more than 20 years in the world. In the meantime, China has also twice translated the international standards for burst test into national standards, which are GB/T13926.4-1992 "Electromagnetic Compatibility Electrical Fast Transient Burst Requirements for Industrial Process Measurement and Control Devices" and GB/T17626.4-1998 "Electromagnetic compatibility test and measurement technology electric fast transient burst immunity test". Currently, the latest draft IEC61000-4-4 standard (FDIS file) has been published.
 
The reason for the change:
 
The actual situation is that the actual value of the repetition frequency of a single pulse in a pulse group is 10 kHz to 1 MHz, but it has been difficult to reproduce this relatively high repetition frequency with a generator that fixedly adjusts the spark air gap earlier, so the standard specifies a lower frequency, A representative dedicated pulse. That is, the test waveform is much different from the actual situation. With the update of pulse forming devices, especially the selection of high-speed high-voltage electronic switches, it is a matter of course to increase the pulse frequency to meet the actual situation, which makes the pulse group immunity test more realistic.
 
(1) Signal generator technical parameter changes
 
Signal generator circuit change
 
In the pulse group generator, the original standard and the new draft standard have a distinct difference in the main components of the generator: the original standard is about the spark gap; the new standard draft is about the high voltage switch. (high voltage switch).
 
2. Changes in characteristic parameters
 
1). The draft standard gives the output voltage range for two different load conditions. The output voltage of a 1000Ω load is 0.24kV – 3.8kV; the output voltage of a 50Ω load is 0.125V – 2kV.
 
2). The draft standard cancels the energy provided by each 2kV pulse on a 50Ω load by 4mJ, replacing it with the comparability of pulse generator performance (see Table 2).
 
 
 
One point can be seen in the table: the increase in the repetition rate of the pulse does not result in an increase in the energy injected into the device under test, since the repetition frequency is increased from 5 kHz to 100 kHz (20 times higher frequency), but the duration of the burst However, it was reduced from 15ms to 0.75ms (the duration was reduced to one-twentieth of the original), so the total amount of pulses injected into the device under test did not change (still 75), and the interference energy injected into the device under test did not change. However, the pulse intensity per unit time has increased. Considering that the fault mechanism of foreign experts on pulse group test is explained as the interference pulse to charge the line junction capacitance, the higher the pulse frequency, the more the number of pulses per unit time, the faster the charge accumulation of the junction capacitance is, the easier it is. The threshold for the line error is reached. Therefore, the new draft standard increases the frequency of testing, which essentially increases the severity of the test.
 
3. Electrical fast transient burst test calibration characteristics and method changes
 
The performance of the generator must be verified to establish a common basis for the performance of all test generators involved in the test. The verification can take the following steps:
 
At the output of the test generator, 50Ω and 1kΩ coaxial attenuators are respectively connected and monitored by an oscilloscope. The oscilloscope's -3dB bandwidth, as well as the 50Ω and 1kΩ coaxial attenuators that embody the test generator's load, require a frequency response of more than 400MHz. 50Ω is the matching load of the test generator; 1kΩ test load reflects a composite load of the generator. Different test generators have the same characteristics under only two extreme load conditions to ensure mutually comparable test results in actual immunity tests.
 
The rise time, duration and repetition frequency of a single pulse are measured during the calibration; and the duration and repetition period of the burst are recorded in detail.
 
 
 
Draft new standard for general test configuration for laboratory type testing
 
Note: l = the distance between the coupling clip and the EUT should be 0.5m ± 0.05m;
 
(A) = location of the power line coupling;
 
(B) = position of signal line coupling
 
Electrical Fast Transient Burst Test In accordance with the new draft standard configuration, ground-mounted equipment, bench-top equipment, and other structural forms of equipment will be placed above a reference ground plane. The device under test and the reference grounding plate are separated by an insulating support of 0.1 m ± 0.01 m thick. The new draft standard stipulates that all equipment installed on the ceiling or on the wall is tested on a benchtop basis. The test generator and coupling/decoupling network are also placed directly on the reference ground plane and maintain a low impedance connection to the reference ground plane.
 
These changes in the draft new standard for electrical fast transient burst testing are particularly important: first place the test generator and the coupling/decoupling network directly on the reference ground plane and connect to the reference ground plane because the burst test is for the subject. The common mode test of the line is the test of adding interference between the line under test and the earth, and the reference grounding plate in the test represents the earth. Therefore, placing the test generator and the coupling/decoupling network on the reference ground plane is determined by the nature of the test. In order not to cause excessive attenuation of the burst interference, the test generator, the coupling/decoupling network and the reference ground plane The connection should be low impedance.
 
The new draft standard for electrical fast transient burst testing indicates that all cables connected to the equipment under test are placed on insulated supports with a height of 0.1 m from the ground. Because the connecting cable of the device under test and the reference grounding plate form a distributed capacitance, the difference in grounding height and the distributed capacitance are different. Different distributed capacitances will also be different for the escape of the high frequency harmonics of the pulse group from the connecting cable, which will directly affect the test results.
 
The new draft standard has greatly improved the test configuration of desktop devices, the test severity of desktop devices and the consistency of test results. According to the original standard test configuration, the bench-top device is placed on a wooden table, the test generator is placed on the reference ground plate (the ground terminal of the test generator is connected to the reference ground plate with low impedance), and the power line with the interference voltage is added. Re-expand from the ground to the power input of the desktop unit. Therefore, the actual ground clearance of the power line should be above 80cm, so that the impedance of the power line relative to the reference plane cannot be fixed (different impedances are placed at different positions), and the high-frequency impedance of the power line is too large (relative to the power line) With reference to the ground plane of 10 cm, the high-frequency components of the pulse group interference on the power line are largely escaped, and the interference actually entering the device under test becomes weak. Therefore, when the same equipment is tested using the test configurations provided by the original standard and the new draft standard, very different results can be obtained.
 
In addition, the new draft standard for electrical fast transient burst testing specifically states that the length of the power and signal lines between the coupling device and the device under test is 0.5 m ± 0.05 m, instead of ≤ 1 m as specified by the original standard. Obviously, the length given by the original standard is not clear, and the range from 0 to 1 m is suitable, but the different line lengths, the high-frequency harmonics of the pulse group are different, and the interference of the equipment under test is actually It is the combined result of conducted interference on the line and radiated interference that escapes into space. Different line lengths, the ratio of conducted and radiated interference received by the equipment under test is different, and the comparability of the test results cannot be guaranteed. Therefore, it is particularly important to clarify the length of the test line and the comparability and consistency of the test results.
 
The new draft standard for electrical fast transient burst testing also stipulates that if the length of the non-removable power cable provided by the manufacturer exceeds 0.5m ± 0.05m, the extra long cable should be folded to avoid becoming a flat coil and placed at the same time. 0.1m from the reference grounding plate. When the power cable specified by the original standard exceeds 1 m, the extra long part is wound into a flat coil with a diameter of 0.4 m, which is placed at a height of 0.1 m from the reference grounding plate. Obviously, the formulation of the new draft standard is more reasonable, and it is easier to deal with the super long line.
 
The test configuration of the rack-mounted equipment (as shown in the figure) was first proposed in the draft new standard for electrical fast transient burst testing, which avoids different test results due to the tester's misunderstanding of the standards.
 
 
 
Electrical Fast Transient Burst Test Rack Mounting Equipment Configuration
 
Note: The coupling clip can be mounted on the wall of the shielded room or on any grounded surface. The coupling clip is also connected to the device under test. For large ground-mounted equipment where the cable enters and exits at the top, the coupling clip should be placed 10 cm above the equipment under test, allowing the cable to pass through the center of the reference grounding plate before sagging.
 
Finally, the draft new standard also requires that lines that do not need to be subjected to fast transient pulse tests be circled and as far as possible away from the test line to reduce coupling between the lines.
 
Regarding the test configuration on the I/O and communication ports, both the original standard and the new draft standard were tested using capacitive coupling clips. However, in the original standard, when two devices were tested at the same time, the distance between the device under test and the coupling clamp was l1=l2≤1m; when testing only one device, l2 should be at least ≥5m, or l2 for decoupling. >5l1. In the new draft standard, when two devices are tested at the same time, the distance between the device under test and the coupling clamp is l1=l2=0.5m±0.05m; when only one device is tested, the device that does not need to be tested A decoupling network must be inserted between the coupling clips.
 
The test configuration of the equipment at the installation site, including the power terminals and I/O and the test on the communication port, remained basically unchanged in the new draft standard and the original standard. The method of pulse injection and the distance between the coupling/decoupling network and the device under test in the non-fixed test equipment connected to the power supply via the cord and plug are similar to those in the laboratory configuration.
 
Changes in electrical fast transient burst test methods
 
Regarding the test time in the test plan, only write for no less than 1 minute in the original standard. In the new draft standard, in order to speed up the test, the test time was chosen to be 1 minute. The test time can be divided into six 10 second bursts, each paused for 10 seconds. In an actual environment, a burst is an independent event that occurs randomly, so it is not inclined to synchronize the burst with the signal of the device under test. The product standard development committee can choose other test durations.
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