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Electrical Fast Pulse Withstand Test (EFT) - Suzhou LCS Standard
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Electrical Fast Pulse Resistance Test (EFT)
Overview of electrical fast transient bursts
This standard mainly introduces the test method of the national standard GB/T17626.4:1998 "electromagnetic compatibility test and measurement technology electrical fast transient burst immunity test", corresponding to the international standard IEC61000-4-4:1995 "electromagnetic compatibility Part 4: Test and measurement technology Part 4: Electrical fast transient burst immunity test".
This standard is the basic standard that specifies the test class and measurement method for electrical and electronic equipment to oscillating wave immunity test.
Analysis of electrical fast transient burst EFT test standard types
The electrical fast transient burst immunity test mainly simulates the switching transient process, such as cutting off the inductive load, relay contact bounce and other transient disturbance types of interference.
Its main features are: short rise time, rich high-frequency content, can reach about three or four hundred trillion; High repetition rate and low energy.
Test verdict
A. Normal performance within the limits of technical requirements.
B. Temporary reduction or loss of function or performance, but can recover on its own.
C. Temporary reduction or loss of function or performance, but requiring operator intervention or system reset.
D. Reduced or lost functions that cannot be restored or in normal state due to damage to equipment (components) or software, or data loss.
Electrical fast transient burst EFT testing of new and old standards
Electrical fast transient burst immunity test as an important part of equipment immunity test has a history of more than 20 years in the world. During this period, China has also twice converted the international standards on burst testing into national standards, namely GB/T13926.4-1992 "Electromagnetic compatibility of industrial process measurement and control devices Electrical fast transient burst requirements" and GB/T17626.4-1998 "Electromagnetic compatibility test and measurement technology Electrical fast transient burst immunity test". At present, the latest draft IEC 61000-4-4 standard (FIS document) has been published.
Reasons for the change:
The actual value of the repetition rate of a single pulse in a burst is 10kHz to 1MHz, but it was difficult to reproduce this relatively high repetition rate with a generator with a fixed adjustment spark air gap, so the standard specified a representative dedicated pulse with a lower frequency. That is, the test waveform is quite different from the actual situation. With the update of pulse forming devices, especially the selection of high-speed and high-voltage electronic switches, it is natural to increase the pulse frequency to match the actual situation, which makes the burst immunity test more realistic for interference situations.
(1) Changes in the technical parameters of the signal generator
1. Signal generator circuit changes
In the burst generator, the original standard and the new draft standard have a clear difference in the main components of the generator composition: the original standard talks about spark gap; The new draft standard talks about high-voltage switches.
2. Characteristic parameter changes
1). The draft standard gives the output voltage range under two different load conditions, and the output voltage of a 1000Ω load is 0.24kV - 3.8kV; The output voltage of a 50Ω load is 0.125V to 2kV.
2). The draft standard eliminates the energy provided by each 50kV pulse in a 2Ω load of 4mJ and replaces it with comparability of pulser performance (see Table 2).
It can be seen in the table that an increase in the repetition rate of the pulse does not cause an increase in the energy injected into the device under test, because the repetition rate is increased from 5kHz to 100kHz (frequency increased by 20 times). However, the duration of the pulse group has been reduced from 15ms to 0.75ms (the duration has been reduced to one-twentieth of the original), so the total number of pulses injected into the test device has not changed (still 75), and the interference energy injected into the test device has not changed, but the pulse density per unit time has increased. Considering that foreign experts explain the fault mechanism of the pulse group test as interference pulse charging the line junction capacitance, the higher the pulse frequency, the more pulses per unit time, the faster the charge accumulation of the junction capacitor, and the easier it is to reach the threshold of line error. Therefore, the new draft standard increases the frequency of testing, which essentially increases the severity of testing.
3. Changes in calibration characteristics and methods of electrical fast transient burst test
The performance of the generator must be verified in order to establish a common basis for the performance of all test generators involved in the test. The following steps can be used for validation:
At the output of the test generator, 50Ω and 1kΩ coaxial attenuators are connected sequentially and monitored with an oscilloscope. The frequency response of the 3dB bandwidth of the monitoring oscilloscope and the 50Ω and 1kΩ coaxial attenuators reflecting the test generator load is required to reach more than 400MHz. where 50Ω is the matched load of the test generator; The 1kΩ test load represents a composite load of the generator. Different test generators can only have the same characteristics under two extreme load conditions to ensure comparable test results in actual immunity tests.
In the calibration, the rise time, duration and repetition frequency of a single pulse are measured; As well as the duration and repetition period of the burst, detailed documentation.
The new draft standard is for general test configurations for laboratory type tests
Note: l = distance between coupling clip and EUT, should be 0.5m±0.05m;
(A) = location of the power line coupling;
(B) = Location of signal line coupling
Electrical Fast Transient Burst Testing According to the configuration of the new draft standard, ground-mounted equipment, benchtop equipment, and other structural forms of equipment will be placed above a reference floor. The test device and the reference ground floor are separated by an insulating support 0.1m ± 0.01m thick. The new draft standard stipulates that all equipment installed on ceilings or walls will be tested as benchtop equipment. The test generator and coupling/decoupling network are also placed directly on the reference ground plate and maintain a low impedance connection to the reference ground floor.
These changes in the new draft standard for electrical fast transient burst testing are particularly important: the test generator and coupling/decoupling network are first placed directly on the reference ground plate and connected to the reference grounding floor, because the burst test performs a common-mode test on the line under test, which is a test that places interference between the line under test and the earth, and the reference ground floor in the test represents the earth. Therefore, the test generator and coupling/decoupling network are placed on the reference ground board is determined by the nature of the test, in order not to cause excessive attenuation of the burst interference, the connection between the test generator, the coupling/decoupling network and the reference ground floor should be low impedance.
The draft new standard for electrical fast transient burst testing states that all cables connected to the device under test should be placed on an insulated bracket with a height of 0.1 m above the ground. Because a distributed capacitance is formed between the connection cable of the device under test and the reference ground, the distributed capacitance is also different at different heights from the ground. Different distributed capacitances, the escape of burst high-frequency harmonics from the connecting cable will also be different, which will directly affect the test results.
The new draft standard changes the test configuration of desktop equipment, and greatly improves the test rigor of desktop equipment and the consistency of test results. According to the original standard test configuration, the benchtop equipment was placed on a wooden table, the test generator was placed on a reference ground plate (the ground terminal of the test generator was connected to the reference ground floor with low impedance), and the power cord superimposed with the interference voltage was extended from the ground to the power input of the benchtop equipment. Therefore, the actual height of the power line from the ground should be more than 80cm, so that the impedance of the power line relative to the reference plane cannot be fixed (different placement positions have different impedances), and the excessive high-frequency impedance of the power line (relative to the layout of the power line leaving the reference ground plane by 10cm), so that the high-frequency components of the pulse group interference on the power line escape a large amount, resulting in weakened interference actually entering the device under test. Therefore, when testing the same equipment using the test configuration provided by the original standard and the new draft standard, very different results can be obtained.
In addition, the draft new 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 0.5m±0.05m, instead of the ≤1m specified in the original standard. Obviously, the length given by the original standard is not clear, from 0-1m are suitable range, but different line lengths, pulse burst high-frequency harmonics escape is different, the interference of the test equipment is actually the result of the conduction interference left on the line and the radiation interference escaping to space. Different wire lengths, the proportion of conducted interference and radiation interference of the tested equipment is different, and the comparability of test results cannot be guaranteed. Therefore, it is particularly important to clarify the length of the test line for 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 manufacturer's non-removable power cable exceeds 0.5m ±0.05m, the extra-long cable should be folded up to avoid becoming a flat coil and placed 0.1m high from the reference floor. Instead of the power cable exceeding 1m as stipulated in the original standard, the ultra-long part is pulled into a flat coil with a diameter of 0.4m and placed flat 0.1m high from the reference floor. Obviously, the formulation of the new draft standard is more reasonable, and the treatment of ultra-long lines is easier.
In the draft new standard for electrical fast transient burst testing, the test configuration of rack-mounted equipment (shown in the figure) is proposed for the first time, avoiding inconsistent test results due to the tester's inconsistent understanding of the standard.
Electrical fast transient pulse group test rack - mounted equipment configuration
Note: Coupling clips can be mounted on the wall of the shielding chamber, or on any grounded surface. The coupling clip is also connected to the device under test. For large ground-mounted devices where cables enter and exit at the top, the coupling clamp should be placed 10cm above the device under test to allow the cable to pass through the center of the reference ground floor before sagging.
Finally, the new draft standard also requires that lines not subject to fast transient pulse tests be enclosed and as far away from the test line as possible to reduce coupling between lines.
For test configurations on I/O and communication ports, capacitive coupling clips are used in both the original and the new draft standards. However, in the original standard, when two devices are tested at the same time, the distance between the device under test and the coupling clamp is l1=l2≤1m; When testing with only one device, l2 should be at least ≥5m, or l2 > 5l1 for decoupling. 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 l1=l2=0.5m±0.05m; When testing only one device, a decoupling network must be inserted between the device that does not need to be tested and the coupling clamp.
The test configuration of the equipment at the installation site, including the test on the power terminal and I/O and the communication port, remains basically unchanged in the new draft standard and the original standard. Only the method of pulse injection and the distance between the coupling/decoupling network and the test device for the non-fixed test device connected to the power supply by the cord and plug were changed similar to those in the laboratory configuration.
Change in electrical fast transient pulse group test method
As for the test time in the test plan, write no less than 1 minute in the original standard. In order to speed up the test, the new draft standard says, the test time should be 1 minute. The test time can be divided into six 10-second pulse clusters, with each interval paused for 10 seconds. In the actual environment, the pulse group is an independent event occurring at random, so it is not inclined to synchronize the pulse group with the signal of the tested device. The product standard setting committee may choose other test durations.
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