Electrical Rapid Pulse Tolerance Test (EFT)

Overview of electrical fast transient pulse trains

This standard mainly introduces the test methods of the national standard GB/T17626.4: 1998 “Electromagnetic compatibility testing and measurement techniques-Electrical fast transient burst immunity test”, which corresponds to the international standard IEC61000 -4-4:1995 “Electromagnetic compatibility Part 4: Testing and measurement techniques-Part 4: Electrical fast transient burst immunity test”.

This standard is the basic standard and stipulates the test level and measurement method for the immunity test of electrical and electronic equipment to oscillating waves.

Analysis of EFT test standard types for electrical fast transient pulse trains

The electrical fast transient burst immunity test mainly simulates switching transient processes, such as cutting off inductive loads, bouncing relay contacts and other types of interference caused by transient disturbances.

Its main characteristics are: short rise time, rich high-frequency content, which can reach about 300 to 400 megabytes; high repetition rate and low energy.

test judgment

A. The performance is normal within the limits of technical requirements.

B. Function or performance is 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. Degradation or loss of functions that cannot be restored or normal due to equipment (component) or software damage, or data loss.

Comparison of new and old standards for EFT testing of electrical fast transient pulse trains

As an important part of equipment immunity test, electrical fast transient burst immunity test has a history of more than 20 years in the world. During this period, my country also transformed international standards on pulse burst testing into national standards twice, 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 testing and measurement technology Electrical fast transient burst immunity test”. Currently, the latest draft IEC61000-4-4 standard (FDIS document) has been published.

Reason for the change:

The actual situation is that the actual value of the repetition frequency of a single pulse in the pulse train is 10kHz to 1MHz. However, it was difficult for previous generators using fixed adjustment of the spark air gap to reproduce this relatively high repetition frequency, so the standard specified a lower frequency., representative dedicated pulse. 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 a matter of course to increase the pulse frequency to conform to the actual situation, which makes the pulse burst immunity test more suitable for practical interference situations.

(1)Changes in signal generator technical parameters

1. Signal generator circuit changes

In pulse burst generators, there is one obvious difference between the original standard and the new draft standard in the main components of the generator: the original standard talks about spark gaps; the new draft standard talks about high voltage switches.

2. Change in characteristic parameters

1). The draft standard gives the output voltage range under two different load conditions, the output voltage for a 1000Ω load is 0.24kV–3.8kV; the output voltage for a 50 Ω load is 0.125V–2kV.

2). The draft standard eliminates the 4mJ of energy provided by each 2kV pulse on a 50Ω load and replaces it with comparability of pulse generator performance (see Table 2).

You can see one point in the table: The increase in the repetition frequency of the pulses does not cause an increase in the energy injected into the EUT because the repetition frequency is increased from 5kHz to 100kHz (The frequency increased by 20 times), but the duration of the pulse burst was reduced from 15ms to 0.75 ms (the duration was reduced to one-twentieth of the original), so the total number of pulses injected into the equipment under test remained unchanged (still 75), and the interference energy injected into the equipment under test remained unchanged, except that the pulse intensity per unit time increased. Considering that foreign experts interpret the fault mechanism of the pulse burst test as interfering with the charging of the line junction capacitor by pulses, the higher the pulse frequency, the more pulses there are per unit time, the faster the charge accumulation on the junction capacitor, and the easier it is to reach the threshold of line errors. Therefore, the new draft standard increases the frequency of testing, which essentially increases the severity of testing.

3. Changes in calibration characteristics and methods for electrical fast transient burst testing

The performance of the generator must be verified to establish a common basis for the performance of all test generators participating in the test. The following steps can be used for verification:

The output end of the test generator was connected to 50Ω and 1kΩ coaxial attenuators respectively, and monitored with an oscilloscope. The-3dB bandwidth of the monitoring oscilloscope and the frequency response of the 50Ω and 1kΩ coaxial attenuators reflecting the load of the test generator are required to reach more than 400MHz. Among them, 50Ω is the matching load for the test generator; the 1kΩ test load reflects a compound load for the generator. Only when different test generators have the same characteristics under two extreme load conditions can they ensure comparable test results in actual immunity tests.

During the calibration, the rise time, duration and repetition frequency of a single pulse shall be measured; as well as the duration and repetition period of the pulse train, and recorded in detail.

General test configurations for laboratory type tests in the new draft standard

Note: l = distance between coupling clip and EUT, which should be 0.5m±0.05m; 

(A)= location where the power line is coupled;

(B)= Position of signal line coupling

The electrical fast transient burst test is configured in accordance with the new draft standard, and ground-mounted equipment, desktop equipment, and other structural forms of equipment will be placed above a reference ground plate. The equipment under test and the reference ground plate are separated by an insulating support 0.1m±0.01m thick. The new draft standard stipulates that all equipment installed on ceilings or walls should be tested as desktop equipment. 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 to the new draft standard for electrical fast transient burst testing are particularly important: first, the test generator and coupling/decoupling network are placed directly on the reference ground plane and connected to the reference ground plane, because the burst test conducts a common-mode test on the line under test, which adds interference between the line under test and the earth, and the reference ground plane in the test represents the earth. Therefore, placing the test generator and coupling/decoupling network on the reference ground plane is determined by the nature of the test. In order not to cause excessive attenuation of burst interference, the connection between the test generator, coupling/decoupling network and the reference ground plane should be low impedance.

The new draft standard for electrical fast transient burst testing states that all cables connected to the equipment under test should be placed on insulating supports with a height of 0.1m above the ground. Because a distributed capacitance is formed between the connecting cable of the equipment under test and the reference ground plane, the distributed capacitance is also different for different heights from the ground. Different distributed capacitances will have different effects on the escape of high-frequency harmonics of the pulse train from the connecting cable, which will directly affect the test results.

The new draft standard has changed the test configuration method of desktop equipment, which has greatly improved the test severity of desktop equipment and the consistency of test results. According to the test configuration of the original standard, the desktop equipment 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 cord with interference voltage is superimposed is extended from the ground to the power input of the desktop equipment. Therefore, the actual height of the power line from the ground must be more than 80cm, so that the impedance of the power line relative to the reference plane cannot be fixed (different impedances have different impedances in different placement positions), and the excessive high-frequency impedance of the power line (relative to the power line. For a layout where the power line is away from the reference ground plane at 10cm), a large amount of high-frequency components of pulse burst interference on the power line escape, resulting in the weakening of the interference actually entering the equipment under test. Therefore, when testing the same equipment using the test configurations provided in the original standard and the draft new standard, completely 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 line and signal line between the coupling device and the equipment under test is 0.5m±0.05m, instead of ≤1m specified in the original standard. Obviously, the length given by the original standard is not clear, and it is within the appropriate range from 0 to 1m. However, with different line lengths, the escape of high-frequency harmonics of the pulse train is different. The interference suffered by the equipment under test is actually The combined result of conducted interference left on the line and radiated interference escaping into space. For different wire lengths, the proportions of conducted interference and radiated interference experienced by the equipment under test are different, and the comparability of test results cannot be guaranteed. Therefore, clarifying the length of the line under test is particularly important for the comparability and consistency of test results.

The new draft standard for electrical fast transient burst testing also stipulates that if the length of the non-removable power supply cable provided by the manufacturer exceeds 0.5m±0.05m, the ultra-long cable should be folded to avoid becoming a flat coil and placed at a height of 0.1m away from the reference ground plane. Instead of when the power cable exceeds 1m as specified in the original standard, the ultra-long part is wound into a flat coil with a diameter of 0.4 m and placed flat 0.1 m high away from the reference ground plane. Obviously, the formulation of the new draft standard is more reasonable and it is easier to deal with ultra-long lines.

In the new draft 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 caused by testers ‘different understandings of the standard.

Configuration of rack mounted equipment for electrical fast transient burst testing

Note: Coupling clips can be installed on the wall of the shielded room, or on any grounded surface. The coupling clip is also connected to the equipment under test. For large ground-mounted equipment with cables entering and exiting at the top, the coupling clip should be placed 10cm above the equipment under test so that the cable passes through the center of the reference ground plate before drooping.

Finally, the new draft standard also requires lines that do not need to be subjected to fast transient pulse testing to be enclosed and kept as far away from the line under test as possible to reduce coupling between lines.

Regarding the test configuration on I/O and communication ports, both the original standard and the new draft standard use capacitive coupling clips for testing. However, in the original standard, when two equipment are tested at the same time, the distance between the equipment under test and the coupling clip l1=l2≤1m; when only one equipment is tested, in order to decouple, l2 must be at least ≥5m, or l2> 5l1. In the new draft standard, when two equipment are tested simultaneously, the distance between the equipment under test and the coupling clip l1=l2=0.5m±0.05m; when only one equipment is tested, a decoupling network must be inserted between the equipment that does not need to be tested and the coupling clip.

The test configuration of the equipment at the installation site, including tests on power terminals and I/O and communication ports, remains basically unchanged in both the new draft standard and the original standard. Only the method of pulse injection and the distance between the coupling/decoupling network and the equipment under test for the non-fixed equipment under test connected to the power supply via cord and plug were changed similar to those in the laboratory configuration.

Changes in electrical fast transient burst test methods

Regarding the test time in the test plan, only 1 minute was written in the original standard. The draft new standard writes that in order to speed up the test, the test time is chosen to be 1 minute. The test time can be divided into six 10-second pulse bursts, with a pause of 10 seconds each time. In the actual environment, pulse bursts are independent events that occur randomly, so it is not preferred to synchronize the pulse bursts with the signal of the equipment under test. The product standard setting committee may choose alternative test durations.

service process