In the EFT test, there are ports such as L1, L2, L3, N, and PE. PE and earth are two concepts. The electrical fast pulse interference is common mode. In the experimental setup chart provided by the standard, it can be seen that the signal cable core from the test generator is added to the corresponding capacitor through the optional coupling capacitor. On the power lines (L1, L2, L3, N, and PE), the shielding layer of the signal cable is connected to the chassis of the coupling/decoupling network. When the EFT test is performed, there are ports such as L1, L2, L3, N, and PE. PE and earth are two concepts. The electrical fast pulse interference is common mode. In the experimental setup chart provided by the standard, it can be seen that the signal cable core from the test generator is added to the corresponding capacitor through the optional coupling capacitor. On the power lines (L1, L2, L3, N, and PE), the shield of the signal cable is connected to the chassis of the coupling/decoupling network, and the chassis is connected to the reference ground terminal.
This means that the burst interference is actually added between the power line and the reference ground, so the interference applied to the power line is common mode interference. For the experimental method using the coupling clip, the electrical fast pulse will pass through the coupling plate. The distributed capacitance between the tested cables enters the test cable, and the pulse received by the test cable is still relative to the reference ground plane.
Therefore, the interference applied to the cable under test by the coupling clip is still of common mode nature. Determine the nature of the interference, then we can take the appropriate measures to make the equipment pass the experiment. Then we can easily see that the X capacitor (differential mode capacitor) used in the power supply filter has no inhibitory effect on EFT interference.
If the device is a metal case, the Y capacitor (common mode capacitor) will act, bypassing the high frequency EFT onto the case and returning to the source through the distributed capacitance between the device case and the reference ground so that it does not enter the circuit.
Mechanism of equipment failure caused by electrical fast pulse interference According to the study of the mechanism of equipment failure caused by pulse group interference by foreign scholars, the energy of a single pulse is small and will not cause equipment failure. However, the pulse group interference signal charges the device line junction capacitance. When the above energy accumulates to a certain extent, it may cause a malfunction of the line (or even the system).
Therefore, there is a time course in the line error, and there will be some chance (there is no guarantee of how much time is left, the line must be wrong, especially when the test voltage reaches the critical point). It is also difficult to determine whether the pulse is applied separately or the pulse is applied together, and the device is more likely to fail. It is also difficult to conclude which device is more sensitive to positive and negative pulses.
Practice has shown that a device is often a cable that is particularly sensitive to a certain polarity at a test voltage. Experiments have shown that the signal line is much more sensitive to the fast pulse interference of the power line.
Effective measures for the device to pass the electrical fast pulse test First, we first analyze the interference injection method: the EFT interference signal is coupled to the main power line through a 33nF capacitor in the coupled decoupling network (the signal or control cable is passed through the capacitive coupling clip). Interference is applied and the equivalent capacitance is 100pF). For a 33nF capacitor, its cutoff frequency is 100K, that is, the interference signal above 100KHZ can pass; and the 100pF capacitor has a cutoff frequency of 30M, allowing only interference above 30MHz. The interference waveform of the electric fast pulse is 5 ns/50 ns, the repetition frequency is 5 K, the pulse duration is 15 ms, and the pulse group repetition period is 300 ms. According to the Fourier transform, its spectrum is from 5K-100M discrete lines, and the distance of each line is the repetition frequency of the pulse.
Knowing the above points, the coupling capacitor that exerts the interference plays the role of a high-pass filter. Because the impedance of the capacitor decreases with increasing frequency, the low-frequency components in the interference are not coupled to the EUT, but only the higher frequency. The interference signal will enter the EUT. When we add the common mode inductor to the EUT circuit (note that the common mode inductor must be added to the main power line and its return line, otherwise saturation will occur and the attenuation interference will not be achieved). Some high frequency interference components can be attenuated because the impedance of the inductor increases with increasing frequency. Therefore, the interference signal actually applied to the EUT has only the intermediate frequency portion.
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