EFT: Understanding Test Methodologies To Avoid Failures
In the previous post, the source of EFT noise is presented in brief. The idea behind this article is to provide a highlight of this topic and hence I'm not going into depth. But we need to know a bit more on the EFT test wave characteristics and the test methodology that is described in IEC 61000-4-4. That might help in understanding what kind of noise might get into the system during testing if proper precautionary measures are not implemented in the design. The waveform below shows the EFT test waveform as suggested in the standard.
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| EFT Waveform Per Standard IEC 61000-4-4 |
The "Repetition Period" above depends on the frequency selected. As recommended in the latest standard, the pulse repetition period could be 5KHz (as it was prescribed in earlier versions) or 100KHz (suggested as more similar to practical, but not mandatory). Hence the repetition period could be 200us (for 5KHz) or 10us (for 100KHz). The "Burst Duration" above is the time duration for 75 pulses, i.e. the burst duration is 15ms when the pulse repetition frequency is selected as 5KHz. Please note that, irrespective of the pulse repetition frequency selected as either 5KHz or 100KHz, the energy imparted by the EFT pulses remain same for both of these cases.
The peak voltage level of the EFT noise pulses could be selected as it is recommended in the product specific standards for the specific channel type (power/IO/Communication) for the specific equipment under test (EUT). For example the peak test voltage levels could be +/-2KV for AC power lines and +/-1KV for other lines (DC power/signal/communication etc). The EFT noise needs to be applied to the EUT for 1 second for each polarity of the peak voltage level tested.
The link below could serve as a quick reference to the waveforms, spectral characteristics, equations and test methodology etc.
Next we will discuss more on the design measures to avoid failures due to EFT noise.
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