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Technical Tidbit - June 2015
ESD and EFT Internally Regenerated by Power Supplies

Suspect conducted emissions plot

Figure 1.
EFT Generated by a Power Supply
Vertical = 1 Amp/div, Horizontal = 1usec/div

Abstract: System power supplies can regenerate ESD or EFT applied to the system into multiple events, even when the applied stress is only a fraction of the breakdown voltage of the insulting barrier of the power supply, generating some very nasty current waveforms and potentially disrupting system operation. This article links my paper written for the Institute of Physics Electrostatics Symposium in Southampton, England in April 2015 on this topic and expands on the material in the paper to include EFT in addition to ESD.

Figure 1 shows a plot of current in the power cord of a small two wire power supply when EFT is applied across the power supply from the AC mains input to the isolated DC output. The supply was not energized and the AC mains connections were shorted together to avoid having a lightning protector affect the current waveform captured in Figure 1.

In Figure 1, a single EFT pulse (~2ns rise time, 50 ns at half amplitude points on the waveform with a peak open circuit voltage of about 800 Volts) was applied and is labeled at the left side of the plot as "Original Stress." The power supply generated the rest of the current waveform. A second EFT pulse is generate about 1/2 microsecond later and is itself embedded in a slow discharge pulse, possibly a corona discharge. The current probe used was a Fischer Custom Communications F-65 1 MHz to 1 GHz current probe with a one Ohm transfer impedance (one Volt output into 50 Ohms per Ampere of current). Since the current probe is AC coupled, the area above and below zero current must be equal and this leads to the plot dipping below the centerline after the slow discharge and above the centerline after the second slow discharge.

The second EFT event, of about the same peak current amplitude as the applied one, and the three slow discharges were all generated in the power supply which had an insulating barrier of almost three times the applied amplitude of the EFT pulse.

The time scale in Figure 1 is orders of magnitude (microseconds compared to tens of nanoseconds) larger than can be explained by the circuit dimensions. The time delay around the loop of the simulator ground lean and power supply leads is on the order of 20 nanoseconds.

This effect is dangerous as EFT pulses occur on AC mains frequently and may not be so easy to control as static generation that leads to ESD.

My Institute of Physics paper "An Unusual Source of Multiple ESD Events in Electronic Equipment," click here to view or download a copy, is focused on ESD as the driving function and shows some very nasty current waveforms compared to Figure 1. I first noticed this effect on the power cord
of a piece of equipment while troubleshooting an ESD problem. I was able to simplify the test setup to just include an ESD simulator and the power supply with the AC mains of the power supply connected to the ground lead of the ESD simulator and the simulattor tip to the DC output side of the power supply. Pictures are shown in the paper. The resulting current waveforms are amazing and described in the paper.

I propose a theory of how such current waveforms can show multiple ESD or EFT pulses of both polarities generated from just one applied pulse by a power supply whose breakdown rating is much higher than the applied stress. The theory is that parasitic resonant circuits in the power supply and its EMC line filter, provide high voltage ringing that can breakdown an insulative barrier in both polarities within the power supply. Work to confirm or disprove this theory is planned and will be published in a future Technical Tidbit.

Summary: Power supplies can regenerate ESD or EFT into multiple events, potentially causing system problems. One theory for explaining how a power supply can generate multiple ESD and EFT events of both polarities of breakdown in response to a single applied stress involves parasitic resonant circuits within the power supply an its EMC line filter. It is important in system designs to avoid power supplies that regenerate ESD and EFT to help maintain system reliability.

Links in this article:
  1. "An Unusual Source of Multiple ESD Events in Electronic Equipment," Douglas C. Smith, Institute of Physica Electrostatics Symposium, Southampton, England, April 2015.
  2. Fischer Custom Communications F-65 current probe

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