EFT Generated by a Power Supply
Vertical = 1 Amp/div, Horizontal = 1usec/div
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
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,
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
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.
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:
- "An Unusual Source of Multiple ESD Events in Electronic Equipment," Douglas C. Smith, Institute of Physica Electrostatics Symposium, Southampton, England, April 2015.
- Fischer Custom Communications F-65 current probe
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