Re: Some ESD protection questions (spark gaps, earth/ground connection issues)
- From: whit3rd <whit3rd@xxxxxxxxx>
- Date: Tue, 28 Aug 2007 12:43:38 -0700
On Aug 27, 2:20 pm, Christian Walter <wo...@xxxxxx> wrote:
... I want to
improve some ESD aspects of the board because we had some problems in
this area in the past.
I have to protect some I/O ports and an USB interface.
The next question I would like to ask what you people think about spark
gaps as a cheap alternative
What I have also read is that a small guard ring at the edges of the PCB
(connected to ground) can help
Another question is how I can verify my ESD protection
without having to go to a special laboratory because of costs.
Surely USB has a grounded shell and provision that the ground
connection
mates first, then the power connections, then the signal? There's no
real
threat of high potentials/spark/currents there, unless your shield
melts
in a lightning strike. If you're canny about the other I/O ports,
they can
be pretty robust, too, and NOT because of TVS diodes or other addons.
Spark gaps (commercially available ones, look like disk capacitors but
with
a little saw-cut slot) will take a lot of current and don't often
fail, BUT
they're intended for rather high breakdown thresholds. Unless you
expect
high currents at high voltages (like an isolated power supply that
floats
at a few hundred volts from ground), the spark gap is going to be too
insensitive, won't protect delicate circuitry well. Low-pressure gas
has better breakdown thresholds, and a Ne lamp can be a very
effective clamp if you can tolerate the (about 100V) turnon threshold.
Grounded-track-surround is a variant of a guard ring. Multiple guard
rings
connected to successive potentials is the hallmark of HV circuitry.
Guard rings are effective if you expect tracking (conduction through
foreign substances after your PC boards get dirty), and I'd expect to
see them used a LOT where a circuit board is expected to have
hundreds of volts per cm kinds of gradients. USB and ' I/O ports'
isn't
really in that range, usually. With multiple guard rings, connected
with spark gaps, you can make your printed circuit board REALLY
intimidating, even without international warning symbols. Techs will
quake every time they open the box...
Typical ESD testing is at low event energy, with voltages (2 kV) that
can
be handled with minor precautions. But, any controlled, safe
environment
for such testing IS a laboratory, by definition. If you don't want to
'go out'
to a laboratory, you build one for yourself.
.
- References:
- Some ESD protection questions (spark gaps, earth/ground connection issues)
- From: Christian Walter
- Some ESD protection questions (spark gaps, earth/ground connection issues)
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