Maker Pro
Maker Pro

Some ESD protection questions (spark gaps, earth/ground connectionissues)

C

Christian Walter

Jan 1, 1970
0
Hello,

I just wanted to ask the more experienced people here for some advice. I
am currently working on a redesign of an existing board and I want to
improve some ESD aspects of the board because we had some problems in
this area in the past. The system is built from two components where the
PCB in questions contains some micro controllers and a USB equipment and
the other PCB is the power supply unit and contains additional internal
modules.

I have to protect some I/O ports and an USB interface. I have chosen to
use a TVS Diode Array (Littlefuse SP0502BA). I have chosen to place the
diode array as near as possible to the I/O connectors of the system.
What I am unsure about is whether to connect the common pins of the
diode array. My first choice would be to use the earthing because if I
connect it directly to the ground plane any discharge current will take
the path from the I/O connector over the diode. From there to the ground
plane and the PCB FCC connector cable to the power supply board and then
to earthing. In my opinion this can cause some signal and system
stability problems although the device on the PCB will not be damaged.
Does anybody support this claim?

The next question I would like to ask what you people think about spark
gaps as a cheap alternative ESD protection for onboard I/O ports. That
is I would add add a signal layer which surround the PCB connections
using a very short distance (maybe 8 thou which is easily doable in a
manufacturing house) and connect this to plane to the earthing. This
would effectively limit the voltage to maybe 1-2kV.

What I have also read is that a small guard ring at the edges of the PCB
(connected to ground) can help to improve ESD performance. Has anybody
used and tried this?

I would be very grateful for some advice because I am still a bit new in
the field. Another question is how I can verify my ESD protection
without having to go to a special laboratory because of costs.

Kind regards,
Christian Walter
 
P

Paul Mathews

Jan 1, 1970
0
Hello,

I just wanted to ask the more experienced people here for some advice. I
am currently working on a redesign of an existing board and I want to
improve some ESD aspects of the board because we had some problems in
this area in the past. The system is built from two components where the
PCB in questions contains some micro controllers and a USB equipment and
the other PCB is the power supply unit and contains additional internal
modules.

I have to protect some I/O ports and an USB interface. I have chosen to
use a TVS Diode Array (Littlefuse SP0502BA). I have chosen to place the
diode array as near as possible to the I/O connectors of the system.
What I am unsure about is whether to connect the common pins of the
diode array. My first choice would be to use the earthing because if I
connect it directly to the ground plane any discharge current will take
the path from the I/O connector over the diode. From there to the ground
plane and the PCB FCC connector cable to the power supply board and then
to earthing. In my opinion this can cause some signal and system
stability problems although the device on the PCB will not be damaged.
Does anybody support this claim?

The next question I would like to ask what you people think about spark
gaps as a cheap alternative ESD protection for onboard I/O ports. That
is I would add add a signal layer which surround the PCB connections
using a very short distance (maybe 8 thou which is easily doable in a
manufacturing house) and connect this to plane to the earthing. This
would effectively limit the voltage to maybe 1-2kV.

What I have also read is that a small guard ring at the edges of the PCB
(connected to ground) can help to improve ESD performance. Has anybody
used and tried this?

I would be very grateful for some advice because I am still a bit new in
the field. Another question is how I can verify my ESD protection
without having to go to a special laboratory because of costs.

Kind regards,
Christian Walter

Envision a complete circuit between the ESD source and your product:
This must include a complete return path. Often, this involves the
protective 'earth' or grounding system. Your goal is to divert as much
of the ESD energy as possible away from your circuits, both to protect
circuit elements and to prevent disruption of equipment operation. If
you insulate your product completely, there will be no ESD to the
equipment, but this is impractical in cases where conductive parts
must be exposed, particularly if those conductive parts cannot be
connected to earth ground. The best defense, then, is to provide
deliberate adjacent paths to ground, for example, the grounded 'ring'
around the perimeter of a circuit board. If possible, design the
product so that grounded conductive parts extend out in such a way
that any ESD conducts to ground rather than into your circuit.

For any ungrounded paths into your circuit that may still take an ESD
hit, you have these basic defenses to consider:
a) series resistance
b) series inductance
c) shunt resistance and shunt breakdown devices such as zeners,
transils, MOVs, spark gaps
d) shunt capacitance

For high speed signals, it may be impossible to incorporate any
appreciable amount of series resistance or inductance or shunt
capacitance without degrading the signal, so you're left with the low-
capacitance tranzorb solution. The common terminal of the tranzorb(s)
should be connected to earth ground with the lowest possible
impedance. If this doesn't do the job, reconsider how you might
prevent direct ESD to signal nodes: shielding, shrouded connectors,
etc.

Keep in mind that the high currents and fast risetimes associated with
ESD will develop high potentials across stray inductances in the
return path, AND there may be significant magnetic field coupling of
ESD currents to adjacent circuitry. Therefore, loop area of the
complete ESD current path should be considered.

Circuit board spark gaps will produce widely varying results depending
on humidity, dust buildup, and other factors, and I think your
estimate of 1 to 2 kV for 8 mils is probably a bit low for most
conditions. Spark gap tubes have the advantage over semiconductor
devices of maintaining relatively shunt voltages even for very high
current levels, so they are often used to absorb more powerful
transients such as those resulting from indirect lightning strikes.

In Summary:
1) Design packaging so that product is either totally insulated from
ESD or any discharges go preferentially to grounded structures,
shunting all ESD current away from internal circuits.
2) Series and and shunt any remaining filter nodes that must take a
direct ESD hit.

As you get farther into this subject, you'll also discover the
difference between insulating systems that dissipate vs collect static
charge and between deliberate low-impedance discharge of ESD versus
dissipating or bleeding off charge to prevent high amplitude ESD.
Paul Mathews
 
W

whit3rd

Jan 1, 1970
0
... 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.
 
C

Chris Jones

Jan 1, 1970
0
Christian said:
Hello,

I just wanted to ask the more experienced people here for some advice. I
am currently working on a redesign of an existing board and I want to
improve some ESD aspects of the board because we had some problems in
this area in the past. The system is built from two components where the
PCB in questions contains some micro controllers and a USB equipment and
the other PCB is the power supply unit and contains additional internal
modules.

I have to protect some I/O ports and an USB interface. I have chosen to
use a TVS Diode Array (Littlefuse SP0502BA). I have chosen to place the
diode array as near as possible to the I/O connectors of the system.
What I am unsure about is whether to connect the common pins of the
diode array. My first choice would be to use the earthing because if I
connect it directly to the ground plane any discharge current will take
the path from the I/O connector over the diode. From there to the ground
plane and the PCB FCC connector cable to the power supply board and then
to earthing. In my opinion this can cause some signal and system
stability problems although the device on the PCB will not be damaged.
Does anybody support this claim?

The next question I would like to ask what you people think about spark
gaps as a cheap alternative ESD protection for onboard I/O ports. That
is I would add add a signal layer which surround the PCB connections
using a very short distance (maybe 8 thou which is easily doable in a
manufacturing house) and connect this to plane to the earthing. This
would effectively limit the voltage to maybe 1-2kV.

What I have also read is that a small guard ring at the edges of the PCB
(connected to ground) can help to improve ESD performance. Has anybody
used and tried this?

I would be very grateful for some advice because I am still a bit new in
the field. Another question is how I can verify my ESD protection
without having to go to a special laboratory because of costs.

Kind regards,
Christian Walter

You might be able to find some useful information on this page:
http://www.emcesd.com/

Chris
 
Top