[SI-LIST] Re: ESD on USB Connector Shield
- From: "Pommerenke, David" <davidjp@xxxxxxx>
- To: <Erin.McPhalen@xxxxxxxxxxxxxxxxxxxxxxxxx>, <Pradeep.RSA@xxxxxxxxxxxx>
- Date: Mon, 10 Jul 2006 11:12:07 -0500
Hi Pradeep,
Please let me add my thoughts on this, as my main research area are
upsets in digital systems caused by ESD. I will comment on the RF
components that cause the disturbance and on how to exactly locate the
traces that are disturbed, forcing you system to reboot.
One should not overlook the transient fields caused by ESD generators',
it is a simplification to only look at the injected current of an ESD
generator.
Normally, the transient fields contain much stronger RF components,
relative to the injected current, (see
http://web.umr.edu/~davidjp/paper/01362865.pdf
fig. 1 for an explanation of the reasons).
The RF components reach many GHz. While most ESD generators inject
somewhat similar currents there are very large differences between ESD
generators' transient fields. The strength of the RF depends on which
brand ESD generator you use. The currents and the fields (guided by
metallic surfaces) will enter your system at gaps around the USB
connector and cause surface currents on the boards, voltages between
"grounds" etc. inducing narrow pulses on many different traces and wires
within your system.
Please check the following paper to obtain an impression on the
transient fields and the variations between generators.
http://web.umr.edu/~davidjp/publications.html
Paper: Characterization of human metal ESD reference discharge event
and =20
correlation of generator parameters to failure levels-part II:=20
correlation of generator parameters to failure levels
Kai Wang; Pommerenke, D.; Chundru, R.; Van Doren, T.; Centola,=20
F.P.; Jiu Sheng Huang;
Electromagnetic Compatibility, IEEE Transactions on
Volume 46, Issue 4, Nov. 2004 Page(s):505 - 511
Figure 3.
At first I suggest to understand why you system reboots. Most likely, it
is not a disturbance to the USB, but the fields of the ESD, guided by
the current densities on the metallic parts couple into some sensitive
lines.=20
I suggest to locate those traces, and to filter these traces, probably
not the USB lines. We have good experience with the method for finding
such traces as it is outlined in
http://web.umr.edu/~davidjp/publications.html
Paper: Finding the root cause of an ESD upset event
Pommerenke, D; Jayong Koo, Giorigi Muchadze
DesignCom 2006, Santa Clara, Feb. 2006
Once you know the sensitive lines I suggest analyzing them: Can you add
RC-filtering to them? Can you change the software such that the system
does not reboot?
In my experience most of the sensitive lines are status lines. They
often can handle a 100 Ohm 50pF filter close to the receiving side of
e.g., a reset pulse.
I hope my information helps in debugging ESD problems. Please contact me
if you would like to discuss this further.
David Pommerenke
David Pommerenke=20
Associate Professor
EMC laboratory
118 EECH - University Missouri Rolla, Rolla, MO, 65409
573 341 4531 - pommerenke@xxxxxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Erin.McPhalen@xxxxxxxxxxxxxxxxxxxxxxxxx
Sent: Monday, July 10, 2006 10:45 AM
To: Pradeep.RSA@xxxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: ESD on USB Connector Shield
Hi Pradeep
I can probably help with question 1.
ESD is a very violent event. The spectral density starts from near DC
and=20
begins decaying at about -20dB/decade at 3 MHz to 300 MHz and -40=20
dB/decade after that. That being said, the standard DC current flow
model=20
is not applied to the paths the discharge takes. In addition the
waveform=20
will take all paths including parasitics to get to ground, the issue is=20
the difference in impedance between your intended path and all available
paths. The issue occurs when the high frequency content is not finding=20
your braided ground the only available path or even significantly lower=20
impedance compared to all other available paths.=20
The discharge can cause significant potential between points on your=20
chassis if it is metal(including across mounting points to the PCB) or=20
across your PCB structure. I have also seen the electric/magnetic
field=20
created by an ESD discharge at the contact point and/or moving through
a=20
ground wire, influence other components near it. I am not sure the=20
discharge is coupling to your USB lines directly but likely taking an=20
unintended path through your PCB. Differential scope probes are very=20
handy in checking what signals are being perturbed by the ESD discharge=20
and decide on a solution from there.=20
Erin McPhalen
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WHICH=20
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PRODUCT, OR TRADE SECRET INFORMATION, WHICH IS EXEMPT FROM DISCLOSURE=20
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Pradeep RSA <Pradeep.RSA@xxxxxxxxxxxx>=20
Sent by: si-list-bounce@xxxxxxxxxxxxx
07/10/2006 02:40 AM
Please respond to
Pradeep.RSA@xxxxxxxxxxxx
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Subject
[SI-LIST] ESD on USB Connector Shield
Hi,
iam currently using a SAMTEC USB-A-D-S-B-TH (two stack)
connector=20
in one of my projects. As per the requirements we are supposed to apply=20
ESD on the areas of the connector that are exposed to the user. Our=20
requirement is +/-4KV Contact discharge.
Query 1
I have grounded (earthed) the connector shield and there is a proper=20
isolation between the shield and the Board ground (Digital ground). When
the ESD pulse is applied on the connector shield, our system restarts.
I tried connecting TVS between the USB data lines, VBUS& DGND to Mains=20
EARTH (Ground). Even after this modification the system restarts (fails
in=20
ESD)
I understand if the shield is properly connected to the Earth then the
ESD=20
pulse should not couple to the USB signals.But this is not happening in=20
our case. Why is this so? And even if it gets coupled to the USB lines,=20
the TVS should bypass it to EARTH. Why is this not happening?
Note: I had used a thick Multistrand wire for Earth connection.
Query 2
On analyzing the USB connector, i found that the metal in the middle of=20
the USB connector is floating and is not connected to the shield. This=20
gets connected to the shield only when an USB Type A plug is
connected.If=20
the ESD pulse is applied on this middle metal plate the system restarts=20
immediately.
Why is this metal (middle one) not connected to the shield of the USB=20
connector? Why does the system restarts when an ESD pulse is applied on=20
it?
Thanks in advance
Pradeep
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