[SI-LIST] Re: ESD on USB Connector Shield
- From: Erin.McPhalen@xxxxxxxxxxxxxxxxxxxxxxxxx
- To: Pradeep.RSA@xxxxxxxxxxxx
- Date: Mon, 10 Jul 2006 08:45:25 -0700
Hi Pradeep
I can probably help with question 1.
ESD is a very violent event. The spectral density starts from near DC and
begins decaying at about -20dB/decade at 3 MHz to 300 MHz and -40
dB/decade after that. That being said, the standard DC current flow model
is not applied to the paths the discharge takes. In addition the waveform
will take all paths including parasitics to get to ground, the issue is
the difference in impedance between your intended path and all available
paths. The issue occurs when the high frequency content is not finding
your braided ground the only available path or even significantly lower
impedance compared to all other available paths.
The discharge can cause significant potential between points on your
chassis if it is metal(including across mounting points to the PCB) or
across your PCB structure. I have also seen the electric/magnetic field
created by an ESD discharge at the contact point and/or moving through a
ground wire, influence other components near it. I am not sure the
discharge is coupling to your USB lines directly but likely taking an
unintended path through your PCB. Differential scope probes are very
handy in checking what signals are being perturbed by the ESD discharge
and decide on a solution from there.
Erin McPhalen
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[SI-LIST] ESD on USB Connector Shield
Hi,
iam currently using a SAMTEC USB-A-D-S-B-TH (two stack) connector
in one of my projects. As per the requirements we are supposed to apply
ESD on the areas of the connector that are exposed to the user. Our
requirement is +/-4KV Contact discharge.
Query 1
I have grounded (earthed) the connector shield and there is a proper
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
EARTH (Ground). Even after this modification the system restarts (fails in
ESD)
I understand if the shield is properly connected to the Earth then the ESD
pulse should not couple to the USB signals.But this is not happening in
our case. Why is this so? And even if it gets coupled to the USB lines,
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
the USB connector is floating and is not connected to the shield. This
gets connected to the shield only when an USB Type A plug is connected.If
the ESD pulse is applied on this middle metal plate the system restarts
immediately.
Why is this metal (middle one) not connected to the shield of the USB
connector? Why does the system restarts when an ESD pulse is applied on
it?
Thanks in advance
Pradeep
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