Hello Charles-
For your first scenario, the air gap distance from shell to inner pins is the
determining factor. For the USB connector I have found it nearly impossible
to zap the pins in the scenario of a “user’s finger” , so my experience
is that this scenario is correct for the USB connector. For your second
scenario, I have some experience that disagrees with this. The cable shield
will not prevent charge build up on the inner wires, and therefore the
host-side connector pins can still receive an ESD discharge from a shielded
cable.
Here are some war-stories that give my experience with the two questions-
“protection from shielded connectors?” and “how much voltage on the
pins?”.
I’ve had several product failures in the past where the ESD was caused by
so-called “cable discharge”. The two most interesting, and I think
pertinent to your questions, were a MODEM port and a USB port. Per usual, the
former is with an unshielded cable (POTS) and the latter is with a shielded
cable. In *both* cases we had ESD upset and/or damage. ESD upset was
independent of the presence of a shield.
(regarding the question “how much voltage?”)
The MODEM problem was reported by our customers and we never found exactly the
voltage they were generating. We were able to duplicate the problem in the
lab with approx. 7kV applied to the POTS cable. So we know they were
developing at least that much voltage in handling the cable. It was
instructive to find that amount of charge on an “insulated” cable and we
subsequently changed our design practices for the MODEM port.
(regarding the “protection from shielded connectors?”)
The USB problem was found in our lab during prototype testing with as little
as 5 kV on the cable shield. During the insertion of the cable into the
shielded USB connector we found that discharge was first from cable-shield to
host-shield, but the cable pins still held charge and subsequently discharged
to the recessed pins within the host connector. The reason? The cable inner
conductors were collecting charge over time through the cable inner
insulation. The rate of charge through the insulator was slow, but fast
enough to charge the wires over “seconds”, and slow enough to maintain
some charge during the “millisecond” long plugging event.
So, from those experiences I have three conclusions,
1- contact with inner conductors of a cable is not
required to charge up the cable
2- the presence of a shield does not protect the
inner pins from ESD discharge
3- the pins of a cable can be charged up to, and
can maintain, at least 7 kV during user handling.
Hope this helps.
Best Regards,
Patrick
Company reg. name: Vestas Technology R&D Americas, Inc.
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From: emc-p...@ieee.org [mailto:emc-p...@ieee.org] On Behalf Of Grasso, Charles
Sent: Tuesday, May 10, 2011 10:20 AM
To: emc-p...@ieee.org
Subject: ESD protection for HDMI/ESata & other shielded cables
Greetings all!
I am interested in the forums collective experience on the degree of ESD
protection
required by HDMI/USB and eSata interfaces given that:
1) The pins are recessed and
2) The I/O has a shielded cable.
I see two scenarios: First : - discharge to the I/O pins – either on the
product connector or
on the cable connector (with the cable attached to the product). It seems to
me that a discharge from a
users finger to a pin will be difficult to achieve as the field will tend to
“jump” to the outer shell of the connector.
Second : - electrostatic induction on the cable i.e. a user takes the cable
out of the bag and induces a charge
on the cable. In this case the predominant field distribution will be
contained on the cable shield. So when the cable is
offered to the product any discharge will jump again to the cable shield.
What’s not clear to me is the esd level induced on the i/o pins as a result
of the external discharge and hence my inquiry.
Thanks!!
Best Regards
Charles Grasso
Compliance Engineer
Echostar Communications
(w) 303-706-5467
(c) 303-204-2974
(t) 3032042...@vtext.com <mailto:3032042...@vtext.com>
(e) charles.gra...@echostar.com <mailto:charles.gra...@echostar.com>
(e2) chasgra...@gmail.com <mailto:chasgra...@gmail.com>
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