[SI-LIST] Re: ESD protection devices

  • From: Geoff Stokes <gstokes@xxxxxxxxx>
  • To: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
  • Date: Tue, 14 Oct 2003 12:29:26 +0100

Nigel

Interesting.  I had not seen these new devices.  (My applications boards
don't include protection for harsh environments, but we still have to look
to customers' real needs.)

On the data line filtering, I don't think I'm in a position to design your
system for you.  ;-)
However, I would have thought you need to compromise between protection and
pulse shape, by adjusting the design on the bench or preferably in
simulation which allows a study of a greater range of conditions.  This
relates especially to the filter design including the bandwidth and
impedance, according to customer requirements.

On the hand-held device situation, I disagree.  The success of the ESD test
depends on the ability of the structure to divert current away from
sensitive devices.  In that test, as I remember, having worked on PDAs
myself, the device is placed on some kind of grounded structure - not too
unrealistic, by the way, compared with the hand-held situation or resting on
a table for example, waiting to be zapped by a few kV.  Anyway the test
probe needs some sort of return path.  The level of the test to European
standard is very specific in terms of probe voltage, capacitance (10 pF or
100pF I can't remember correctly) and source resistance (100 ohms?), and
even, if my hazy memory is any good, the specification of the ground table.
Even if you think you don't currently need to work to specific standards
(ESD is part of the  EMC Directive and is a requirement for CE approved
products - it's changed since I looked last), I'd strongly suggest you see
one as this could help with testing and design - e.g., see the latest IEC
61340 series.  If you have to connect external data lines during the test
the situation becomes somewhat more sensitive.  The ESD performance as I
remember relied heavily on electrostatic screening and consideration of
current paths within the structure.  You might have to consider performance,
such as data glitches (signal integrity) as well as damage protection.  Also
the discharge has a fast part and a slow part...

Good luck!
Geoff

> -----Original Message-----
> From: Nigel Hughes [mailto:nigel.hughes@xxxxxxxxxx]
> Sent: 14 October 2003 11:40
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: ESD protection devices
> 
> 
> Hi Geoff,
> 
> thanks for your comment.  There exist some pretty low 
> capacitance devices 
> out there now days (both zeners and varistors - eg. 
> http://www.amotech.co.kr/kor/pdf/smd_varistor_avlc_series.pdf & 
> http://www.protekdevices.com/products_lowcap.htm).  However 
> as the fastest 
> data rate I am concerned with is a max of about 200kbps I 
> don't think this 
> is a big issue.  By the way for this data rate, have you a 
> suggestion of 
> what sort of 3dB frequency should be chosen for the EMC/ESD 
> filter at this 
> port?
> 
> I also thought I would mention something that is so often 
> over looked in 
> the consideration of ESD protection - as the design I am 
> concerned with is 
> for handheld battery operated equipment, there is no real 
> return path for 
> the ESD to flow in and hence I don't think this concept can 
> not be used 
> with (small) battery operated equipment when looking at 
> signal integrity 
> issues.
> 
> Nigel.
> 
> At 10:22 14/10/2003, you wrote:
> >Nigel, Steve
> >
> >I would add that protection devices always degrade the 
> system performance
> >somewhat.  Transzorbs have a capacitance of 1000pF or more 
> at a few volts,
> >so are not normally suitable for signal bandwidths of 
> greater than around 1
> >MHz.
> >
> >The protection sometimes has to be a compromise, using 
> shielding, resistors
> >and diodes for high speed interfaces, with special 
> consideration to ESD
> >return currents via protection diodes and filtering in the 
> power supplies.
> >RF people use filtering such as a shunt choke and/or 
> sometimes spark-gap
> >devices to achieve low capacitance, or special low 
> capacitance limiter
> >diodes which are not appropriate for DC coupled or low frequency
> >applications.
> >
> >Regards
> >Geoff
> > > -----Original Message-----
> > > From: steve weir [mailto:weirsp@xxxxxxxxxx]
> > > Sent: 14 October 2003 09:45
> > > To: nigel.hughes@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
> > > Subject: [SI-LIST] Re: ESD protection devices
> > >
> > >
> > > Nigel, varistors have the slowest response of available
> > > devices as they
> > > have always been fairly inductive.  Varistors also have
> > > traditionally had
> > > the least well controlled hold and breakdown voltage
> > > tolerances.  General
> > > purpose zeners have a soft VI curve, and limited energy handling
> > > capacity.  Transzorb type devices are the most expensive but
> > > handle high
> > > energy surges, have comparatively very flat VI curves, low
> > > inductance, but
> > > alas higher prices.  Vendors have device performance curves
> > > and lots of
> > > application notes.
> > >
> > > At 09:30 AM 10/14/2003 +0100, Nigel Hughes wrote:
> > > >Hello Forum,
> > > >
> > > >I am trying to compare and evaluate ESD protection devices.
> > > According to
> > > >the device physics, what are the things that limit the speed
> > > at which a
> > > >zener or varistor responds?  I have heard previously 
> that a varistor
> > > >responds quicker but read that the device itself is damaged
> > > when an ESD
> > > >event passes through it.  However others say that with the
> > > new multi-layer
> > > >structure of surface mount devices, the damage is negligible
> > > (if occurring
> > > >at all).  Has anybody got a better perspective of what the
> > > truth actually
> > > >is.  A bit of background supporting information would be helpful.
> > > >
> > > >Thanks,
> > > >Nigel.
> > > >
> > > >=============================================================
> > > ==========
> > > >O O O Nigel Hughes                      Tel +44 1763 266266
> > > >O O O Terminals                 Fax +44 1763 261216
> > > >O O O TTPCom Ltd.
> > > >           Melbourn Science Park, Cambridge Rd., Melbourn,
> > > Royston, Herts
> > > >SG8 6HQ UK
> > > >           nigel.hughes@xxxxxxxxxx
> > > >=============================================================
> > > ==========
> > > >
> > > >
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> ==============================================================
> =========
> O O O Nigel Hughes                      Tel +44 1763 266266
> O O O Terminals                 Fax +44 1763 261216
> O O O TTPCom Ltd.
>           Melbourn Science Park, Cambridge Rd., Melbourn, 
> Royston, Herts 
> SG8 6HQ UK
>           nigel.hughes@xxxxxxxxxx
> ==============================================================
> =========
> 
> 
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_________________________________________________________

Zetex Semiconductors - Solutions for an analog world

EID Award Winners for  'Best Use of Technology' 2003 for the 
AcoustarTM ZXCW8100 End-to-End Digital Audio Amplifier Controller

http://www.zetex.com
_________________________________________________________

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