[SI-LIST] Re: ESD protection devices
- From: Nigel Hughes <nigel.hughes@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 14 Oct 2003 11:40:29 +0100
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
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> > Royston, Herts
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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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