R:[SI-LIST] : Zener used to clamp Vcc?

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From: Vigliarolo Roberto (Vigliarolo@asamrt.interbusiness.it)
Date: Mon Feb 28 2000 - 03:07:21 PST


Ray...

I don't understand your suggestion regarding the use of a Schottky diode to
clamp transient overvoltages on power supply: where do I need to connect the
Schottky diode?

Chris...

I routinely use zener diodes on my power supplies as a form of protection
against transient overvoltages but I use the TVS type (Tranzorb, Transil,
etc.). They use very big chips and thermal mass inside the package so the
can absorbs huge amount of energy before going short circuited.
At that point if the protection of the power supply you use has a lower
I_square_t than the _square_t of your short circuited zener the power
supply should go off and during repairing you only need to replace the TVS
and find the cause of the overvoltage (may be a fault in the voltage loop of
your power supply).
If the I_square_t of your short circuited zener is lower than the
protection of the power supply you use your TVS will open and you end up
with no protection against overvoltages and possibly a lot of damage to your
board.
You need to trade off the diminished reliability of using one more component
(it can fail on its own) with the benefits of having protection against
transient overvoltages.
Only one time I had a reliability problem caused by TVSs but it was due to
damage to the package during the assembly bending progess (it was a leaded
device).
If you are interested you can go to ST site (www.st.com) and search for an
application note titled "Protection by Transil: how to ensure absolute
safety".

Regards,

Roberto Vigliarolo

 Ferrovie dello Stato SpA
 Tecnica e Ricerca
 Unificazione Tecnica
 Viale Lavagnini, 58
 50129 Firenze
 Italy
 
 Tel: +39-055-235-3848
 Fax: +39-055-235-3581
 Email: Vigliarolo@asamrt.interbusiness.it

> ----------
> Da: WAUGH,RAY (HP-SanJose,ex1)[SMTP:ray_waugh@agilent.com]
> Risposta a: si-list@silab.eng.sun.com
> Inviato: venerd́ 25 febbraio 2000 23.45
> A: 'si-list@silab.eng.sun.com'
> Oggetto: RE: [SI-LIST] : Zener used to clamp Vcc?
>
> Chris...
>
> The Zener will do fine if the spikes you are clamping are not too fast
> (sub
> nano-second). Fast spikes must be clamped with a Schottky diode
> (picosecond
> fast).
>
> 400mW refers to the continuous power your diode can dissipate if you
> maintain the leads at 25 degrees C. That rating has no meaning when you
> are
> considering short duration current spikes. You can safely dissipate far
> more than 400mW in that diode if you only do it for a couple of
> microseconds. It really is a fairly complicated thing to figure out
> current
> carrying capacity (which is what you are really interested in).
>
> Ray
> ------------------------------------
> ray_waugh@agilent.com
> ------------------------------------
>
> -----Original Message-----
> From: Chris Bobek [mailto:cbobek@cadence.com]
> Sent: Friday, February 25, 2000 2:20 PM
> To: Si-list
> Subject: [SI-LIST] : Zener used to clamp Vcc?
>
>
> Hi,
>
> I was instructed to use a 5.6V Zener diode on the 5V Vcc plane of our
> board to clamp the voltage in the case of voltage spikes on the power
> supply. The Zener diode they chose is a 400mW diode. In your
> experience, is this the best way to handle power supply spikes? Is
> 400mW enough to accomplish anything? Our power supply is a 500W supply,
> I believe. The board typically draws 2A.
>
> I'm not sure what power supply spikes they are referring to. When I
> look at Vcc on a scope, the rising edge is a nice, quick, R-C charging
> curve. There's a lot of capacitance on our board and supply that
> dampens this response.
>
> Thanks for your help,
>
> Chris
>
>
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