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

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From: WAUGH,RAY (HP-SanJose,ex1) (ray_waugh@agilent.com)
Date: Fri Feb 25 2000 - 14:45:42 PST


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
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-----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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