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

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From: Tom Dagostino (tom_dagostino@mentorg.com)
Date: Mon Feb 28 2000 - 14:57:16 PST


I'm curious what the actual function of the Zener diode will be. A simple
model of a Zener diode is a reference supply of 5.6 volts in this case, with
an ideal diode with Vforward of zero volts and a resistor in series with it.
Parallel with these 3 components is a capacitor. In devices like the one
you are proposing to use the R value is on the order of 2 to 5 Ohms and C is
100 to 200 pF. If you are trying to clamp narrow transients generated on
the board with a single Zener diode, where are you going to place it? To be
effective the Zener has to be near the source of the spikes. Otherwise the
spikes may see the entire power supply bypass system before it sees the
Zener. If the spikes are narrow they should be taken care of by the supply
bypass system.

If you are trying to protect the board from spikes coming from the power
supply this seems like too small of a device to protect the board. To get
any action from the Zener you will have the output impedance of the Zener to
contend with, in this case 2 to 5 Ohms. Your 500 watt supply can source 100
amps. For any reasonable spike from the supply the 2 Ohms series resistance
will not effectively protect the board from over voltage. I suspect the
Cdv/dt of the bypassing system will do a better job.

True, the diode may be able to dissipate 30 watts for a few microseconds but
to get 30 watts you will have almost 3 Amps flowing in the Zener. The
terminal voltage of the Zener will be over 10 volts (5.6 volts plus (3 amps
* 2 Ohms), likely to blow any devices on the board. To clamp you need low
impedance compared to the impedance of the source power supply in this case.

Tom Dagostino
ICX Modeling Group
tom_dagostino@mentor.com
503-685-1613

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of WAUGH,RAY
(HP-SanJose,ex1)
Sent: Friday, February 25, 2000 2:46 PM
To: 'si-list@silab.eng.sun.com'
Subject: 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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