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'[EE]: Zener Diode Speed'
2001\03\09@122859 by Sean Breheny

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Hi everyone,

Quick question: how fast are zener diodes, typically? Specifically, I'd
like to know how long it takes for it to reach equilibrium state when
reverse bias is applied.

No, this isn't for a voltage spike protection circuit, it's for a
modification to an existing MOSFET driver to reduce the gate drive
voltage, and make it less dependent on supply voltage.

Thanks,

Sean

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2001\03\09@153959 by Dan Michaels

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SB wrote:
>
>Quick question: how fast are zener diodes, typically? Specifically, I'd
>like to know how long it takes for it to reach equilibrium state when
>reverse bias is applied.
>

Slow.

Typical zener capacitance for Vz < 10v is in the range 100-1000 pF,
according to my databook from Microsemi. And lower Vz --> higher cap.

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2001\03\09@164340 by Sean Breheny

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Hi Dan,

Thanks. Is the capacitance the usual limiting factor (instead of carrier
transport delays, etc. in the semiconductor)? In my case, the diode would
be fed by a ~1K resistor, so a 1000pF cap would result in a time constant
of about 1 microsecond, which would be totally acceptable for this
situation. I would only be concerned if the time constant were more than
several microseconds. Is that the case?

Sean


On Fri, 9 Mar 2001, Dan Michaels wrote:

{Quote hidden}

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2001\03\09@170044 by Spehro Pefhany

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At 04:44 PM 3/9/01 -0500, you wrote:
>Hi Dan,
>
>Thanks. Is the capacitance the usual limiting factor (instead of carrier
>transport delays, etc. in the semiconductor)?

Yes.

>In my case, the diode would
>be fed by a ~1K resistor, so a 1000pF cap would result in a time constant
>of about 1 microsecond, which would be totally acceptable for this
>situation. I would only be concerned if the time constant were more than
>several microseconds. Is that the case?

Yes, but the capacitance is far from constant, it's much higher when the
voltage across the junction is low (which is probably when you'd like it
to be low). Lower current zeners have less capacitance. eg. 1N75x

A clamp formed by a zener biased with a resistor and a switching diode
to the clamp voltage (and to ground, if you need it) might be better for your
purposes.

Best regards,

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2001\03\09@175944 by Bob Blick

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On Fri, 9 Mar 2001, Sean Breheny wrote:
> Quick question: how fast are zener diodes, typically? Specifically, I'd
> like to know how long it takes for it to reach equilibrium state when
> reverse bias is applied.

Hi Sean,

Zeners are fast enough for your application. The capacitance is the
dominant effect, although it changes with applied voltage. That's what I
remember, and just to refresh my memory I tested it on a 1N4742A zener
right now. It's a 1 watt 12 volt zener. On a cap meter it measured about
100pF. When I put the zener in series with 10k ohm and applied a 40 V P-P
square wave, it clips as expected with fairly symmetrical rise and
fall. Replacing the zener with a 100pF cap showed similar edges(no
clipping of course).

Cheers,

Bob

P.S. Zeners as mosfet gate clamps are good. :-)

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2001\03\09@232410 by Dan Michaels

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Sean wrote:
>Hi Dan,
>
>Thanks. Is the capacitance the usual limiting factor (instead of carrier
>transport delays, etc. in the semiconductor)? In my case, the diode would
>be fed by a ~1K resistor, so a 1000pF cap would result in a time constant
>of about 1 microsecond, which would be totally acceptable for this
>situation. I would only be concerned if the time constant were more than
>several microseconds. Is that the case?
>

Hi Sean, I don't really know specifics about the internal physics of
zeners, just looked in the databook, and saw the graphs. Different
families/manufacturers may have slightly different characteristics.
A couple of years ago, I tried tying a 5.1v zener thru a 330 ohm
resistor to a PIC pin, for OV protection, but it killed the
responsiveness that I needed. It may be ok for your app. You might
just try it and see how it goes.

- dan michaels
http://www.oricomtech.com
==================

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2001\03\10@135746 by Sean H. Breheny

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Thanks to Dan,Bob, and Spehro for the quick answers. As it turns out, I may
be making an even greater change to the design, but if not, I will try
using the zeners.

Thanks,

Sean

At 11:24 PM 3/9/01 -0500, you wrote:
{Quote hidden}

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