The system mannequin matches the printed knowledge—that ain’t essentially so

The great people from whom you obtained your SPICE instrument have made all types of assumptions in establishing their system fashions. Nevertheless, a few of these assumptions won’t agree with a few of the printed knowledge sheets for the simulated gadgets. We are able to see this peril simply by one quite simple instance.

These well-known phrases of the character “Sportin’ Life” in George Gershwin’s opera “Porgy and Bess” have an applicability right here that Mr. Gershwin by no means envisioned.

Please think about this excerpt from a revered provider’s datasheet for the 1N4148 diode in Determine 1.

Determine 1 An excerpt from the 1N4148 datasheet. Supply: John Dunn

One would possibly assume from this chart that if 100 µA of reverse present movement have been acceptable, one may apply as much as 100 volts of reverse voltage throughout the diode. With that assumption in hand, I can image myself fortunately utilizing the 1N4148 at 100 volts as I’m going forward with a SPICE simulation of my design. Then to my embarrassment, I’d discover that my simulation does not work.

In searching for the explanation for my failure, I would arrange a SPICE simulation to hint the attribute curve of the 1N4148 which might yield the lead to Determine 2.

Determine 2 Attribute curve examination for the 1N4148 used at 100 volts. Supply: John Dunn

I’d uncover that the 1N4148 in my model of SPICE reveals reverse breakdown with an software of solely 80 volts which is nowhere close to the 100 volts of my provider’s knowledge sheet.

Taking a look at two regular state circumstances would supply the breakdown affirmation (Determine 3).

Determine 3 A affirmation of the reverse voltage breakdown of the 1N4148. Supply: John Dunn

At 79 volts, the 1N4148 simulation carries a bit greater than 14 µA but when I take that reverse voltage as much as 81 volts, the present shoots as much as a couple of ampere.

Only for enjoyable, I took the reverse voltage as much as 100 volts and the simulation present rose to 30.91 amperes. Attempt to image a puny little 1N4148 making an attempt to hold that a lot present. The system could be dissipating greater than three kilowatts.

What was that flash?

The thing lesson of all that is that in case you are pushing a element to anyplace close to a most ranking and you might be utilizing a simulation instrument to guage your design, guarantee that your simulation fashions conform to your assumptions and conform to printed knowledge about every element’s rated maximums.

They won’t.

John Dunn is an electronics guide, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York College (MSEE).

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