The device model matches the published data—that ain’t necessarily so

The good folks from whom you obtained your SPICE tool have made all kinds of assumptions in setting up their device models. However, some of those assumptions might not agree with some of the published data sheets for the simulated devices. We can see this peril just by looking at one very simple example.

Those famous words of the character “Sportin’ Life” in George Gershwin’s opera “Porgy and Bess” have an applicability here that Mr. Gershwin never envisioned.

Please consider this excerpt from a respected supplier’s datasheet for the 1N4148 diode in Figure 1.

Figure 1 An excerpt from the 1N4148 datasheet. Source: John Dunn

One might assume from this chart that if 100 µA of reverse current flow were acceptable, one could apply up to 100 volts of reverse voltage across the diode. With that assumption in hand, I can picture myself happily using the 1N4148 at 100 volts as I go ahead with a SPICE simulation of my design. Then to my embarrassment, I would find that my simulation does not work.

In looking for the reason for my failure, I might set up a SPICE simulation to trace the characteristic curve of the 1N4148 which would yield the result in Figure 2.

Figure 2 Characteristic curve examination for the 1N4148 used at 100 volts. Source: John Dunn

I would discover that the 1N4148 in my version of SPICE exhibits reverse breakdown with an application of only 80 volts which is nowhere near the 100 volts of my supplier’s data sheet.

Looking at two steady state conditions would provide the breakdown confirmation (Figure 3).

Figure 3 A confirmation of the reverse voltage breakdown of the 1N4148. Source: John Dunn

At 79 volts, the 1N4148 simulation carries a little more than 14 µA but if I take that reverse voltage up to 81 volts, the current shoots up to more than one ampere.

Just for fun, I took the reverse voltage up to 100 volts and the simulation current rose to 30.91 amperes. Try to picture a puny little 1N4148 trying to carry that much current. The device would be dissipating more than three kilowatts.

What was that flash?

The object lesson of all this is that if you are pushing a component to anywhere near a maximum rating and you are using a simulation tool to evaluate your design, make sure that your simulation models conform to your assumptions and conform to published data about each and every component’s rated maximums.

They might not.

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

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