max AC coupled input on MOSFET with SPICE - Page 1
Forum section: electronics / projects

author: Gibson486
author_rank: Frequent Contributor
timestamp: March 26, 2020, 01:44:13 am
content: Dumb question....The max Vgs on a mosfet, does that still apply for AC coupled signals? I was playing around in Spice and i biased my mosfet, but it allows me to put AC signals at the gate that exceed the max Vgs. I am guessing that in real life this would lead to magic smoke? Or does this actually work?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 26, 2020, 05:29:36 am
content: Vgs is instantaneous, though exceeding it transiently (for some ns or us at a time, at very low duty cycle) is usually possible without damage.What's it matter? You aren't getting any amplification above about +8V, or below 1V, for most types. Bias+drive much beyond that range isn't going to accomplish much..?Note that it's Vgs, not Vg, and voltages are always relative between two points. A rather beginner note, but as you didn't provide any info about your simulation I'm adding this just in case.SPICE doesn't model breakdown by itself, so you're free to, say, put 1kV on the gate, or -1kV on an NPN base; just don't expect anything interesting to happen in the model, and don't expect it tonotdo something interesting (magic-smoke-related, say) in the real world.(Breakdown can be added to the model by strategically placed zeners; BJT breakdown can be repetitive so this can be useful, but gate breakdown is abrupt, damaging and permanent, so there's not much point in modeling it.)Tim

author: Zero999
author_rank: Super Contributor
timestamp: March 26, 2020, 12:13:47 pm
content: Yes, SPICE will merrily carry on simulating, even if the components are being pushed to smoking point. Some simulation packages have attempted to warn or stop, if components get damaged, but the only one I've used did a pretty poor job. It would stop working if the current rating of a transistor was exceeded, even by 1µA for just 1µs, which was no good as any transistor can cope with such a short duration transient. I'd just disable that feature, because it was annoying.One should look at the voltages, currents, power/energy dissipation of different parts of the circuit. Yes, I mentioned energy and power separately which might seem silly but for very short transients, it's more important than power. For example, a TRIAC might be able to withstand a surge of 100A, a power dissipation of a couple of hundred Watts, over a very short duration, but only rated up to 10A continuously.

author: Gibson486
author_rank: Frequent Contributor
timestamp: March 26, 2020, 12:22:10 pm
content: I amplified to just under a 1kV pk-pk sine signal, but I wanted to buffer it to make it lower impedance out. LtSpice is letting me buffer it through a push-pull amp with the that signal at the source. I knew that may not fly in the real world. I guess I have to make the push pull buffer with the gain built in somehow. During this isolation shutdown, I am trying to make a mini and cheaper Trek High Voltage Amplifier (like thishttps://www.trekinc.com/products/PZD700A.asp). I thought that it was just a transformer, but I do not see a transformer with that high of a ratio giving their claimed current output.