They seem to have good lifetimes. You can't fire an avalanche stack at a very high rate!
There is a Zetex appnote about using them in series and parallel. The cool thing about a series stack is that a small pulse, TTL class, can trigger the bottom one, and the whole stack zippers.
The Zetex parts are designed to avalanche. They are made in Russia, probably on an ancient diffusion line. They have really low Fts, like
40 MHz, which good avalanche transistors seem to.-- John Larkin Highland Technology, Inc lunatic fringe electronics
Aaaand...it's dead. Oh well, fun while it lasted. :^)
No visible damage, but the three high side are ~shorted.
Just a show of the last version: 10pF Miller caps, which kicked dV/dt up to
17k/us or thereabouts (that's 100ns at 1.7kV).Tried 220 ohm damper resistors in series with the 10p's, but that wasn't actually helpful/useful (actually, may've been harmful? no idea). (The ringing was something else.)
Ferrite beads (type #31) on the gates, and then bigger ones on the gate drive pairs (emphasis on the middle ones, because, guess why!), fixed the
100-400MHz screaming pretty nicely, as confirmed by near-field loop "sniffer".I realized early on, my conspicuous lack of high voltage, wideband probes... well, that won't do.
So....I know!
Probe circuits are traditionally drawn with "speed up caps" across the divider resistors, and nothing else, but that's dumb. It's an impedance equalizer, going from ~megs at DC, down to whatever the transmission line ratio needs to be at HF. It looks like a capacitor at middle frequencies, and a resistor (of one value or the other) at the asymptotes. In this case,
100:1 into 50 ohms is 5k, and 5k/10 is 500, so the distributed resistors are 500 ish.Failure to observe the correct design process results in heavily ringing waveforms (the coax is shorted by caps at either end, so resonates like an inductor), and in mistakes, like having to solder your resistors in after-the-fact in a layout that wasn't made to fit them. :-)
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website:
You maybe. I've blasted a 2N3904 at 100kHz. :)
And I said with current. This is in the spec sheet too: you don't get too many peak kilowatts, before the life curve goes from "she'll outlive me" to "you have one second of run time, better make it important".
Yup. Might also be neat to build a Marx generator with them, though again, you have the problem of current. Any transmission line you hang on the output is going to be hundreds of ohms, and after a few kV, you get into transistor-popping currents, no problem.
And then the problem spirals geometrically out of control.
To make something like IEC 61000-4-4 EFT, you need about a hundred of the poor things, in series-parallel. Such an innocent noise signal -- but it's brutally spiky.
It's too bad HOTs are no good. They have really low fT and were made on ancient diffusion lines (or maybe some epitaxy too).
That at least leaves some HOTs useful for SRD, as you've found.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
Much shorter lifetime -- though, in their intended use, a slightly bigger bang. ;D
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
Find that appnote Tim. They'll avalanche 60A/20ns for >10E11 cycles. See e.g.
Clifford Heath.
Oh, nice! I'll have to check Altium out. :)
Your schematic looked a bit Inventorish. I took an AutoCAD class at the junior college last year, and the prof also made us use Inventor a few times. I found it a bit cumbersome, and I just preferred AutoCAD.
Thanks,
Michael
2N3904 likely isn't as robust, but also, notice how much it drops with pulse width -- returning to the 61000-4-4 example, the nominal pulse width is 50ns, but that's only to the 50% level of an exponential decay. Diodes Inc. defines pulse width as half-cycle sinusoid,
That simple hyperbola seems to suggest a deeper truth: that the conductive channel has constant voltage drop, and the delivered energy needs to be limited to a constant.
Let's see. If the curve is an exponential decay, with a half-life of 50ns, then the time constant is 72ns, and the total area under the curve, divided by the amplitude, is simply the time constant. So it's more than 50ns, but less than 100 at least, not quite as bad as I had been thinking.
There is quite a large disparity between the "no failure" and modest-life curves. For this curve, it's about 16A vs. 60A.
Which would be switching impedances of 19 ohms, and 5 ohms respectively, which is pretty damn low! And, a stack of ten gets you 3kV into 50 ohms -- supplying up to 10 hours of continuous EFT duty!
Not as bad as I remember calculating before.
Still, if you need long life, the point about parallel strings stands -- that's a ~4x reduction in load current, so you need a four strings of ten to switch it.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
No, I use the latest rev of Altium Designer. However my PCB engineer, Chuck Fisk, who retired a few years ago, stayed with Protel. Re: colors, I'm constantly honing my choice. I dislike Altium's default ground symbol, but haven't figured out how to change it.
--
Thanks,
- Win
Remember my advice: "You should have two sets of drivers and gate transformers, one each for high and low sides. Then you can create an adjustable deadtime."
Switching one side on at the same time the other is going off, can create a damaging rail-rail shoot-through. The shoot-through name refers to fast hidden high-current spikes. High di/dt means high V = L di/dt voltage spikes, blow out gates.
--
Thanks,
- Win
Avalanching old 2nXXXX parts, like in the old Tek samplers, gave pulses that were some fraction of the supply voltage, half maybe. The Zetex things really turn on basically saturate, when they fire.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
AN8:
Cheers, James Arthur
High enough di/dt will also blow up wires. I've seen metal-case transistors lose internal wiring that way.
My experience with 2N3904 is it's about 10 ohms "on", which squares more or less with typical data on RC and RE. Given that those parameters will be a bit lower due to the huge charge injection.
Recovery time is about 5-10us.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
Put your vote in:
I like the default colors well enough, but the libraries are horribly ugly. So I've got my libraries with everything drawn the way I like.
I've taken to making discretes shaded light blue, and ICs yellow, using descriptive symbols or pinouts where possible. (So TL431 is a yellow blob with a zenery symbol inside, suggesting its dual nature of transistorishness and ICness.)
Here's a very discrete schematic...
I also rather like making shiny 3D models, though it's a pretty bad way to create them. I tend not to put in quite this much detail, unless I'm doing it on my own time.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
Like I said, the dead time is huge -- it's driven with a short pulse, then zero, then an opposite pulse, then zero. There's no opportunity for shoot-through. It also helps that the off-side gets reverse bias at the same time, increasing the noise margin even further!
Still, because of dV/dt, and because of common mode coupling in the transformer, it might be the problem. Dunno. (But if that's the case, then splitting the transformer would actually make it worse, because of the lack of reverse bias. Hmm.)
Measuring the actual gate waveforms, accurately, is unlikely to succeed, so it's hard to do more than speculate...
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
I've seen nuclear bombs lose internal wiring that way. :^)
(Exploding bridgewire, and slapper: supposedly these are used to ignite the charge. The limited availability of JL's krytron seems to corroborate this well enough, as do other available documents.)
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
Incidentally, I picked up some of the cheaper (not quite) alternative, FZT857. Rated for 350V, but this one actually snaps beyond 550V!
Seems to avalanche consistently, though appears to have 11.3 ohms ESR when "on".
Output waveform is interesting, and I don't think it's due to circuit parasitics (I've got a big fat 1.5nF silver mica on there, so it's not without stray L).
Circuit is: +HV -- 100k charging resistor -- 1.5nF to GND -- FZT857 (C to E) -- 50 ohm BNC (terminated). There's 2.2k B-E to make it click.
Rise time is 6ns, up to a plateau at 420V for 14ns. (Vce starts at 580V before breakdown, so the transistor is dropping a lot of voltage and current during this time.) Time constant is 92ns, suggesting 61.3 ohm loop resistance, 50 of which is the termination, so the transistor appears to account for about 11 ohms.
Recovery (recombination) time about 30us. Varies with size of the pulse (i.e., size of capacitor).
...
Oh, scratch that... it seems I broke it. It's still avalanching, but now it's doing it at 440V. I wonder if the plateau is not so much a "buildup" phase, but a "she can't take anymore o' dis, captain" phase, that causes damage. (Can semiconductors run out charges to free, so the current saturates?)
I'm sure a large part of the cost, of the FMMT417s, is just aging. Remember back in the days when the manufacturers had huge racks of toobs glowing away? Yeah, like that...
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
-- Never piss off an Engineer! They don't get mad. They don't get even. They go for over unity! ;-)
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