In fact, most HOTs (Horizontal Output Transistor -- 1500Vcbo, 5-20A, Vce(sat) 2-5V @ hFE = 2.5) handle 1600-1800V at breakdown.
Most trigger in avalanche rather slowly (~100ns), though I've seen a few that go quickly (
Try making less current available. Maybe they'll turn off.
Probably no relation. DSRD is a step-recovery effect.
Grehkov did observe both the DSRD effect and a very fast avalanche mode in rectifier diodes. Combined, DSRD driving avalanche, he got phenominally fast multi-kilovolt edges. But his avalanche mode had to be driven by a really fast edge, to over-voltage the junction before it realized it.
Does anybody still make HOTs as such?
Modern transistors tend to not avalanche well. Bad doping profile or something.
If you drive a 2N7002 hard enough, it will turn on in under 1 ns.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
There's a reason they mostly used a driver transformer o n those. They need to be FORCED off otherwise dv/dt cooks them with that inductive load. Mayb e you don't have that inductive load but if you want the Vce to rise fast y ou really have to work to turn it off, because you are asking for the same thing. Just because it is not inductive means nothing, at the higher voltag es all the same rules apply.
I am not saying use a transformer, that is only good for a fixed frequency and you haven't bled out the little piece of information yet. If you could rig like a -2.5 volt source for the bottom of a totem pole type driver that should turn it off just fine. Keep the resistances low and you'll only nee d maybe 3.5 volts for the positive side of the driver circuit. I you go wit h higher impedances you'll probably need a speedup cap which then makes it frequency (and other things) dependent. At this point I assume you want inf initely variable frequency and duty cycle, up ti the transistors limits of course.
Like a quenching resistor on a SPAD? Can you trigger at some lower voltage and have them turn off? (I thought the 2n3904's were OK... did you try 2n4401's)
George H.
FYI, this is an... unconventional mode of operation.
Only the falling edge matters.
The transistor is behaving somewhat like a spark gap. Corona and all. C-E leakage (from applying a high voltage C-E) acts somewhat like corona, with the current flow being fairly small over most of the range, then increasing sharply as you approach breakdown.
At breakdown, the transistor randomly becomes very conductive. In fact, it becomes too conductive for the amount of current applied, so the voltage discharges down some amount.
For zener diodes at low bias, this looks like a noisy relaxation oscillator, with 10s of mV of noise.
For BJTs, it's rather more exaggerated, with the noise being in the several-volts range. See:
And yeah, the probe wasn't compensated correctly, so the falling edge appears to descend below GND. Oops...
Transformer coupling can be useful for triggering things (a pulse is more likely when the base is driven forward sharply):
If I used a HOT in conventional switching, it would be impressive to achieve falling and rising edges under 100ns. A good avalanche transistor (in a similar voltage class) is 10ns. The 2N304 seems to be under 1ns pretty consistently, and classic 2N2369 pulse generators are typically measured in the 200-500ps range.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com wrote in message news:95a7e768-37e9-4c58-8532-2f85ca12289a@googlegroups.com... There's a reason they mostly used a driver transformer o n those. They need to be FORCED off otherwise dv/dt cooks them with that inductive load. Maybe you don't have that inductive load but if you want the Vce to rise fast you really have to work to turn it off, because you are asking for the same thing. Just because it is not inductive means nothing, at the higher voltages all the same rules apply. I am not saying use a transformer, that is only good for a fixed frequency and you haven't bled out the little piece of information yet. If you could rig like a -2.5 volt source for the bottom of a totem pole type driver that should turn it off just fine. Keep the resistances low and you'll only need maybe 3.5 volts for the positive side of the driver circuit. I you go with higher impedances you'll probably need a speedup cap which then makes it frequency (and other things) dependent. At this point I assume you want infinitely variable frequency and duty cycle, up ti the transistors limits of course.
What do you need this for? If you don't need a lot of voltage swing, there are better ways to get clean, fast edges nowadays.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Just research.
If you need a possible application, think about an IEC 61000-4-4 EFT pulse generator. You'd need a lot of 2N7000s for that... :)
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
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