M/A-com SOT-23, MUR420, 1N4007 M/A-com, Metelics, M-Pulse, MicroMetrics. All SRD makers start with the letter "M" http://www.scottyspectrumanalyzer.com/srd/srd.html ______________________________________________________________________ My favorite Grekhov diode is the c-b junction of a power transistor! Probably nobody else in the world knew that when I discovered it. A qualitative understanding of semiconductors are enough to train one's instincts. I did suspect that this particular transistor was a diffused junction device, and that diffused junctions can have SRD doping profiles. But I didn't need to understand the gory details. John ______________________________________________________________________ There's also the Grehkov drift step-recovery effect: a diode that normally would be a mediocre, soft snapper (classic mode, dc forward bias followed by rapid current reversal) can become a very nice, very fast step-recovery diode if it's only forward biased for a few hundred ns. The charges don't have time to spread out much, so are swept away very cleanly when the current reverses. This can play nicely into the typical anti-shoot-through gate drive timing of a fet totem pole: current happens to be flowing into the motor, bottom n-fet is on, conducting "backwards", drop the bottom n-fet gate drive, wait a couple hundred ns for anti-overlap, during which time the drain swings negative and the substrate diode conducts, then the upper fet turns on, fet-fet current builds up big time, lower fet substrate diode snaps off, zowie! John ______________________________________________________________________ The higher-voltage power diodes, like a 4007, tend to act like PIN diodes. They can make decent drift step-recovery diodes, too. Try this some time: apply 48 volts in the forward direction to a 1N4007 or 1N4009. The current will ramp up pretty linearly to numbers like 50 amps in 100 ns or so. Now apply a lot of reverse current, like 50 amps, through a small inductor. It will conduct in the reverse direction for maybe 50 ns then snap off, making a huge (kilovolt maybe) spike across the inductor, just a couple ns wide. Lotsa EMI. Power rectifiers can do the step-recovery thing even at 60 Hz. That can cause mysterious problems elsewhere in a box. A Russian guy, Grehkov, discovered the DSRD effect and another, even faster plasma breakdown effect, in common power diodes. John >How the hell do you do that, for just the 100ns? Surely any device >that could deliver the 48 volts and 50 amps would produce heaps of > EMI all by itself? We just used some power mosfets. The forward bias supply was 48 volts and the reverse was 400. We used a different diode, something we could cool properly, to get the rep-rate up. But lots of cheap power diodes will snap like gangbusters. http://www.highlandtechnology.com/DSS/T220DS.html ftp://jjlarkin.lmi.net/T220_Neon.jpg We designed this for the LEAP tomographic atom probe. Didn't sell many, but it was fun. Lots of things turn out to be fun but don't sell many. John ______________________________________________________________________ John Larkin wrote: > A 1N4007 can also be used as a drift step-recovery diode and as a > plasma avalanche diode. Together, two can generate a kilovolt edge > with a 100 ps risetime. Okay, so I'm intrigued already. I have all the hardware available--two 1N4007s and a 3 kV adjustable power supply! How do I build one? Cheers, Phil Hobbs Google "Grekhov diode." A lot of the papers are for members only, but this one gives the general idea: http://www.ece.jhu.edu/~pps/ECE777/ADMAT/CircDev/Pulse-GENERATORS-SHAPERS/sub-nano-pulse1.pdf > Look up Grekhov Diode. It's pretty much a regular rectifier used as a > HV SRD. The trick is to forward bias it (like, 50 volts forward!) for > a few 10's of ns, then reverse it hard. Grekhov discovered both the DSRD and the plasma avalanche effects in cheap power diodes. The core of the DSRD effect is that, if a PIN diode is forward biased for not too many nanoseconds, the carriers don't have time to float all around the place so the charge profile is good for a nice reverse snap. HP did the same thing in their classic 1430 12-GHz sampling head, circa 1965 roughly. This box used the DSRD effect, in a semiconductor that one would not expect to be used in an application like this... http://www.highlandtechnology.com/DSS/T220DS.html We bias the snap diode +48 volts (yes, forward direction) for about 80 ns before we turn the drive around for the snap. It was originally designed for use in a LEAP atom probe. John ______________________________________________________________________ On 28 Oct 2003 19:38:42 -0800, (E-Mail Removed) (Dyan Ali) wrote: >Hello, >I'm trying to design an ultrafast pulse generator, one that operates >at 3.4kV, has rise times and fall times ~1 nsec and a pulse width of >about 5-6 nsec.I need the pulse rate to be 10Hz. I've got hold of a >few papers by R.J.Baker (available at >http://cmosedu.com/jbaker/papers.htm) but they provide info on >designing broader pulses, maybe because the power MOSFETs he was >working with were inherently slower. Is such a design possible? Could >someone help me out? >-Dyan Fets are too slow for your risetimes. There are a few ways to do this: 1. A stack of avalanche transistors, probably dumping a coaxial energy-storage line into your load. You'd need about 10-20 of them in series, depending, probably the Zetex SOT-23 (!) parts. There are papers around on how to do this. 2. A triggered spark gap or a krytron tube. Very simple. 3. A DSRD (drift step-recovery diode) or Grehkov avalanche diode. 4. A coaxial reed relay, again dumping a storage line into your load. The relay can be inside a tube that *is* the coaxial system. Cheap, clean, *lots* of jitter if it matters, maybe reasonable life at 10 Hz. 5. Maybe even a regular relay. This actually works sometimes, even at these speeds. 6. Buy it from the Russians; they are very good at this sort of stuff. If any of these are interesting, let me know and I can find a few paper references at work tomorrow. Any of these would be a *serious* development project, except maybe 2) or 4) or 5), which wouldn't be as terrible. Measurement will be interesting. I currently make a 2KV thing that's sort of in the ball park... http://www.highlandtechnology.com/DSS/T220DS.html which used DSRDs, and we're working on other techniques for bigger stuff. What's your application? What's the load like? John >I'm actually trying to build my own driver for a KDP Q-Switch, so that >translates to a capacitance of 5pF (at 1kHz). The thing is that we are >trying to ubild one such device rather than buy it. >Dyan OK, cool. You only need half the voltage at the pulser if you use a longish coax feed and let the pulse voltage naturally double at the end. And I presume you can't tolerate much jitter, so relays are out. A krytron can give you jitter close to 1 ns, and avalanche transistors will be even better. There should be lots of journal papers on stuff like this (RSI, NIM, etc), and Zetex has some app notes. John ______________________________________________________________________ Actually, it acts more like, no, it acts exactly like a battery. The voltage is almost constant until the charge is gone -- that means the diode is actually producing energy (positive voltage, negative current) as the charge leaves. FYI, a simulation of 1N4007 driven at If = 1A and turning off at -100A/us reaches a peak current I_rr of 6.978A in 69.7ns before turning off. A proper high speed diode like MUR420 does it in 1/3 the time, and accordingly about 1/9th the charge (Q_rr). Real recovery time of a 1N4007 is probably a lot worse (4us+?). Tim The higher-voltage power diodes, like a 4007, tend to act like PIN diodes. They can make decent drift step-recovery diodes, too. Try this some time: apply 48 volts in the forward direction to a 1N4007 or 1N4009. The current will ramp up pretty linearly to numbers like 50 amps in 100 ns or so. Now apply a lot of reverse current, like 50 amps, through a small inductor. It will conduct in the reverse direction for maybe 50 ns then snap off, making a huge (kilovolt maybe) spike across the inductor, just a couple ns wide. Lotsa EMI. Power rectifiers can do the step-recovery thing even at 60 Hz. That can cause mysterious problems elsewhere in a box. A Russian guy, Grehkov, discovered the DSRD effect and another, even faster plasma breakdown effect, in common power diodes. John A step-recovery diode needs a hyperbolic doping profile, which regular diffusion can make, on purpose or not. The SRD was discovered accidentally by Boff at HP, in the 60s. There's some history in the old HP Journals. They were originally called "Boff diodes" but the name didn't catch on. A modified doping profile can make a "soft recovery" power rectifier diode that still stores a lot of charge but doesn't snap. John ______________________________________________________________________ > I have a diode that I want to model, it's a 85HF20. > <http://www.vishay.com/docs/93529/85hf.pdf> > This is one of those standard stud mounted rectifiers. If you're concerned about the "forward recovery time" thing, I doubt that a model will be very useful. You'd really need to test an actual part to see how it behaves. To complicate life, several of the different voltage rated versions may at various times come off the same wafer. In general, higher voltage parts have wider junctions, approaching pin diodes for, say, 600 volt parts, and they tend to be slower turning on. I've slammed 48 volts across a 600 volt "fast recovery" diode and it took a couple hundred nanoseconds to ramp up to 50 amps. What's your circuit? John The diode in question is inside this: http://www.highlandtechnology.com/DSS/T220DS.html It's a drift step-recovery (Grehkov) diode, or rather a commercial power diode used in DSRD mode. I assure you that we know exactly what we're doing here, and the diode behaves as described. The inductance of the drive circuit (+48 volts followed by -400) is around 8 nH. The pulser was desiged to rip ions off a microtip in a tomographic atom probe. Google the obvious and learn something. Start here: http://www.avtechpulse.com/papers/thesis/8/ John Yikes. High voltage pn diodes have big lightly-doped (intrinsic) regions so act like/are PIN diodes. They turn on and off slow. SiC diodes are a lot faster, assuming you have too much voltage to use schottkies. Infineon? Cree? You'd probably need a few in parallel to handle that current and keep the parasitic inductance down. John ______________________________________________________________________ >>> Some of the 4000 series behave more like PIN diodes. >> >> The higher-voltage ones ARE pin diodes. They can make excellent drift >> step-recovery diodes and impact avalanche diodes. Need 1000 volts in >> 100 picoseconds? >> > >Do you have some examples and pics? That would be interesting, I have >use the 1N4007 as a RF PIN diode but never as SRD. Google "Grehkov diode". He's a Russian guy who discovered the DSRD and impact avalanche effects in cheap power diodes. Also look for papers and patents by Thomas E. McEwan. We did one water-cooled DSRD pulser that makes -2KV pulses, about 2 ns wide, at 500 KHz. We forward-bias a secret diode at +48 volts for about 80 ns, to let the current build up to 50 amps or so, then reverse-bias it from a 400 volt supply and wait for it to snap. Here's the pulser head... http://www.highlandtechnology.com/DSS/T220DS.html with the serious parts bolted to a gold-plated copper block. A water-flow cold plate gets bolted to the bottom. I can show you innards privately. The HV, high power PIN diode turned out to be unusual. John Yes. We did wind up with a reel of FR804's that snap nicely, maybe decent PIN diode material, but we went to a bigger part in the end, higher voltage and more power dissipation. It was fun, but we only sold a few. ______________________________________________________________________ GeoAdmin » Sun Aug 08, 2010 9:12 am I'd suggest you to try for starters the ones based on avalanche transistors. Zetex has nice ones that work very well, a paper on those can be found here: http://www.diodes.com/_files/design_note_pdfs/zetex/dn24.pdf drift step recovery diodes are pretty simple devices, but I haven't seen a fast enough radar based on them with decent results. This however is my very own opinion and I haven't seen them all obviously, so if you feel comfortable with those then perhaps is worth trying them.