Diode Recovery Tester - Page 1
Forum section: electronics / projects

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 02, 2017, 06:30:02 am
content: Ever looked at a diode datasheet, and wondered how reverse recovery is measured?Ever wondered about not-often-discussed forward recovery?https://www.seventransistorlabs.com/Images/Diode_Recovery_Tester.pdfThis project more or less replicates the conditions specified in most datasheets. It's very adjustable: pulse duration, forward current, ramp rate, and commutation voltage are all adjustable. (Well, test power is external, so you can use an adjustable supply for that.)Connect an inductor between the labeled terminals, then connect a diode.Warning: short circuit current is not limited, beyond what the transistor can do. Testing a diode backwards may explode it. (That said, a 1N4007 will handle 80A for a few microseconds just fine.)The snubber circuit clamps open-circuit voltage, so the circuit does not need to be stopped while swapping out diodes. (TVS are impressively robust and handle this service easily!)There are only small differences between the circuit shown, and what I've built. The big one is the transistor, where I used some beefy salvaged IGBT (RJH6075 -- it "doesn't exist", but it's probably 600V, 75A, and 200 or300nC). It's suitable for most diodes (which are tested in the 100-1000A/us range, for whatever reason), but too slow to see interesting behavior.Observations:- I haven't seen any spooky (SRD-like) behavior, on any of the diodes I've tested, in the operating range I was testing at. Dang.- Zener diodes are generally pretty fast (t_rr < 50ns), and quite sharp (sharpness factor < 0.3?). Might be useful?- Most diodes do have t_rr varying with t_on, but Irm varies too. This seems an inefficient way to sharpen a pulse. That is, unless Irm gets disproportionately large at high dI/dt, which seems likely, and presumably leads to the region of SRD-like behavior.- Some junctions have t_rr varying with t_on, even for t_on > 10us. I interpret this as, it takes a few recombination times for charge to "soak" in and reach steady state. (Accordingly, it should take as long for Vf to stabilize, but Vf is ~logarithmic with charge(?), so it would barely be noticeable.)- I think the slowest diode I have is HIB120 (high voltage, 3A size; can't find any data), t_rr > 5us.- Horizontal output transistors are unimpressive as diodes (unsurprising given their ~us t_stg, I guess).- I have some surprisingly fast diodes: MUR880E for instance, lives up to its datasheet. Others I don't remember at the moment, but have salvaged from power supplies and TVs, with similar performance. And schottky of course, which do indeed exhibit nearly (or actually) zero recovery.Ooh, now I can test if SiC schottky exhibit nasty recovery drool at pulsed currents (i.e., where Vf is high enough to forward bias the guard ring PN junction).Tim

author: flash2b
author_rank: Regular Contributor
timestamp: October 02, 2017, 12:21:30 pm
content: I also build one back in 2009. This is not a high current one like yours.Please use this (unfortunately in dutch) link:https://www.circuitsonline.net/forum/view/76535Some pictures of the results with my tester using my 60MHz scope which I had during that time

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 02, 2017, 12:46:24 pm
content: Nice! Looks like a good companion for testing small signal diodes. (This one is for high current testing, as many volts and amperes as you care to run the transistor and diode at.)A small change to the circuit and I've got dI/dt up to 1350 A/us. Here's a DSEP29-06A at 10A, 60V, and 1350 A/us:Ch1 is 10x probe, 20V/div. Ch3 is 5A/div.I'm not sure how much of that is stray inductance in the diode, and in the current shunt resistors. Probably at least the voltage step is diode stray (which I'd eyeball around 20nH; a 14V step at 500 A/us is 28nH, so that's probably most of it).Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: October 02, 2017, 03:03:56 pm
content: For small diodes one does not need a special tester. Just a square wave generator with 50 output will do. The amplitude sets the current level - usually symmetric for forward and reverse, but an offset can change that. For first indication it is enough even for a 3 A diode. T_rr sometimes changes with current but usually not that dramatic, except for schottkys when you start to activate the PN guard.Some photo-diodes (those with good NIR performance) are really slow, t_rr can go up to the 20 µs range. Of cause you won't test these at high currents.There are also sometimes weak very slow processes, like charging some defect states. These can take rather long to reach equilibrium. I remember an experiment (though more LDR like photo conduction and not reverse recovery) at the university with Germanium. There it took some 10-50 ms to come back to equilibrium when tested at lower temperatures (e.g. - 100 C). So slow one might no notice it for a fast cycle to get a flicker free picture on a scope.

author: PA4TIM
author_rank: Super Contributor
timestamp: October 02, 2017, 03:39:00 pm
content: I had to test a diode today for trr. Used my R&S AFGU and a 50 ohm dummyload. I used +5 and -10V at first, then +3 and -5 and that made no differenceBut my FG has also a 5 ohm output. And that gave a whole different output. The trr did not change a lot but the trace was "wilder" The load was stil 50 ohm so I do not know what was going on. (had no time to investigate it) ( a diode with known trr measured in the normal 50 ohm mode, 147ns and the datasheet stated 150ns)T3sl4co1lThat is the real deal, roast those bastardsNice tester, that is a serious piece of kit.