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Re: Terry's DRSSTC - Bench Test

Original poster: Steve Ward <steve.ward@xxxxxxxxx>

Hey Terry,

Just a comment about deadtimes and IGBT rise and fall times.  For the
BIG coil, i sorta fine tuned my gate drivers to give a 50nS dead band,
not much at all, but enough to see that there is no cross conduction.
Secondly, i went with NO gate resistance at all and was able to drive
the bricks *quite* fast, at about 100nS rise and fall times.  You
mention that the very fast transition times might might "tear the
IGBTs up", i assume this has to do with discharging the miller
capacitance?  I wasnt aware there was any negative effect to doing it
"faster", unless there is enough inductance for a dI/dt event to cause
some sort of transient within the IGBT?

I built a really cheesy SMPS to power all of my gate drivers... might
be something to think about later.  All it really is, is a half-bridge
running 40khz driving a transformer (old flyback core!).  To make all
4 outputs to the gate drivers "identical", i simply took a length of
cat-5 cable and wound about 8 turns on there, and seriesed each pair,
so that each winding was really 16 turns.  This method has been tested
up to 750VDC buss voltage, and no failures yet ;-).

Looks like its almost time to start driving coils ;-).


On Fri, 21 Jan 2005 22:29:31 -0700, Tesla list <tesla@xxxxxxxxxx> wrote: > Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx> > > Hi Again, > > I thought I would add some more test results ;-) > > I checked the overall drive current to H-bridge output delay: > > http://drsstc.com/~terrell/pictures/BenchTest-05-IO-Delay.gif > > 424nS... Sort of "so so"... I "think" I know how to wind a CT to go > faster (that is where all the delay is). But it is good enough not to > worry much with it right now. > > Now this is cool!! I took "out" all the crossover dead time and just ran > it full speed (each half of the H-bridge is a scope channel): > > http://drsstc.com/~terrell/pictures/BenchTest-06-FullSpeed.gif > > Looks just fine!!! > > Check out this cross over!! > > http://drsstc.com/~terrell/pictures/BenchTest-07-FullSpeed.gif > > Big high power IGBT die going from turn off to other side conduction in > darn near 50nS!!!! Symmetry is close to 5nS!! Surprising since the gate > drive wiring is really sloppy and I am floating the gate drive power > supplies that are "riding" on this too!! But the impedances are very low > and the "electrical "mass"" is small too. Too small to make a good antenna > at these frequencies ;-) Cheating a little since the load is nil, but it > seems the controller alone is able to control cross conduction and all very > well. > > Cross conduction is a zero issue it "seems". I really like my controller > circuit ;-)) > > http://drsstc.com/~terrell/schematics/Controller.gif > > U3a,b and the four gates there just make darn sure nothing is going to go > cross conducting. The +- CT also insures full symmetry. > > But the close crossover pushes losses up (but the "little batteries" I am > using don't seem to mind at all.)... I wonder if I should just "go for it" > and let the IGBT temp sensors worry if it dissipates too much heat and back > off from there if needed... But I have to slow down the mess too or Miller > shoot through will blow it all up... > > I wonder if one could "feedback" the actual H-Bridge output drive signals > to the controller. So if say shoot through did hang it up, the controller > would "know" and just "not" turn on the other side, thus saving the > day... With high speed current trip and temperature monitoring, that is > just about the point to where the H-drive "can't" be blown up!! I suppose > a microcontroller could "on-the-fly" optimize things like dead time too and > even do a "black box" function to record the last uS of data if the bridge > ever did fail... > > Here it is at 500kHz: > > http://drsstc.com/~terrell/pictures/BenchTest-08-500kHz.gif > > Here at 500kHz CW, the drives and my low-Z amp just can't keep up so the > "test equipment" is going away... But given that everything is going > haywire, the signals are still perfectly symmetrical and cross over is a > "don't care"... > > The TLP250 will over heat if they are run at say 500kHz CW. Pulsed low > duty cycle stuff is no problem and I had no problem with them tonight, but > that is an issue with them to watch if you are playing with them at CW high > frequency. There 25kHz rating is a "thermal thing"... The gate > capacitance of my IGBTs looks mean at first but quickly > diminishes... That's why I like International Rectifier stuff. They know > all the cool tricks ;-)) > > I did note that my 1000uF HF electrolytic caps on the gate drive boards > seems to be running about 4C over air temp... Maybe the lamp was too close > or something, but I'll watch them incase there is an issue there. > > So the problems are as expected but I seem to have full control over > them. I don't know where to "set" anything to deal with them > though... But I'll figure it out ;-)) I just got 32 more IGBTs buy UPS > today "if" I mess up :o))) Tonight's stuff was just all low power battery > stuff, so maybe it will get more interesting still :o))) But the test > results so far are A++++ > > Cheers, > > Terry > > > At 08:21 PM 1/21/2005, Terry Fritz wrote: > >Hi All, > > > >Tonight I ran everything on the bench just off four 9V batteries to check > >operation: > > > >http://drsstc.com/~terrell/pictures/BenchTest-01.JPG > > > >It all worked pretty much just as expected. Here is the output at 40kHz > >and 300kHz: > > > >http://drsstc.com/~terrell/pictures/BenchTest-02-40kHz.gif > > > >http://drsstc.com/~terrell/pictures/BenchTest-03-300kHz.gif > > > >I have the dead times set really high and the load is only a 1k ohm > >resistor so things look a little odd, but this does verify remote control > >to bridge output operation. Yea!! > > > >Interesting to look at the IGBT turn on and off times: > > > >http://drsstc.com/~terrell/pictures/BenchTest-04-Times.gif > > > >The turn on is super fast! You can see the blip at about 10V where the > >doping regions are tearing themselves on! The load is trivial here, but > >that will have to be slowed down or the IGBTs will tear themselves apart > >under a real load. Just a trivial bigger gate resistor... Turn on is > >rather dull since with such a low load you are just seeing the output > >capacitance and the output MOV discharge curve. A little cross conduction > >would fix that >:o) > > > >Look at the gates!! > > > >http://drsstc.com/~terrell/pictures/BenchTest-04-Gates.gif > > > >The "pumped" TLP250's and the opto-isolated floating gate drivers have no > >problem at all pushing the gates! Infact, they drive "too" > >fast... Again, more gate drive resistance will slow that to a tame > >speed. Not sure what the bonus down spike is on the turn off... Might be > >due to the drive power supplies floating or inductance. It only helps > >here ;-) Since I am just running off batteries, the gate drive signals > >here are far less than the actual 27V drives: > > > >http://drsstc.com/~terrell/pictures/IGBT-DriveCards-01.gif > > > >So I need to tighten the dead time and play with gate drive resistance. A > >few more measurements and then I will start bolting it all into the box! > > > > > >Interesting that Antonio found that the freewheeling thing did not seem to > >help much. The added electronics to do that is pretty complex and I will > >not miss it ;-)) > > > >Steve Ward's data and experiments were really exciting!!! With 27V of > >gate drive I can run pretty high currents at high speed ;-)) I was > >planning on running very low currents and slow (the exact opposite), but > >it can run just about anything... > > > >Steve Conner's PLL circuit is really cool too! I think one would have to > >preset the free running frequency pretty close to Fo so it could sync and > >lock in "really fast" (only have 10 cycles according to Steve Ward). I > >think the start up would also have to be synced with the free running > >oscillator too so the very first turn-ons start together so the PLL error > >signals get off to a good start. > > > >I am still looking into getting those MMC cap PC boards. Not too many > >people want them so it is sort of on the edge. I think I can drop the > >order to 16 and still get them for like $37 which might be just right. > > > >Cheers, > > > > Terry > > > > > >