Re: Terry's DRSSTC - Bench Test

["Tesla list" <tesla@xxxxxxxxxx>]

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 ;-).

Steve


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
> >
> >
>
>