RE: Terry's DRSSTC

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Mccauley, Daniel H" <daniel.h.mccauley@xxxxxxxx>


Huey with all the waveforms.

LETS SEE SOME ARCS!!!!  :)

Dan


>Hi,
>
>These days, the voltage waveforms (blue) from my DRSSTC look like this:
>
>http://hot-streamer.com/temp/DRSSTC-ModeSwitch-01.gif
>
>The used to look like this:
>
>http://drsstc.com/~terrell/pictures/FullSystem-02.gif
>
>Ever since I tried running with the deadtimes removed on the
>IGBTs, the
>corners of the voltage waveforms have gotten very noisy and
>the ~400nS flat
>portions of the waveform have gone away.  I put the dead time
>back in but
>"things have changed".  I have checked and rechecked all the drive
>electronics and the dead times still seem to be there even if
>one really
>looks close even at the output waveform (the current blips due
>to lack of
>current drive).  So the corners are noisy and the turnoff is
>slow and messy
>now.  I am also getting spikes that go well over the rail
>voltages (like
>100V!) which should not be possible.  The IGBTs seem to have
>lost their
>"warp"...  I double checked all the test equipment and probes too.
>
>I am thinking the anti-parallel diodes have gone away.
>Strange since one
>would think the IGBTs themselves would get beat up from tiny
>dead times and
>fast switching rather than the separate diodes....
>
>So I am going to switch out the set of IGBTs and see if it all
>returns to
>normal.  If so, I will have the die checked to find what blew up.
>
>Interesting that the thing still runs fine from an overall
>perspective.  If
>the diodes are gone, it is just pulling down the voltage with
>the opposite
>IGBT now instead of doing it the nice way.
>
>So for the smaller high speed co-pack IGBTs, running with very
>low dead
>time my not be a good idea.  I also have just 5.1 ohm gate
>resistors in
>there but that does not seem to be a problem according to the
>literature....  I think I just have to get the IGBTs out and
>start testing
>them specifically to pinpoint what is going on.
>
>So I "think" I blew something up!!  But it still "runs" fine
>:o))  Hard to
>say but I think the noise my be getting a little worse with
>time which is
>probably to be expected...
>
>I just played with the coil at low power last night and there
>are all kinds
>of mysteries there to be played with ;-))  The very first
>burst seems to
>really "make the sparks",  Tuning and all that mode stuff have
>a pretty big
>effect too...  I wonder about our tuning of the primary to a lower
>frequency in regular coils.  Perhaps is not so much a streamer loading
>compensation but rather forcing the coil into the lower mode
>where things
>seem to run better...  It does seem to help the DRSSTC, but
>those effects
>should be the same in any coil!
>
>Much to ponder...
>
>Cheers,
>
>	Terry
>
>
>