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Re: Recent s.s.t.c work

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

Ken, what is the impedance of your gate driver? I have never seen waveforms that slow and with that peculiar rising edge. The falling edge looks normal. Is your gate drive transistor (the one that pulls positive) falling out of saturation perhaps??? The turn on is usually faster than turn off...

The really odd thing is that your gate voltage slows down before you even hit the 0V line!!! That is *not* right. Have you checked each driver? Maybe this one driver is damaged and also causing your imbalance in primary current from hafl-cycle to half-cycle?

Good luck... in any case, i think that those large turn on delays are fix-able. My gate drivers can supply LOTS of current, as the mosfets i use in it onl have a 55m ohm Rds. I dont use any resistance on the IGBT gates, so i switch at the maximum speed possible.

Steve


On 10/17/05, Tesla list <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx> wrote:
Original poster: "K. C. Herrick" <<mailto:kchdlh@xxxxxxx>kchdlh@xxxxxxx>

Update #2:  Adding 20 uF to the 3 uF drive-xfmr coupling capacitor
did bupkus for the phase shift between the H-bridge supply current
and the primary current.  So I looked further and took the photo at
<http://www.hot-streamer.com/temp/KCH_TCH7.jpg>http://www.hot-streamer.com/temp/KCH_TCH7.jpg
It appears that the "phase shift" that has been puzzling me is
nothing more than the to-be-expected turn-on delay in the
IGBTs.  Surprise, surprise!

That seems quite clear from the photo:  Turn-on of a pair of the
IGBTs occurs about 1 us after the beginning of the + gate
excursion.  And turn-off of that pair's reverse-diodes begins about 2
us after the turn-on.  Do I have that right?

So...how is one to compensate for that?  Does one need to?  Can't do
it (that's what I think)?  Anyone...?

KCH

And by the way, I realize why the waveforms in my photos appear to
overlap the scope bezel:  What appears to be the bezel is actually a
frame in the phosphor of the CRT, not the exterior bezel of the scope.