Re: Passive Ballasting for DRSSTC - My thoughts before Ed Wingates Teslathon

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Jimmy Hynes <jphynes@xxxxxxxxx>



On 8/27/05, Tesla list <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx> wrote:
Original poster: Terry Fritz 
<<mailto:vardin@xxxxxxxxxxxxxxxxxxxxxxx>vardin@xxxxxxxxxxxxxxxxxxxxxxx >

At 01:12 PM 8/25/2005, you wrote:
>.........  But you guys tell me.  What are your thoughts.
>I'm open ears.

I have not done a lot of real math on this, so the numbers and
speculations are just "pie in the sky" right this moment...

But assume that IGBTs fail due to over current, over voltage, and/or
over power...  Probably a good assumption :o))


I'd classify it a little differently. There's overvoltage (which can 
be pretty much eliminated by useing primary feedback to ensure soft 
turn off, and a proper layout), and then there's overheating.

Overheating can be separated into transient heating, and steady state 
heating. Transient heating is either caused by trying to get too big 
of a bang (just limit on time), or overcurrent (active current 
protection stops this). "Overcurrent" just means the voltage drop 
becomes huge and overheats it quickly. Steady state heading is just 
the result of inadequate heat sinking.

But consider the secondary power arc to ground...  The secondary
suddenly has a low resistance (lower loss) path to ground...  The
"power" has to go somewhere"...  It goes to the streamer, secondary R
losses, primary R losses, and IGBT losses...  (maybe "lost" IGBTs ;-)))


"somewhere" can be back into the electro lytics, if everything is low loss.

The streamer losses probably go way down even though the arc "looks"
brighter...  I^2R, but R to ground is many orders of magnitude less
now...  I^2 probably can't keep up...  Power gets thrown "elsewhere"...



The arc completely discharging any energy in the secondary, and if it 
happens near the end of the bang, then its a much better way of 
sucking the energy out of the coil. If you're talking about a 6" arc 
to ground, the story may be a little different

Secondary pure R losses are higher, but still not very significant
overall...  The secondary coil is still far from significant heating
effects even with a couple of thousand watts input...

Primary R losses...  I have not noted a primary coil/cap ever getting
red hot, or even warm in this case...


My primary used to get hot even when not arcing to ground..

IGBT losses... Loud explosion with hot white flames...  Hmmmmm!!!  A
weak spot!!!

But, I think it is a "primary circuit Q factor" thing...  When the
secondary shorts to ground through an arc - voltage, current, and
power "buildup" in the primary loop seems to be the "killer"...  Just
a "little" primary resistance will drop the primary Q like a
rock!!  I guess I find it hard to imagine that "skinny wire" has more
R than IGBTs...  But IGBTs are the masters of "low R"
silicon...  Added resistance her might place a "ceiling" on primary
loop over stresses...


It depends on how you define a "little". To drop the current to safe 
levels, you're gonna get a lot of heating that way. If you have gone 
through the trouble of active limiting, then it isn't possible to 
drive the IGBT out of saturation, and its actually easier on the 
IGBT, since it takes an early rest.


So when the coil is making pretty streamers, primary R is just not a
big deal.  But with a shorted secondary, primary R might make all the
difference in the world as a place to find "low Q" to protect the
IGBTs...  Primary to secondary coupling must be a BIG factor here too....

There may be a "Primary Q, Secondary Q, and Coupling" equation out
there, somewhere, that would govern all this...  I think it is a "two
hump" thing so things would get funky...

"That Teslathon" is probably going on right now, so interesting to
here the "results"???...

Cheers,

        Terry