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SISG IGBT Timing Resistors

Original poster: "Mark Dunn" <mdunn@xxxxxxxxxxxx>

Last nite I decreased the IGBT Timing Resistors to 680 ohm in my SISG
array(4 boards - 12 SISG circuits - 10.8KV threshold) on my 4.5" dia
coil.  This gives an IGBT "on" time of 107 uS.  For my coil 1st notch is
40 uS and full decay around 180 uS.

Most recently, I had been operating with a 1.1 Kohm resistor(173 uS).

The improvement was significant.  MOT Primary Current dropped from ~15
amps to ~12.2 amps
and voltage increased from 98-99 VAC to 105-106 VAC.  Spark length
increased a few inches.  Have not measured exact length, but roundabout
4 foot.

So to review the Resistors tested so far:

Ohm   "on" time
4.7K	738 uS
2.2K  342 uS
1.1K  173 uS
680   107 uS

I have seen a steady improvement as I progressed down the list.  Note
that the 680 ohm is the first case that turned off the IGBT before decay
was complete.

For those of you experimenting with these boards, I suggest you
temporarily solder the IGBT Timing resistor(R5) to the bottom of the
board to make it easy to change until you are satisfied with the choice
for your application and then permanently mount it to the top of the
board.  Makes it easier to change.

IGBT cooling does not appear to be much of an issue at this time, but we
are only pushing 1300 VA into the system.

Note that when this coil was a DC Resonant charging type, it took
upwards of 1800-2000 VA to achieve the same performance(spark length).
Thus, it appears that SISG has improved efficiency by nearly 30%.  Of
course, these numbers are VA not watts so we are not comparing real
power only apparent.

Also note that I am not using a Variac so it is not a matter of simply
increasing voltage(in which case current would rise as well).  Current
in my system is limited by an inductive ballast, which remained
unchanged during the test.  Thus, the relatively large change is
current/voltage documented above was purely due to the change in IGBT
"on" time.  I find this very interesting.  I had always wondered how
much effect varying the pin size and disc diameter would have with RSG,
but such an experiment was too labor intensive for me.