RE: Off Line - DRSSTC (OL-DRSSTC) - 4 = 5

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Jason,

If I use a 100 ohm resistor across the gate, I need to have 30mA 
going through it to hit the 3V turn on voltage of the IGBTs.  I am 
using 100:1 CTs, that gives 3 amps of primary current to turn on an IGBT.

So when the primary current gets to 3 amps, I am starting to turn on 
IGBTs...  Then it is a matter of what the primary current 
is.  Assuming it is 460 amps peek, as the models suggest:

I = 460 sin (2 x pi * 105000 x t)

or

3/460 = 0.0065

0.0065 = 2 x pi x 105000 x t

t = 10nS worst case...    !!:D

The models are 100nS based and they show a "little cross conduction 
spiking"....  But that is well inside their area of "vast modeling errors"...

Suppose the resistors are 10 ohms...  Then I need 30 amps...  The 
time is then 100nS...  We have to remember that the gate capacitance 
is pretty high too and we have to charge it over time...  But our 
currents are pretty high from the direct connected CTs too ;-)))  But 
I need to "start" the monster with the energy from the trigger 
circuit too....  So I can't get too carried away with low gate 
parallel resistance...

We can play tricks like adding transorbs, diodes, caps... in series 
with the gates...  It is "very" unexplored and the models are not 
terribly good here without solid real data to guide them...  The 
IGBTs are "slow" too.  And the gate capacitance is very 
"dynamic"...  Jimmy warned us of all this...  But those CTs are 
giving us darn "high power signals" to run everything off of!!  CTs 
on a 460amp, 100kHz, 4kV line...  We got the "gate drive power" ;-))))

So, God only knows....  But there are tons of tricks to "fix it" so 
it works and they are all cheap and simple ;-)))))  But we need to 
"actually wire the darn thing up now" to know where to go and what 
tricks we need to pull out....  But agood guide is just to "not think 
too hard" about it all ;-))

Glade so many are interested in all this!!  I hope they solve a few 
of these problems TOOooo  :o))))

Cheers,

        Terry


At 09:42 PM 9/29/2005, you wrote:
> Hi Terry,
>
> What switching delay or switching current do your simulations show for
> the IGBTs in relation to the primary current ?
>
> Jason.