Re: OLTC Update - Primary circuit resistance.

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi All,

Eddie mentions that the IGBTs may be able to take higher currents than
rated.  Darren disagreed since "spot heating" on the die may be a problem.
Hard to say...  At these points the data sheets and the manufacturers say
"your on your own.  Good luck! :o))  The "cheap" IGBTs don't use copper on
silicon or any of that fancy stuff.  Ones that do, appear very similar but
are rated at 280 amps instead of 204.  I would guess we are nearing the
point that the conductors in the die will simply fuse.  Spot heating may
not be too bad since the die are running very cold.  Like 30 C.  So we have
a lot of room for heating!  A lot depends on how well the IGBT and package
are made.  IMHO International Rectifier is the best in the business there.
We really just have to test them and find out...  It will be interesting to
see how much margin is really there.

IR usually over rates CE voltage by about 100 volts and that is a known
hard limit that we can't go over especially with our currents.  Far far
beyond any avalanche current rating!


Marco mentions that I am running the gates at pretty high voltage (18V).
IGBTs like these usually "break" at about 50 volts on the gate.  No problem
nearing the 20 volt spec there.  The 2 9V batteries will quickly go down to
about 16 volts.  That is only 6 volts over the full 200 amps on spec and I
am a little worried about noise getting on the gates too so more margin is
nice.  Shoot through from the Ccg capacitance is a concern and that is why
I have 100 ohm gate resistors to slow the turn on way down compared to what
it "could" be.  


Bert mentions "As previously mentioned, a possible downside is that, through
transformer action, some of the secondary's energy will be extracted
during secondary ringdown, resulting in partial recharging of the tank
caps through the internal HexFRED diodes."

The "theory" is:  In order for the primary to take energy back from the
secondary.  The primary has to be "tuned".  When the IGBT opens, it
destroys the primary as a tuned circuit.  Thus the primary will not be able
to take back the energy.   We will really just have to see what happens
There.  there is a "plan B" of two IGBTs in a CE-EC configuration.  But
we'll see if we even have to worry about it.

Cheers,

	Terry