Re: OLTC Update - Primary circuit resistance.

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

Original poster: "Eddie Burwell by way of Terry Fritz <twftesla-at-qwest-dot-net>" <eburwell-at-columbus.rr-dot-com>

At 11:14 PM 8/18/02 -0600, you wrote:
>Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

>However, the current is "high"!  In my 50 volt testing tonight, I must have
>been hitting 500 amps peak!  It may turn into a 120VAC coil instead of a
>240VAC coil...  Or, get rid of that charging reactor...  Definitely must be
>very careful of running without a secondary or a miss-tuned secondary.
>MicroSim says I could hit 5000 amps in a nice slow ringdown.  That would
>destroy the IGBTs...
>
>Even in the full coil MicroSim warns of 6000! Amps now....
>
>This changes things a bit, but it is wonderful in the long run :-)))  As
>odd as it sounds, I may have to run my coil off 120 VAC without resonant
>charging (but at higher break rates?)...  An even simpler coil.  Or add
>primary loss.  In the first stage, I may have to run it on a variac just to
>"reduce" the voltage!  I may be able to pump up the break rate, but hat
>means more currents that I was not expecting...  Many complexities are
>added here, but they are good ones ;-))
>
>I seem to have far more power than I expected :o))
>
>The whole idea was to pump 2000 amps at about 30kHz through a primary coil
>coupled to the secondary.  I may have vastly overdone it ;-)))  With only
>0.0006 ohms of primary resistance, I could use a 1.5 volt battery :o)))
>
>Cheers,
>
>	Terry
>
Terry,

  One thing I have contemplated but have not tested is I believe that if you
push an IGBT past it's maximum current ratings it may not necessarily die. I
think the reason the manufacturers place the maximum peak current rating
where they do is that if the current exceeds a certain point the IGBT
latches on like a SCR. In other words the ratings are spec'ed to maintain
gate control. 
  The important thing for semiconductor health is die temperature. We could
start out by assuming that the pulse is so brief and intense that the
thermal conductivity of the package can't remove the heat generated during
the actual pulse. If that is the case, any energy dissipated in the device
during the pulse will go directly to heating the die. We can measure the die
and approximate it's mass. Using the specific heat of silicon we could then
calculate the energy necessary to elevate the die temperature by some
amount. After the pulse the die will cool down until it is hit with the next
pulse.
  I have a feeling this approach should be valid especially since IGBTs
don't have di/dt failure modes(at least for halfway sane levels of di/dt)
like SCRs do. The main question is if there is some other failure mode that
kicks in above Icm. If the IGBT latched on while the current was above Icm
and the regained gate control below Icm the OLTC may seriously be in
business. The other snag I can think of is that it's not just one pulse that
the IGBT is dealing with, it's a sequence of decaying pulses. The first
pulse will bump the die temp up the most. The last of the pulses as
quenching is approached will probably be of little consequence as far as die
heating is concerned.

Perhaps it's time to destructively test some single devices under the heavy
scrutiny of test equipment?


Eddie Burwell