Re: OLTC II Initial calculations

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

Original poster: "Stephen Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>

At 11:44 07/05/03 -0600, you wrote:
>Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>
>
>
>Hi Steve,
>
>Emitter inductance is indeed a big issue.  If we have 3150 amps at 58kHz and
>want only say 2 volts of gate disturbance, we can calculate the inductance:
>
>L = 1.74nH
>
>Yipps!!  That is not very much inductance!!

It's pretty scary isn't it... I suppose there are ways that Powerex could 
have done it. If they laid out the wiring so the emitter inductance was 
almost identical for each die, then made a Kelvin connection to one die, 
the disturbance would be cancelled out for all the dies. That's if you 
don't count mutual inductance between the gate wiring and the 
emitter/collector wiring. I'm assuming (hoping) that they did something 
like that. However I suspect that crap current sharing is the main reason 
the ratio of pulse rating/continuous rating is less than for single devices.


>My Pearson 101 goes up to 50,000 amps so maybe I can test it...  I think I
>have all the stuff here.  Not sure how to drive the beast...  120nF gate and
>24nF Cies is not easy...  How were you thinking of driving it?

With a totem-pole driver made from two 60 amp MOSFETS =|>  I have a circuit 
for this that switches in 0.5us but it's heckuva complex and I should be 
able to simplify it somewhat. I'll post a schematic when I've got something 
usable. It would be great if you could do some tests because I just don't 
have the equipment to deal with this :(


>Maybe Emitter inductance is not such a big deal after all!!  It gets lost in
>the reverse transfer current...  Not sure how to handle gate resistors
>either...  Have to turn the gate on with a ramp instead to eliminate the
>resistor...

As far as I understand, gate resistors are used to damp out parasitic 
oscillations. The IGBT capacitances, gain of the IGBT when in the linear 
region, gate circuit inductance, etc, form an oscillator. If you can keep 
the gate circuit inductance low enough, the risetime fast, and deliver 
stupid levels of drive current to slam it through the linear region before 
it knows what's hit it, you can use a very low value resistor 
(non-inductive natch) or nothing at all.


>Be careful about making the secondary too big with large numbers of turns 
>of thin wire on cardboard or
>the secondary "Q" my have trouble like mine did.  Normally, secondary Q is
>not a big deal but it can catchup to you.

I'm hoping that the "Stubby" secondary (tm) will have a higher Q than a 
taller thinner type. According to my ham radio handbook the aspect ratio 
for optimum Q is 2:1. I was surprised by how well the Mini OLTC secondary 
worked even though it had 1500 turns of 40swg (whatever that is in awg) and 
a 800 ohm DCR. Ringdown traces showed that it had much less losses than the 
primary circuit. I did wind it on a very thin (<1mm) polycarbonate former 
however. For this one I hope to snag some thin walled 10" PVC or ABS pipe. 
I know they make this for fume cupboard ducting.

Steve C.