RE: Jimmy's DRSSTC 2

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

Original poster: "Steve Conner" <steve.conner-at-optosci-dot-com> 

 >The results of the thermal simulation make it look too easy. I'd like
 >someone to check my numbers. I really hope it's right.

They certainly do look suspiciously... good :P I've been doing calculations
on some similar 300A 1200V bricks. (the Semikron SKM300GB123D)  The maximum
pulse rating on these IGBTs is 600A for 1ms, and the datasheet says the
pulse width is limited by junction temperature.

That suggests that 600A for 1ms will raise the junction temperature from
25'C to 125'C. (The fact that the pulse rating goes down to 440A at 80'C
case temperature reinforces this hypothesis)

Hence (making the assumption that IGBT forward voltage doesn't change with
current) any values of (Average current * time) that work out to (600*1e-3)
should have the same effect on junction temperature. In practice the
temperature may be higher with shorter pulses as the heat has less time to
escape to the baseplate.

So, 1800A average for 333us is the same as 600A for 1ms.

In a H-bridge powered from 650V DC with sine wave load current, this works
out as...

Peak current= 1800*2/0.61 (since it's a sine wave but each IGBT only
conducts half the time)= 5902A

RMS current= 5902*.707=4173A

Energy delivered to load= Vrms*Irms*time= 650*(4/pi)*4173*333e-6= 1149
joules

This is an ABSOLUTE THEORETICAL MAXIMUM that takes no account of switching
losses or poor current distribution causing hot spots within the die! And it
assumes that the IGBT case is at 25'C.

But it's still a heck of a lot bigger than the 200J you plan to use. So
either your thermal simulation results are reasonable, or we're both crazy.

However. I would not use this particular IGBT under these conditions..
because the datasheet also says that its short circuit rating is 2000A and
it's not guaranteed to survive more than 1000 of these 2000A pulses. I would
keep my peak current down to 1000-1500A and that limits me to 50-70 joules.

Good luck!!!!!

Steve C.