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RE: OL-DRSSTC 7 - It's Alive!



Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Jason,

At 07:13 PM 10/11/2005, you wrote:

Terry,

Thanks again ! I had neglected to take into account the switching losses
when I did my thermal calcs, thinking that the duty cycle was low enough
to ignore it. I had had noticed that my heatsinks temps increase
dramatically with non zero current switching so those switching losses
must really add up fast.

> Hi Jason,
>
>
> I see.  The "peak power" in that case might be enough to
> super heat the die over about say 200C for an instant.  That
> will explode the die.  When it is not switching, there is no
> switching loss in the equation.  But if the die has say 320
> amps 350VDC for 1uS, the peak power is over 100kW(!!) and 0.1J.
>
> Figure 6 in the data sheet puts the pulsed power thermal
> resistance at 0.007 c/w absolute minimum.  Assuming we can go
> from 25C to 175C and survive, the pulse power is 150C / 0.007
> = 21kW.  Thus, anytime the IGBT sees that, destruction is
> almost assured!!  There is no fudge or safety factor in that
> number and the destruction is "instant".

So from the data sheet, this is based on a 10us pulse ? If the pulse is
shorter then the IGBT should be able to cope with higher peak
dissipation levels ?

Nope! At that point the heatsink, package, and 90%of the die itself could be stone cold. But, the upper ~5% of the surface of the die is at 200C and only the tiny thermal capacity of that silicon sliver is controlling the temperature. You are blasting the junctions right off the otherwise cold die at that point. You might be able to get away with a little more, but the fundamental limit to the instant power is at hand... Over a short time, the aluminum/copper on those die will "shribble up" and fail even if it does survive more than "once"...



>
> Assuming the voltage drops by 1/2 and the current is 1/2 at
> the worst power peak in the switching and the bus voltage is
> say 320V...
>
> 1/2 x 320 x 1/2 x Ipeak = 21000    Ipeak = 262 amps.  Pretty close to
> your 320 amps!!

Given switch off would normally occur in less than 200ns is the 21KW
figure still correct ? I would have thought to work it out we would need
to calculate the Junction temp after each half cycle and add in the
switching loss for each switch cycle in the burst. The data sheets
switching loss graph (figure 8) only extends to about 60 amps though so
accuracy would be a problem.

It is mostly a function of "energy" with is independent of time in this case. Maybe it could be fiddled with, but this is getting near the limit for sure!!


..........

No smore smoke would be good, those IRG4PF50WDs are hard to get at a
decent price here in Australia. I'll keep working on the thermal

DigiKey has 1866 in stock... So many IGBTs, so little time >:o))

Cheers,

        Terry


Cheers,

Jason.