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 ?
>
> 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.