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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi, when you do the thermal design of a DRSSTC bridge, do you take into account "ripple" of die temperature at the interrupter frequency, or do you just average it out?
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
the thermal impedance between the die and the case, then the case and the heatsink will do the averaging for you. there's no way to keep the die temp from spiking, the best you can do is draw the heat out of it as slow as the junctions will allow.
this is the main problem with pulsing igbt's. if you look at the datasheets, they all say "pulse current *limited by device temperature". if you had a way to draw the heat quickly directly out of the die, then the pulse current could be infinite.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Actually, transient thermal analysis is done quite a bit, especially when it comes to RF amplifiers (pulsed) and MOSFETs and IGBTs in pulsed applications. Yes, there isn't much you can do thermally to really help with the transient heating of the die (as opposed to the steady state average heating), but what you do get out of those analyses is a range of "safe operating pulse" waveforms that can be run without damaging the die.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, you need to take the ripple into account to calculate the true peak die temperature. IR datasheets include graphs of transient thermal impedance for various pulse lengths that you can use for this purpose. You calculate the average case temperature, then add the peak pulse power, multiplied by the transient thermal impedance at the burst length you're using.
An interesting feature of the transient thermal impedance curves is that the allowed energy for a short pulse decreases significantly when pulse width is shortened. So it seems, that the energy dissipation in the die is more and more localized for shorter pulses.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Thanks all for the replies. I did some calculations based on the transient thermal impedance diagram, and the peak die temperature is indeed considerable at lower pulse widths.
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DRSSTC transistor cooling, thermal design
