# Air cooling and the thermal resistance.

Discussion in 'Electronic Design' started by overgeo, Jul 25, 2006.

1. ### overgeoGuest

HI i'm designing an inverter, the characteristics are, DC bus voltage
300V, Irms 30A, pwm frequency 100Hz, and a maximum junction temperature
of 80ºC, I have used this igbt calculator
http://www.powerdesigners.com/InfoWeb/calculators/IGBTCalc/igbt_calc.shtm
and i get 160 Watts of power losses and a resistance for the disipator
of 0.117 ºC/w, are these results more o less correct?, i have chosen
this igbt IRG4PSC71UD, in the datasheet says that maximum power
dissipation at Tc = 100ºC is 140W, the igbt will be able to dissipate
my power losses?

I have looked for disipators but i haven't seen disipators with this
thermal resistance so the solution i'm gonna use is a disipator with
air cooling, and i would like to know how is the diminution of the
thermal resistance of a disipator using air cooling?, and finally if
someone has some suggestions about the use of air cooling please tell
me, thanks in advance, and sorry if my english sounds a little bad, i'm
spanish.

2. ### Guest

Have a look at the Farnell catalogue or the Martson web-site

http://www.hsmarston.co.uk/heatsinkselector.htm

Farnell carry a couple of Marston heatsinks that can offer around
0.1C/W thermal resistances to ambient when coupled to a 120mm fan.

It is difficult to mount a single dissipating device on such a heat
sink without having as much thermal resistance from the device to the
heatsik as you get from heatsink to ambient - carbon fibre cloth is
good, but it is also electrically conductive.

3. ### Nico CoeselGuest

No, that power rating is based on a die kept at 25 degrees with a
magic cooler (read 'magic' as: it doesn't exist in the real world). It
is a number to use in calculations.

Another thing to worry about is thermal dynamics since you are working
with pulsed signals. Try to read some about it, you'll see the amount
of power a device can dissipate is much less than its rated capacity.

4. ### overgeoGuest

ha escrito:
Hi thanks for your answer, i will use a disipator of that page or
something similar that i'll find in my country. I will keep in mind
that the resistance device heatsink will be as bigger as the heatsink
ambient, thanks.

Nico Coesel ha escrito:
dissipation
(http://www.ixysrf.com/pdf/switch_mode/appnotes/1aprtheta_power_dissipation..pdf)
and you're right the specifications are not very real, but i didn't
found anything related with power dissipations and pulsed signals the
only thing that i found was the thermal response.

The solution to this problem that i have thought is to change the igbt
or paralelling, i think the easiest is to change the igbt, i was
looking to the products of international rectifier, and the IGBT with
the biggest dissipation is this IRGPS60B120KD, the datasheet says that
Pd at 25º is 600W and at 100ºC 240W, do you think that this will be
suitable for my case?. In addition i have seen the modules
(SKM75GB123), but i will like prefer to use a discrete igbt, thanks