Quote:
Originally Posted by CamLight
How many watts of heat do you estimate each FET will have to dissipate?
LOL, sorry about all the design questions. I'm just fascinated with high-power thermal-transfer design and it always leads to lots of questions.  |
I dig the thermal stuff too, and did a bunch of calculations and testing previously on the 500A Revolt prototype in my car.
See my posts on these pages:
http://ecomodder.com/forum/showthrea...tml#post153760
http://ecomodder.com/forum/showthrea...ng-9325-2.html
http://ecomodder.com/forum/showthrea...-6404-214.html
Some basic heat generation formulas for mosfets were used that approximates both conduction and switching losses (just google 'mosfet heat dissipation' or something).
At cruising speeds, ~200 motor amps, the total calculated heat generation was roughly 300W. Roughly 2/3 of that were from the 10 mosfets, so that's only 20W per mosfet. When I went through the calculations, I think I was predicting a junction temp somewhere around 80C at these steady state conditions in the hot AZ weather. The heat generation is roughly proportional to motor amps (duty cycle has an effect too), so conceivably 400A would have twice the heat generation. Accelerations would also create excursions into higher temperatures, though they often occur after coasting or sitting still for several tens of seconds which allow enough time for the controller to cool down a little bit.
Paul's SR version will use the same number of mosfets, but I believe he is lowering the gate resistance to decrease the turn on/off times. This should reduce switching losses significantly and reduce the total heat generation. Additionally, any output above 500A *should* only last for several seconds. A dynamic analysis would be more appropriate to estimate how much time at full power it would take to hit the thermal cutback threshold. Getting rid of the thermal pad would surely benefit this dynamic thermal performance. Even still, I read a post that the Zilla 1k has a hot-day rating of 1000A for 10 seconds - the SR version may have something similar.
All this being said, I agree that it's all about cooling. The soliton1 claims 1000A continuous with proper water cooling. My setup is a basic finned heatsink with 3 wimpy computer fans. I think there is a lot of room for improvement if you were to take the time and effort. Someone mentioned heat pipes - I've been thinking about that as well as it should be fairly easy and effective, but what I have now seems to work!