Heatsink Choice

Wetmelon

Hello again

I'm trying to decide on the correct heatsink for my project, but I'm not sure exactly what I'm looking at. On Digikey, each heatsink has values for Thermal Resistivity and Power Dissipation. This is the datasheet information for the MOSFET:

Tj = -55 to +175 C
Rds(on) = .02 ohm
Rjc = .75 C/W
Rcs = .5 C/W
Rja = 62 C/W

I'm driving 40A at 12V, so my power = 40*40*.02 = 32 W

Temperature Rise (Rjc) = .75*32 = 24 C
Temperature Rise (Rcs) = .5*32 = 18 C
Total Temp Rise (Rjc+Rcs) = 18+24 = 42 C

Available Temperature Rise (150-42) = 108 C

Max Thermal Resistivity of Heatsink = 108/32 = 3.75 C/W


Is the Thermal Resistivity the only thing I have to be worried about? I found a suitable heatsink, but it says its power dissipation is only 9W @ 60C.

Wetmelon

What do you think? Do I need to worry about the "Power Dissipation" value, or just Thermal Resistivity?

mheslep

Stick w/ the thermal resistivity figure. The sink you've selected is insufficient in ambient, unforced, air. If you want to operate the FET w/ a max junction of 150C in up to, say, 30C ambient then you need an effective thermal resistance of ~2.4 C/W. I suspect a common PC fan will move your sink's resistance down a degree C per Watt, just guessing.

Also, since you're running close to the margin, the above applies if the FET is rarely switched. If you are switching w/ any frequency at all you should include the increased power dissipation during the switch time. And of course be sure you have an adequate gate drive circuit, since if the FET switches slowly for some reason you'll need spares.

Wetmelon

Crap, I forgot to add in ambient temp. Depending on where this goes, it could be up to 35 or 40C ambient.

Damn.

I'm adding a fairly high-speed fan to the case, giving me nearly 600 LFM of flow. From the charts & graphs I've seen, it should bring my value down a pretty decent amount (5-6 C/W?). I simply didn't have room on the board for the nice big 2.4C/W heatsink, so I'm going to have to give it what I can and blow a lot of air over it. It's also only going to be switched on for a second or two to start the motorcycle's engine then switched off. Yes, I'm using a MOSFET as a starter relay? Got a problem!? ;D The other one I need to sink is going to be driving up to 30A the entire time the car is powered on.

mheslep

You stated the thermal resistance of the heat sink was 3.75 C/W. I assumed that was still air. Is it? If so, it can not decrease by 5-6C/W.

No problem, just make sure you drive it fully on to the rated Vgs.