I need to do a calculation, Google search or whatever, but I'd rather watch the NE Patriots playoff game, so I'm going to toss my problem over to you guys. Perhaps it can only be solved with a bench measurement. Has one of you been there and done that?
I have three DPak MOSFETs located on 2.5 square inches of copper on both sides with heat-transfer vias. Additional heat-sinking is required, and I'm evaluating a 35x60mm flatback heatsink with fan to remove heat and increase the maximum allowed dissipation.
I've located a decent heat-spreader layer, by t-Global. I can easily place it on the DPaks with the heatsink on top. We know the heatsink will have a poor thermal path to the DPak tabs. Alternately the heat transfer layer and heatsink can go on the other side of the board, arguable with better heat transfer from that side's copper. Which scheme will work better? Can we add performance numbers to the evaluation?
If I use Aavid 0.5-inch-high 7109 DPak PCB-mount SMT heatsinks on top, with a fan, we might be able to handle more than 5 watts total for the three parts. I'd like to do much better than that. The MOSFET channel-to-tab Rth is 6-deg-C/watt. My co-conspirator says I should stand up the part and treat it like a TO-220. Ugh! Infineon offered an upside-down TO252, with metal exposed to the heatsink. Don't wanna try bending leads and playing that game.
Any advice or comments will be appreciated.