We need to drive a motor with around 40 Amps. The mosfet has an Rdson of about 7 mohms at 90 degrees Celsius. That is a whopping 11.2Watts of heat generated on the poor mosfet.
We are very space constrained so initially we thought we'd use a surface mount mosfet, such as D2PAK. Is it at all possible a surface mount mosfet to handle so much heat? We thought of mounting the mosfet on a large copper pad (that sort of diminishes why we chose the D2PAK in the first place though, since we can't use that board space anymore), and placing many thermal vias on that copper pad all the way to the backside of the board, and on the back, again on a large copper plane, mount an heatsink. Can we dissipate the heat in this fashion? Would the vias on the board be an effective thermal path?
Another option is to use TO220. But we can't figure out a good way to cool TO220's in our restricted space. There are individual heatsinks for TO220s on the market, but without forced airflow, most of them are capable of cooling the device down to about 80 degrees above ambient at 11.2W. That is a bit too much.
I would like to hear your experiences in cooling mosfet packages, any ideas would be appreciated.