Electronic – Heatsink calculations for a TRIAC

When you see a heatsink with a screw, that is because the chip it is mounted to has a hole for a screw mount.

Sometimes the plate that you have to heatsink is going to be a different voltage then the surrounding board, so you need to use something that is electrically isolating, but thermally conductive. These replace the need for conductive paste.

If you have the area of the regulator/IC that generates heat at a voltage like ground, and the other connections for your heat-sink will be ground, you can connect them directly, and you normally want to use a form of thermal paste. I have attached a heatsink without thermal paste and still had a device temperate at almost 90 degrees C. After adding thermal paste it measured at 5 degrees C above room temperature. Often being able to connect your heatsink to ground via solder helps dissipation as it dissipates to the ground plane.

In computers you have a very specific task, cooling the Processor. In electronics it can be a very large range of tasks, and often you are willing to pay more to cool something because you design calls for it, or you are willing to pay less because your design does not need some very nice thermal paste.

In general, you are going to just want to use thermal paste, you do not have to worry about insulating your heat-sink if you leave it floating in the air, or ensure where it mounts to the board there is not a voltage connection. This keeps things simpler for a tinkerer. For any chip you get that you think needs heat-sinking, read what they suggest to do (on the datasheet), and follow it.

Last but not least, thermal paste for computer processors will work for ICs.

To directly answer your first question: No, not even close. A little more copper around a surface mount part isn't going to get rid of 11W of heat. No way.

One answer may be to parallel multiple FETs. Not only does that cut the total dissipation by the number of parts, but the dissipation on each FET is reduced by the square of the number of parts. So if one FET dissipates 10W, then two parallel FETs would dissipate 5W total, and each FET would dissipate only 2.5W.

That's in theory. In practise they won't share the load exactly equally, so you have to design for a bit worse than that per FET. The good thing about paralleling FETs is that they have a positive temperature coefficient. The Rdson goes up with temperature. This helps them ballance somewhat and prevents runaway of a single part, like could be the case with bipolar transistors.

Ultimately you have to decide what you really want. Switching 40A is going to make some heat. One way or the other you're going to have to deal with that. You can tell us you're space contrained all you want, but ultimately the physics will dictate a certain amount of space, surface area, forced cooling, or whatever. It may not be possible to meet all the constraints. Not all combinations of small size, high current, and low cost are possible.