Some time ago, I built a soapbox-sized battery testing device. One of its functions was to discharge batteries at a specified current, and it handled most of the common battery sizes well, but its maximum discharge current was quite limited. Thus I didn't need to worry about heat dissipation.
I'm now designing an enhanced version, and this time I want to support draining middle-sized batteries (like the ones in cordless screwdrivers, they are around 12 Wh) in reasonable time. I'd be happy if I'm able to dissipate around 10-15 watts of heat in a device with the same form factor (think 85×60×40mm or similar). The heat-producing element is a TO-220 MOSFET. I'm thinking about either heatsinking it (and selecting a plastic enclosure with vents), or using an aluminum box, with the MOSFET dissipating directly to the enclosure. Would that work? Which approach is better (and of course, other ideas are also welcome)?
I'm quite new to this, so links, recommended reads, calculators and stuff would also be greatly appreciated 🙂
Best Answer
A soapbox is hardly a standard unit of measurement, but 12 W doesn't require anything too huge, even without forced air, provide you can at least let natural convection be, well, natural. Here's how you calculate the heatsink you need.
I've picked the datasheet for IRF510 as an example. It's a very common TO-220 MOSFET and should work for your application.
The first thing you will see is that the datasheet lists power dissipation as 43 W. This of course requires an ample external heatsink, but it should cover your application with a healthy margin.
The absolute maximum junction temperature \$T_J\$ is listed as \$175 ^\circ C\$, and let us assume ambient temperature is \$35 ^\circ C\$. That means the temperature can't rise more than \$175 ^\circ C - 35 ^\circ C = 140 ^\circ C\$. And to be safe, let's add a safety margin and design for no more than \$ 100 ^\circ C \$ rise.
The datasheet lists the maximum junction-to-case thermal resistance as \$ R_{\theta UC} = 3.5 ^\circ C/W \$. That is, for every watt, the junction temperature will rise \$ 3.5 ^\circ C\$ assuming the heatsink can magically remove all heat. At 15 W, that's a rise of \$ 3.5 ^\circ C/W \cdot 15 W = 52.5 ^\circ C \$.
We are hoping for no more than a \$ 100 ^\circ C \$ rise, so we will have to find a heatsink that won't raise the temperature more than another \$ 100^\circ C - 52.5 ^\circ C = 47.5 ^\circ C \$. That means our thermal resistance budget for the heatsink is \$ 47.5 ^\circ C / 15W = 3.17 ^\circ C / W \$.
This is pushing the edge of what can be done with natural convection, but it's doable. Thumbing through my Mouser catalog I can find an Ohmite heatsink FA-T220-64E with a thermal resistance of \$ 3 ^\circ C / W \$ with natural convection. It's the biggest one they sell for TO-220. It's about 1 x 1.6 x 2.5 inches and Mouser will sell you just one for $2.17 plus shipping.
Strictly speaking, I haven't taken into account the thermal resistance of the transistor case to the heatsink. The IRF510 datasheet gives a typical value of \$ 0.5 ^\circ C / W \$ for a greased surface, which at 15 W will mean another \$ 7.5 ^\circ C \$ rise in junction temperature. But remember we included a margin of \$ 40 ^\circ C \$ and assumed a rather high ambient temperature of \$ 35 ^\circ C \$. We should be safe.
Even so, something may obstruct your heatsink, so you may do well do build in some sort of thermal protection. You can implement this yourself, but there are also MOSFETs out there with thermal protection built in. If you do this, you don't need such a margin, and you may very well be able to dissipate more than 15 W if you don't mind the possibility that the thermal protection kicks in.
And, it bears mentioning that even though the transistor shouldn't fail, it will get mighty hot. A plastic box with poor ventilation is probably no good. You will have to keep fingers away for sure. If you want to keep things cooler I'm afraid you have no choice but forced air or spreading the heat over more area: big power resistors, multiple transistors, etc.