> how hot they get in use
Depends on the heatsink.
TO-220 without a sink and with more than a Watt in it will get VERY hot. The maximum heat is a fraction of the power amp's full-power consumption. So for any stage-level amp, you need a heatsink. I won't check FYL's 40% number; that looks close enough for what is a nonlinear problem where we want to err on the safe side.
1 Watt in 1 square inch runs about 100 degrees C rise. (Which is why a naked TO-220 cooks at something over 1 Watt.) In audio we customarily strive for half that, 50 deg C (or 60 deg C, fine in this case). That means 2 or 1.7 square inches per Watt. Taking 17 W dissipation you would like 34 or 28 square inches. Both-sides of 17 square inches. Or at least 4"x4" area.
You can't push heat through thin metal to the far end of a chassis. You would like length/thickness below 10:1. For ordinary metal gauges, a 2" push (4" across) is about all you can hope for.
> and if they use the wafer/grease or a thermo pad for better conductivity?
With a bare-tab device you MUST insulate from any grounded or finger-touchable part.
With a VERY large heatsink, the insulator is a bottleneck. The gold-standard was thin oxide grease and mica. I understand the best silicones are comparable.
With a largish device on a small heatsink, the bottleneck is the exposed-to-air surface area, NOT the device to sink area. Air is really bad. Even a crappy solid/liquid is much-much better.
On a chassis-metal sink, the real bottleneck is the ring of thin metal just outside the device mounting area. The TO-220 metal tab is about 0.15 square inches, and someone thought it needed that much. The perimeter is near 1.5 inches. If the chassis metal is under 0.1 inch thick, the sink initial cross-section is less than what the TO-220 has. That works as good as a toilet pipe that narrows from 3" to 2": you get a backup. Worse, because the TO-220 is copper (under the silvery flash) and you got steel or at best aluminum. AND the path through sheet-metal is a lot longer than the 1/4" from TO-220 die to tab edge.
OTOH that tab was sized for 80+ watts and you are only doing like 17W.
On cheap or self-protected LOW-voltage devices there is a simple test. Power up, heat up. Put your finger on the device and the sink. If they are the same temp, your junction is good; if the sink runs cooler than the device you need better junction-stuff. Obviously this would be deadly on a 400V system; save it for your LM7812 regulators and such.
Topic: To use or not to use a VVR? (Read 14607 times)