On Wednesday, November 13, 2013 11:07:47 AM UTC-6, David Riley wrote:
You're not going to get significantly better thermal performance out of a TO-3 attached to a small heatsink than you will out of a TO-220 attached to a similarly sized one (though it is obviously better than zero benefit).
Check the datasheets and run a thermal simulation... Its huge like a factor of ten sometimes. Max junction temp 150C and junction to package thermal resistance of 0.5 C/W will always beat max junction temp 125C and package thermal resistance of 5 C/W. The point of my long explanatory post was it doesn't matter anyway for our project's power levels because the short ckt protection is kicking in long before the thermal protection ckt is kicking in, so thats why its OK to use the TO-220 3 amp regs. As long as the heatsink can keep up, the junctions will be chilling.
Now if we were turning 14 volt car power into 5 volts, then the numbers aren't so pretty. That would be like 10 watts per amp, and 3 amps would mean 30 watts, and a cheap TO-220 linear reg at 5 C/W package design would mean the junction roasts at 150 degrees hotter than whatever the heatsink is at (whoops) so you'd have to use dry ice cooling on the heatsink for the junctions to survive, but a TO-3 mounted junction would be a mere 15 degrees warmer than the heatsink, which is nothing.
Its all moot anyway; TO-3 regs aren't going to be made anymore, maybe ever again, the market is flooded with non-operative TO-3 counterfeits, switcher modules are cheaper than TO-3 + heatsink cost now, high end modern TO-220 packages work almost as well as ancient TO-3 designs and we aren't thermally limited anyway.