Here's a short heatsink lesson, heavily simplified.
Doped silicon semiconductors start coming undoped at over about 150C. Depends on silicon doping levels, processing, etc., etc., but 150C is what most power devices claim. Other things like uC CPUs don't like to get over 85-90C before the internal operations start going funny, but that's a special case. ==> The datasheet lists the maximum chip temperature.
In the case of the 7806, Fairchild only wants you to let it get to 125C when it's operating. How much wattage is that?
You can't tell without knowing a lot of other stuff. If the chip package is bolted to a metal plate and the metal plate is externally heated to 125C, then the available self heating power is zero - you can't even dissipate a milliwatt without going over the 125C limit. And if the package is bolted to an external plate which is cooled with ice water to 0C, then you can dissipate a lot?
Yes. But you have to know the numbers. Heat flow is modeled like current flow. Temperature is analogous to voltage and heat flow (watts) is analogous to current. You can compute the temperature something will get to by computing the amount of watts flowing and the thermal resistance
of the stuff it passes through. To get from the semiconductor chip to the outside world, the heat has to pass through
- the chip to package thermal resistance
- the package to heat sink thermal resistance
- the heat sink to ambient temperature resistance.
On Semi also makes a 7806, and they spec theirs at 150C operating. The datasheet http://www.datasheetcatalog.org/datasheet2/9/0p4t1g1lw0spoo5ukh2s1uxchzky.pdf
also lists two thermal resistances. One is the resistance from junction to ambient (that is, just the case sticking up in the air) and another of junction to case. The junction-to-ambient (JTA) is 65C/W, meaning if you make the chip generate one watt, the junction will rise 65C higher than the surrounding air. If the air is 25C, then the junction is at 90C.
Put another way, with 25C air, the most power you can dissipate internally is (150C-25C)/65 c/w = 1.923W.
However, if you bolt that thing to a heat sink with a thermal resistance of 0.5 c/w and either lap the metal surfaces smooth or use heat sink goo to get the heat transferred over the gap between the two down to 0.7C/w, then the total resistance junction to ambient is the junction to case resistance of the package (5c/w) plus the interface plus the heat sink, or a total of 6.2c/w, and the allowable power is (150-25)/6.2 = 20W.
For all power devices, how much you can dissipate is determined by
- the max junction temperature (from the datasheet)
- the junction-to-case thermal resistance (from the datasheet)
- the case-to-sink thermal resistance (how well can you mate the semiconductor case to the heat sink surface to let heat flow across?)
- the heat sink thermal resistance to ambient air (or water, whatever)
- the temperature of the surroundings that you're going to reject the heat to.