Heat dissipation question


Joined Jun 4, 2014
The Tc value applies if the case temperature is maintained at 25C (implies an infinite heatsink).
The Ta value is without a heatsink. I;m never quite sure how to interpret this as it clearly depends on the air flow and any heat producing components nearby.
On a finite heatsink, you use the junction to case value and the heatsink value (plus insulator if used) to calculate the total junction to ambient thermal resistance.


Joined Oct 7, 2019
Picture one: often there is a foot note "infinite heat sink" for 40 watts, "in air" for 2 watts.
You can not afford a infinite heat sink.
Two watts in free air has the junction at the milting point. Also 40 watts with infinite has the temp right on the edge of not working.

Picture two: The silicon will increase 62.5C for every watt of energy with out a heat sink. Two watts = (62.5 X 2) + 25C room temp = 125+25=150C on the silicon.
Bolt the transistor to a block of Aluminum the size of your house. (almost infinite) The silicon will increase by 3C/watt while the case remains at 25C.


Joined Jun 6, 2011
The Tc value applies if the case temperature is maintained at 25C (implies an infinite heatsink).
Actually, it doesn't imply an infinite heat sink, just that the case temperature is equal to (or less than!) 25C.

As an example, imagine a paltier or liquid cooler attached to the case. As long as the cooler keeps the case <= 25C (regardless of ambient), the part will support 40 watts.

One could also operate the device at 40W with a reasonably good heatsink and an ambient temperature far enough below 25C that a 25C case temperature is maintained.

Audioguru again

Joined Oct 21, 2019
I think that it is bad for a transistor or IC to have a wide temperature swing between being turned off and turned on, specially if it happens frequently. The temperature swings cause contractions and expansions over and over which will result in fractures. Use a heatsink large enough or a fan so that the temperature swings are not frequently wide.

Audioguru again

Joined Oct 21, 2019
We were never told why the transistor gets hot and what it is used for.
I hope it is not at the output of a class-A audio amplifier and room heater.