Vtyjkyuk

I am wondering why manufacturers mention two different maximum power dissipation values for transistors like 2N1613:

Does it have anything to do with whether a heatsink is used or not?

Normally transistor power dissipation is quoted for T_A and T_C. Renesas explain it as follows:

The specification at T_A = 25°C in the power ratings refers to the total power dissipation of a discrete semiconductor element in an environment with an ambient temperature of 25°C. In this case, the thermal resistance from the heat source to the ambient air is expressed as R_th(j-a).

The specification at T_C = 25°C in the power ratings refers to the total power dissipation when the semiconductor element (case) itself has been forcibly cooled, i.e., when temperature of the package surface is kept at 25°C.

Note that the ratings may include the note "with infinite heat sink". However, in actual use, it is very difficult to make the package surface temperature exactly 25°C, and if you also take derating into account, the allowable power will in fact be somewhere in between T_A = 25°C and T_C = 25°C.

In your example, the maximum power dissipation when T_C = 25°C is 3.0 W and the maximum power dissipation when T_A = 25°C is 0.8 W. Note that when the ambient temperature is 25°C the case temperature is likely much higher, so the maximum power rating must be lower.

Adequate cooling makes a big difference.

They're limits for different operating conditions:

So the first limit is when the case temperature ($T_C$) is held at 25 C by some means (for example with a heat sink and forced air cooling, or just by operating the part with low duty cycle).

And the second one is when the ambient temperature ($T_A$) is 25 C, so the case will be heating up above that temperature.