Question: I refer to a 2N3055 power transistor, the spec. sheet gives the following: 

Max. current 15A, Max. Voltage 50V, Power rating is 115 Watts. 

* What is the power rating calculated on?
* If it is E\*I then the power would be 750 Watts, this is of course incorrect, so how is it calculated please.?

Answer: 
You can have maximum current through the transistor and you can have maximum voltage across it but you can't have both at the same time. This is specified in the "safe operating area" power curve.

[![enter image description here](https://i.stack.imgur.com/yAQU1.png)](https://i.stack.imgur.com/yAQU1.png)

*Figure 1. The safe operating area of a BDV66C transistor. Source: Wikipedia [Safe operating area](https://en.wikipedia.org/wiki/Safe_operating_area).*

In Figure 1 we can see the following:

1. 16 A at about 12 V. Pmax = 192 W.
2. 5 A at 50 V. Pmax = 250 W.
3. 0.2 A at 120 V. Pmax = 30 W.

Taking the two maxima from the graph one might have expected that the device could handle 16 A at 120 V = 1,920 W which is not the case.

SOA is defined as the voltage and current conditions over which the device can be expected to operate without self-damage. The SOA figures are, of course, subject to other constraints such as maximum junction temperature and so the power dissipated as heat, even when inside the SOA, must be conducted away to maintain a safe operating temperature. There is more information in the linked wiki article.
Total Power Dissipation @ Tc = 25°C
Derate PDmax=115W Above 25°C by 0.657 W/°C for case temp, Tc so **Rjc= 1/0.657= 1.52'C/W**

This implies the max junction temp, **Tj = 175'C+25'C=200'C** but this accelerates failure rate significantly, so a **good design has Tj<< 100'C**.

That assumes an infinite heatsink which is not possible.

So to compute Tc , you need Tja heatsink rating like 0.1 'C/W for a super CPU cooler with Fan or 1'C/W for a large passive heatsink then add internal ambient rise above room temp (or outside temp)

Then Temp rise of junction is Rja\*W= Rjc+Rca(total) \* Watts = Tj rise