What are the test condition for power Transistors?

[Shahriar]

Hi
If you take a look at a TIP122 datasheet  (which is a power transistor) it is written 8A - 65Watt.
As I remeber the power of a transistor is defined as so:  P = VCE . Ic. it means I can draw more than 3A when the voltage of the collector is 10V and the emitter voltage is around 3.5V.
But when I just draw 1A from TIP122, it gets so much hot (Although it has heatsink) that I didn't dare to leave it more and the voltage of Emitter increases 0.01V every two seconds. I guess this is because the CE resistance Decreases as the temperature Increases.
the same thing happens for 2N3055. In datasheet, it has written 15A, 105Watt. It means we can draw
nearly 10A in 30V Stabilized power supply in project section when the collector is 32V and the  output is 22V.
My Question is In what test conditions the manufacturer has written these bullshits???

ThanX in advance
Shahriar

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[audioguru2]

Hi Shariar,
If your TIP122 transistor gets hot when it dissipates only 6.5W then either your heatsink is too small, you didn't use thermal grease between it and the heatsink or you used an insulating washer between it and the heatsink.

All power transistors have their absolute maximum power rated at a case temperature of 25 degrees C which is about room temperature. There must be perfect contact of the transistor to the heatsink, the size of the heatsink must be infinite, or have a cooling fan blowing on it.
The datsheet describes the transistor's thermal resistance and provides a formula and a graph for you to select a suitable heatsink for the power dissipated. 8)

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