why the specified maximum power dissipation of a power transistor at an ambient temperature of 25 celcius is much less than maximum power dissipation at a case temperature of 25 celcius?
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That's not how it works. The dissipation is rated at 25c and gets derated by some fraction of a watt for each rise in case temp above 25c.
How are you going to keep the case at 25 degrees C when the transistor is hot?
The case will also get hot then the chip will be too hot. So you must reduce the power.