These power ratings are heating Watts, not output Watts. The transistor creates heat that must be dissipated away. The amount of heat is the voltage across the transistor multiplied by the current through it. The datasheet says 90W when its case is held to 25 degrees C somehow. The BD139 is allowed to heat with 8W when its case is held to no more than 70 degrees C somehow. The 625mW transistor is usually used without a heatsink so it must not exceed its maximum allowed temperature.1) how much power i can get with 90W transistor and a big heatsink. 45W ? or 75% ?
2) how much power i can get from BD139 with some heatsink ?
3) how much power i can get from 625mW transistor without heatsink ?
A 625mW transistor will be at its maximum rated temperature when dissipating 625mW in 25 degrees C air. I would use it to maybe 470mW (3/4 of its max temperature).
A BD139 or BD140 needs a pretty big heatsink then can dissipate 8W. The datasheet says its thermal resistance from the ambient to its chip is 100 degrees C per Watt so without a heatsink then the chip will be at its max rated temperature with 1.2W when the ambient air is 30 degrees C. I would use it to 0.9W.
To use a heatsink then the datasheet says the thermal resistance from its chip to its mounting base is 10 degrees C per watt, (thermal grease is an additional 0.5 degrees C).
The rated power of 1.2 W without heatsink involves a junction temperature of 150 °C and about 140°C case temperature. The hand calculation also gives > 70 °C case temperature for 0.5 W. If you touch it, you'll still find it "very hot". So what's not true, particularly?This is not true, a BD gets very hot at 500mW Pd.
Bonus question -> how much power i can get with 90W transistor and a big heatsink. 45W ? or 75% ?
another bonus one -> how much power i can get from BD139 with some heatsink ?
another bonus one -> how much power i can get from 625mW transistor without heatsink ?
This is so confusing.
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