Hi friends i am trying step down converter using transistor.
Application:
- Micro controller power-supply = 3.3 volt
- Input battery supply range = 10 - 13 volt
- So i use BD139 for stepping down from 13 to 5v and the using 3.3v regulator.
Circuit works fine and i connected my controller. But the problem is BD139 is heating up-to "45-49 degree". My controller consume only 50ma current. But i don't whether this heating is normal or abnormal.
Answer
11-13V to 5.25V @ 50mA linearly regulated is 0.2875W to 0.3875W dissipated.
If ambient is 25C, then a temperature of 49C is a rise of 24C above ambient.
That works out to a junction to ambient thermal resistance of 126C/W. The BD139 has a listed thermal resistance of 100C/W without heatsinking. So that seems pretty close.
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