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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.

enter image description here

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.

Bud
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2 Answers2

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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.

DKNguyen
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  • Ok thank-you for you reply friends. Now i gone step down from 50 to 12v, using mosfet. And i have a problem in that. I will post that as a new one. – Bud Aug 28 '19 at 06:31
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the answer of @DKNguyen is correct.
it is normal. so please attention : Pout = 50ma*5.2v = 265 mw Plost= 50ma * (13-5.2)V = 0.400

TO-220 thermal resistance = 100C/W

T = 25C + 0.4*100 = 65c

it is posible then we have to use heatsinking

Mahdi KhosroAnjom
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