Transistor Silicon die size, package and thermal

[mlokhandwala]

I am trying to ask two questions here

1. I am pretty confused about how to calculate the heat a component will generate, especially in the context of a transistor or LDO. I and using a Freescale microcontroller which controls a ballast transistor to help me drop incoming voltage from 12.5V to 5V. I am drawing possibly about 50mA from the transistor which is a BC68PA NXP SOT1061 package. The PCB is a 2 sided FR4 and the footprint is the minimal with no extension for heat sink. The transistor heat up to an extent that it cannot be touched by hand. 12.5-5 * 0.50 = .375W. I am not expecting this kind of heat. I can increase the heat sink area but my problem is ultimately the quantum of heat would be the same within the enclosure which has no air flow even if the transistor itself may feel less hot right ? How can I calculate the heat being generated based on the Watts only?

2. There is a reference board from Freescale (TRK-S12ZVHY64) with the same MCU and the same transistor BCP68 except it's an SOT223 package and it's dropping exactly the same voltage. I am not sure about the current but it could be the same too as it is running a large LCD and several LEDs. This however seems to generate no heat whatsoever and feels like room temperature when touched.

So the questions really are

1. How can I calculate the actual heat generated by the transistor. Is it purely and only based on Watts dissipated as given above or is there something more to it? What would be the rule of thumb/easy way to calculate such heat in *C from Watts ?

2. Is there a difference in the actual semiconductor die size between packages like SOT1061 and SOT223 which makes SOT223 generate less heat or is it only that the outer package is different and bigger ?

 

[Audioguru]

Heat is caused by the voltage across it times the current in it. Its ability to throw the heat away depends on its physical surface area in open air (not enclosed) and its heatsink that is in open air.
Your problem is that your transistor is wasting a lot of voltage across it.

If the transistor is switched completely on and completely off quickly with Pulse-Width-Modulation then the average output voltage will be reduced like you want but the transistor will remain cool.

 

[mlokhandwala]

I agree with what you are saying, and therefore I am considering moving to use a buck converter, however that does add cost and the cost may be a waste considering that the MCU is supporting this external ballast and my current requirement is really small (50mA tops).

Any idea about the second question...

2. Is there a difference in the actual semiconductor die size between packages like SOT1061 and SOT223 which makes SOT223 generate less heat or is it only that the outer package is different and bigger ?

 

[Audioguru]

Voltage times current makes heat. Physical size of the die has nothing to do with it.