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18th August 2015, 13:17 #1
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Thermal Calculations (heat rise)  Help
I'm trying to determine if a simple voltage regulator circuit will operate within its allowable temperature range.
The device package is TO252 (0.4 deg C/W), soldered to PCB that will act as a heatsink.
Power dissipated by the device is (125) * 0.25 = 1.75 Watts
Using TI's thermal calculator, it appears that if I have 8"^2 of copper, the regulator will be hot but should be within it's range.
However the PCB will be mounted in a sealed (gray) metal enclosure with a surface area of 0.5ft^2 and located outdoors in full sunlight. The maximum ambient temperature in this location will be 45 deg.C
Assuming 14W/ft^2 of solar energy, the enclosure would need to dissipate a total of 1.75 + 7 = 8.75W
Based on this calculation: http://www.hoffmanonline.com/stream_...09&pRID=162533
It appears the temperature rise in the enclosure would be about 6 deg C (seems low?)
If that's correct the maximum temperature on the board would be about 90 deg C
Does this look correct?

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18th August 2015, 16:16 #2
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Re: Thermal Calculations (heat rise)  Help
All thermal resistances add to Tj which is the important metric to keep below 85'C in a good design.
YOu can test it under a 300W Work Lamp using a Halogen lamp at < 0.5m distance or so to simulate solar power for 250W in a 0.25 m2 area.
A better solution would be to use a spring clip to the case inside to conduct heat directly to the case rather than indirect via the topside copper with solder mask and thermal insulating Epoxy dielectric.
The best is to eliminate the heat loss with a 3 terminal switching regulator from OKI/MURATA that avoids the VI power drop loss.
   Updated   
Have you considered these? or similar? 85~90% eff at 12V at 0.25A out
http://www.digikey.com/productdetai...116ND/1926159
I wonder if a White powder coat is better than grey?
More details on solar cooling
http://www.pentairprotect.com/wcssto.../WP00005B.pdfA best design is easily achieved with good test specs™
A better question deserves a better answer. ™
... so include all your acceptance criteria ( values, % tolerance) and assumptions in your question or any design.
... Tony Stewart EE since 1975
 slightly north of Toronto

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18th August 2015, 16:37 #3
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Re: Thermal Calculations (heat rise)  Help
Hi,
Power dissipated by the device is (125) * 0.25 = 1.75 Watts
and
the enclosure would need to dissipate a total of 1.75 + 7 = 8.75W
If there is no power consuming outside the box, then you can calculate with the total voltage of 12V and 0.25A = 3W.
A 85% switching regualtor will reduce this to: 5V * 0.25A / 0.85 = 1.47W
Klaus

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18th August 2015, 16:59 #4
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Re: Thermal Calculations (heat rise)  Help
The document above mentioned, says that:
"Over much of the United States, the approximate peak values of solar radiation striking the Earth’s surface is 97 W/ft^2"
Therefore the calculation you did seems quite optimistic, perhaps not correct.
Part of the world that you live in, You are the part that you're giving ( Renaissance )
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