+ Post New Thread
Results 1 to 4 of 4
  1. #1
    Full Member level 6
    Points: 2,916, Level: 12

    Join Date
    Aug 2012
    Location
    .
    Posts
    376
    Helped
    7 / 7
    Points
    2,916
    Level
    12

    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 TO-252 (0.4 deg C/W), soldered to PCB that will act as a heatsink.

    Power dissipated by the device is (12-5) * 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.

    Click image for larger version. 

Name:	w1.png 
Views:	2 
Size:	34.0 KB 
ID:	120298

    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?

    •   AltAdvertisement

        
       

  2. #2
    Advanced Member level 5
    Points: 34,834, Level: 45
    Achievements:
    7 years registered Created Blog entry Created Album pictures
    SunnySkyguy's Avatar
    Join Date
    Sep 2007
    Location
    Richmond Hill, ON, Canada
    Posts
    6,729
    Helped
    1679 / 1679
    Points
    34,834
    Level
    45
    Blog Entries
    7

    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/product-detai...116-ND/1926159

    I wonder if a White powder coat is better than grey?

    More details on solar cooling
    http://www.pentairprotect.com/wcssto.../WP-00005B.pdf
    A 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



    •   AltAdvertisement

        
       

  3. #3
    Super Moderator
    Points: 77,923, Level: 68
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    15,796
    Helped
    3593 / 3593
    Points
    77,923
    Level
    68

    Re: Thermal Calculations (heat rise) - Help

    Hi,

    Power dissipated by the device is (12-5) * 0.25 = 1.75 Watts
    this is the dissipation of the regulator.
    and
    the enclosure would need to dissipate a total of 1.75 + 7 = 8.75W
    But now the circuit consumes 0.25A x 5V = 1.25W, as long as the power is not pissipated outside the box, with a motor or any other outside power consuming device.

    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



    •   AltAdvertisement

        
       

  4. #4
    Super Moderator
    Points: 29,666, Level: 42
    andre_teprom's Avatar
    Join Date
    Nov 2006
    Location
    Brazil
    Posts
    8,866
    Helped
    1117 / 1117
    Points
    29,666
    Level
    42
    Blog Entries
    9

    Re: Thermal Calculations (heat rise) - Help

    Quote Originally Posted by Jester View Post
    "...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
    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"
    At other countries located at similar lattitudes should not vary so much.
    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 )



--[[ ]]--