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Thermal Heating of DM648

Mark Depp
Intellectual 765 points

 

Hello All

I am using a custom board made with schematics of DM648EVM by Lyrtech. The baord is fully functional in all respects. But for standalone application the board fits in a box close to RF circuitry. After 2hrs of operation I noticed the PCB front and back planes of custom board going very hot. Then I used mettalic spacers b/w RF circuitry and my custom board shorted with the heat sink. This caused good power dissipation as well as lowering of temperature but not excellent. I want more lowering of temperature. Any ideas?

 

 

 

 

 

 

 

 

 

  • 0
    TI__Intellectual 1670 points

    These parts are sensitive to enclosed areas and will sometimes need heat sinks.  If the processor heats, it will take more power, which will cause more heating, etc.  This can become a cycle and the part can get very hot.  Cooling the part adequately with a heat sink will lower temps and power considerably.

    It's difficult for me to recommend a heat sink since I have no idea what your box, environment, or acceptable temperatures are.  The best way to choose a heat sink is to look at the thermal properties of the part, and do the calculation for the environment with the thermal resistance of the heat sink.  This seems difficult at first, but it's actually fairly easy math. 

    First, you find out the power dissipation of the part, at the clock frequency you're running at, with the selected MAC and peripheral usage.  This is difficult to predict, but since you're already using the part, you can just measure power.  Once you find (or estimate based on the system power) the power going into the part, you know how much you'll have to cool.  If the part is dissipating 3W, then that is your heat load, for example.

    We have an application note that covers much of this process at:

    http://focus.ti.com/lit/an/spraal9/spraal9.pdf

    Since you have a heat sink for the RF portion of the circuit, I'm thinking you may already know how to select them.

    Also know that for a part such as this (where the package is not specifically designed for a heat sink), cooling the bottom of the PCB directly under the part may be just as effective as cooling the top of the package.  To do this a conforming thermal pad is required between the board and the heat sink to conform to the surface of the PCB with the parts on it.

    Feel free to ask if you have any more questions, but make sure to include more specifics about the enclosure and what temperatures you are seeing.  Thermal solutions require a knowledge of the enclosure as well as the materials used to determine where heat will go, and how well it will be dissipated.

    Keven

  • 0 in reply to KevenC
    Prodigy 10 points

    In our products, we are esperiencing a strange behaviour on the last lot of DM648.

    The same lot, same pcb, same application  (same FW etc etc etc..) and same conditions of use (testing in lab).

    We have some DM648 with the case temperature raising up to 70 °C (ambien temperature 20°C), and some other with case temperature not over 50°C.... very strange we cannot find any difference.Do you have any suggestion on what could be good to test or investigate?

    Thanks

    Omar