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Thermal Isolation

Discussion in 'Circuit Simulation & PCB Design' started by Mikebits, Nov 26, 2016.

  1. Mikebits

    Mikebits Well-Known Member

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    Greetings; I am designing a voltage reference board and the reference voltage ic is a lm399. The lm399 uses a built in heater to keep the temp of the part constant.
    The heat from this part can affect the surrounding components which in turn affect the overall accuracy of the design. What I would like to do is confine the heat generated by the lm399, to just that part and not allow it to just spread across the board. My question then is can anyone recommend methods to do what I need, that is thermally isolate the lm399. One method from a app note was to cut slots around the part. Are there any other suggestions?

    Thanks.
     
  2. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Mount the IC with long leads to (well above) the PCB so the only thermal path is through the small leads.
    Use a thermal shield such as shown on page 7 of this data sheet.
     
  3. Mikebits

    Mikebits Well-Known Member

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    Hi Crutch, the 399 already comes with that thermal shield. Thanks
     
  4. dave

    Dave New Member

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  5. spec

    spec Well-Known Member Most Helpful Member

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    Hi Mike,

    0.5ppm voltage/temp stability- pretty impressive. You must be doing a pretty fancy circuit.:cool:

    The only solution I can think of is to bury the LM399 in a heatsink and them maintain the temperature of the heatsink surface by peltier cooling and a thermal pipe.
    It sounds complicated, but the practical implementation should be simple because of the small size and availability of standard coolers and heat pipes.

    spec
     
    Last edited: Nov 27, 2016
  6. OBW0549

    OBW0549 Member

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    In addition to cutting the slots, you could wrap additional thermal insulation around the part. This will minimize the amount of heat the internal temperature regulator needs to dissipate to maintain its temperature at the setpoint.
     
  7. spec

    spec Well-Known Member Most Helpful Member

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    Hi OBW,

    there is a danger that the chip may rise above the temperature set-point with insulation.

    spec
     
  8. crutschow

    crutschow Well-Known Member Most Helpful Member

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    The only power dissipated, besides the heater, is the current through the 7V zener so, if that current is kept low, it's unlikely that it will overheat for any reasonable level of insulation.

    Then I don't think the amount of heat released to the surrounding circuit will be significant.
     
  9. spec

    spec Well-Known Member Most Helpful Member

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    For the voltage reference not to overheat, heat must be lost and that heat will affect the surrounding components, that is Mike's problem.

    If you put an object generating any heat, never mind how small, in a perfect insulator, the object will approach an infinite temperature at a rate defined by the energy input and the average specific heat (think that is the right term) of the object.

    spec
     
  10. OBW0549

    OBW0549 Member

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    Yes, if there's too much insulation; the trick is to insulate just enough that the on-chip temperature controller stays on, but runs at a relatively low power level.
     
  11. Mikebits

    Mikebits Well-Known Member

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    While researching this issue further, I came across this little app note showing some methods for thermal control see Image below.
    I also found this info useful from Maxim: https://www.maximintegrated.com/en/app-notes/index.mvp/id/4083

    These JEDEC standards also provide good info, I have been able to locate some of the standards listed below with Google:

    JEDEC Specification Titles
    JESD51: Methodology for the Thermal Measurement of Component Packages (Single Semiconductor Device)
    JESD51-1: Integrated Circuit Thermal Measurement Method—Electrical Test Method (Single Semiconductor Device)
    JESD51-2: Integrated Circuit Thermal Test Method Environmental Conditions—Natural Convection (Still Air)
    JESD51-3: Low Effective Thermal Conductivity Test Board for Leaded Surface Mount Packages
    JESD51-4: Thermal Test Chip Guideline (Wire Bond Type Chip)
    JESD51-5: Extension of Thermal Test Board Standards for Packages with Direct Thermal Attachment Mechanisms
    JESD51-6: Integrated Circuit Thermal Test Method Environmental Conditions—Forced Convection (Moving Air)
    JESD51-7: High Effective Thermal Conductivity Test Board for Leaded Surface Mount Packages
    JESD51-8: Integrated Circuit Thermal Test Method Environmental Conditions—Junction-to-Board
    JESD51-9: Test Boards for Area Array Surface Mount Package Thermal Measurements
    JESD51-10: Test Boards for Through-Hole Perimeter Leaded Package Thermal Measurements.
    JEDEC51-12: Guidelines for Reporting and Using Electronic Package Thermal Information.
    JEDEC51-7/12 pdf link here.
    I just thought I would share my findings in case someone else can use it.

    Thanks for the input:)
    thermaleffects.PNG
     
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  12. OBW0549

    OBW0549 Member

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    Just curious, but why did you choose the LM399? Back in its day it was just about the last word in voltage references, but that was a long time ago. Today references such as the ADR4550B and the LT6655B achieve the same 2 ppm/°C (max) temperature stability as the LM399 but without any internal heater; also, their noise voltage is about 100X lower than the LM399.
     
  13. Mikebits

    Mikebits Well-Known Member

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    Good question, and I will try to give a equally good answer.
    First let me preface with the following: The LM399 is my brassboard/predecessor design for my final design which will use the ovenized LTZ1000, which is considered the reference to end all references:), So I wanted to use a part with similar technology to learn lessons on a lower cost design as my eventual design will use high dollar resistors and the venerable but very expensive LTZ1000.
    So with that said, I chose the LM399 based on tempco (.5ppm/degC) not 2ppm as you stated above, secondly, Long term stability(LTS) LM399 is (8ppm/√kH), compare ADR4550 (25ppm/√kH),. Note: Initial accuracy of LM399 actually sucks compared to other references, but LTS spec directed my decision to go with LM399.

    As far as output noise, I plan on filtering the noise using a output buffer with filter, I have a reference design showing how to do this, and can post it if anyone wants it. What the heck, posted below:)
    Some other pitfalls of LM399, Warm up time for LM399 is 3 seconds as opposed to 160 uS of ADR4550.
    Other factors affecting choice was component package. From what I read, the output voltage of SMT packages are more sensitive to mechanical stress, and thermal gradients created by neighbor components. Lastly, I already purchased the LM399 parts, and created time consuming CAD symbols, (silly reason I know).
    refnoisefilter.PNG

    Well, that was my thinking at the time I went down this road.
    P.S. I would like to model thermal behavior of my design with LTSpice, but I think I will need help with some items. I will follow up this post with some questions/issues that I encounter as I can't recall at the moment.
     
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  14. OBW0549

    OBW0549 Member

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    All things considered, the LM399 does seem like the best choice, then.
     

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