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Linear vreg Thermal spreader

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Mosaic, Jul 13, 2017.

  1. Mosaic

    Mosaic Well-Known Member

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    This is useful enough to bear repeating.
    If u have a design and discover your v.reg is running too hot due to a large voltage drop, offloading the heaitng with a couple resistors in parallel can save the day.
    I had a design that spec'd the D2 PAK size LM317 surface mount part. But due to limited PCB space it was downgraded to the smaller D-PAK case.
    Eventually I discovered the unit was running at about a 72°C delta:arghh:.

    No space for a heatsink...what to do?:confused:
    The Vreg was dropping 8.4V @ 120mA.

    I squeezed in a couple 39Ω , 0805 resistors in parallel and placed them 1mm away from the 1/4" spade terminal Vcc input for heat dissipation. I added a soldermask stop to permit thick solder on the 1mm PCB trace length. The resistors drop about 2.4V and have a 50°C delta. The LM317 also has a 50°C delta now.:) Without the 1/4" spade terminal the resistors have a 75°C delta.

    The 50°C delta is useful as the module being designed might have to sit in subzero temps and not form condensation on the PCB. Sealed Enclosure Air volume is only 0.02L
     
  2. crutschow

    crutschow Well-Known Member Most Helpful Member

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    I assume you mean the resistors are in series with the regulator input, not in parallel.
     
  3. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    If the load on the regulator varies, say +- 25% of its average load, you could put a resistor around (in parallel with) the regulator. That might help if the regulator is already operating close to its dropout voltage.
     
  4. dave

    Dave New Member

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  5. ci139

    ci139 Active Member

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    39/2*(0.12,0.06,0.03,...) ≉8.4 -- e.g. which is not the case this time
    see Power Sharing Resistor p.53
     
    Last edited: Jul 14, 2017
  6. Mosaic

    Mosaic Well-Known Member

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    Actually, since my load is about 120mA, I did both series (20Ω net) and parallel (500Ω net) resistors to obtain a 35°C improvement in the V reg op temp.
     
  7. ci139

    ci139 Active Member

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    i've used parallel resistance as an extreme last measure to quiet a noisy supply's switch element in few occasions
     
  8. AnalogKid

    AnalogKid Well-Known Member

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    As long as there is a firm minimum load current, parallel will work.

    ak
     

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