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chicken and the egg, or, diode V/I & Tj curves

Discussion in 'Electronic Design' started by [email protected], Mar 20, 2007.

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  1. Guest

    I'm trying to figure out how hot the junction of a diode will get. I
    know that 1.75A will be passing through it. I know the ambient
    temperature willbe 25C. I look at the V/I table in the datasheet. It
    has several curves depending on junction temperature. Looking at the
    curves, you can see that the drop across the diode for a given current
    changes depending on junction temperature. But junction temperature is
    dependent on the power dissipated inside the diode, which is
    calculated using the drop across the diode, which is affected by the
    junction temperature....AHHHHH!! (brain exploding sound).

    So it seems to be chicken or the egg. How do I find the "equilibrium"
    point?

    Its the MBRD650 (data sheet at www.irf.com)
     
  2. Eeyore

    Eeyore Guest

    With or without forced cooling ? Will it *really* be 25C btw. Are there no other
    hot components near it ?

    In addition to which you need to add in how well the leads are 'heatsunk' !

    Ok, it's D-pak so it's designed to be heatsunk to the pcb. Look for an
    application note.

    It's a 6A device so 1.75A simply isn't going to make it sweat.

    Graham
     
  3. John  Larkin

    John Larkin Guest

    You could do one of those old classic load-line sorts of graphical
    solutions, but that would take a lot of setup.

    So iterate:

    Calculate Theta_ja, based on the diode T_jc and the external
    heatsinking.

    Guess a temperature.

    Look up the voltage drop.

    Multiply by 1.75 to get the power

    Multiply by T_ja to get temp rise

    Add ambient to get junction temp.

    Compare results to your guess and make a better guess and do it all
    again, until you stop changing or straddle the solution point.

    The hardest part will be finding T_ja.

    John
     
  4. Guest

    I like load lines. How exactly would I do a load line for this kind of
    problem? What is the line? Power dissipated versus Tja?
     
  5. Ian

    Ian Guest

    Tj max =150 deg C
    Max Vf at 25 deg C at 3A 0.7V
    Max Theta j-a no heatsink 80 deg C/W
    Max dissipation 1.75 *0.7 = 1.225W
    Tj < 123 deg C

    So you're safe at room temp even without a heatsink.

    Putting copper in the board round the part will have far more
    effect than tolerances of Vf or variations with temperature.

    Regards
    Ian
     
  6. Eeyore

    Eeyore Guest

    You need to know the thermal impedance of your pcb pattern.

    Which you don't or apparently don't realise the importance of. Hence the
    question is moot.

    Graham
     
  7. Noway2

    Noway2 Guest

    You make a good point here. Rather than trying to calculate how hot a
    component will get running in the application, you are better off
    calculating the worst case and seeing if you are safe. If you can
    handle the worst case, which is often times easier to calculate, then
    you have reasonable assurance that things will work in practice.
     
  8. Jim Thompson

    Jim Thompson Guest

    The way we "big boys" do it...

    Run 1mA thru diode
    Place in oven and measure (and record) forward drop versus temperature

    In system, run load current until forward drop reaches equilibrium
    Abruptly switch to 1mA and measure forward drop
    Consult table you made from above

    For chips I often add a diode that is used for nothing but temperature
    recording.

    ...Jim Thompson
     
  9. Do you connect the diode to I/O pins, or use some internal trick to
    read it?
     
  10. Jim Thompson

    Jim Thompson Guest

    For the prototypes it's usually connected to a package pin, though it
    may only be a probe pad.

    It's normally not used in the final product.

    The latest game in town uses a MUX to look at a variety of internal
    points thru a single test pin.

    ...Jim Thompson
     
  11. John  Larkin

    John Larkin Guest

    Tja is constant, and you need to figure that out for any solution.

    The graph paper axies will be power dissipation (Y) versus temperature
    (X). There will be two curves to plot, call them "diode" and
    "heatsink."

    The heatsink curve is easy. It is 0 power at 25 deg C (or whatever you
    call ambient) and rises straight-line with slope 1/Tjc.

    For the diode curve, get a table of diode voltage drop vs temp at 1.75
    amps and multiply by 1.75 to get some points of power dissipation vs
    temperature. Plot those points and eyeball-interpolate the curve.

    (Or use Excel, curve fit, and spoil all the fun.)

    The intersection of the curves is the equilibrium operating point. I
    think.


    |
    |
    |
    |
    |
    |
    |
    |
    |
    | /
    | / ------------ diode
    | / ------/
    | ------+------/
    | -------/ /
    | /------/ /
    |---- /
    | /
    | /
    | / heatsink
    | /
    | /
    | /
    | /
    | /
    -------------------------------------------------------------
    25c


    The diode curve may actually slope up or down, depending on things. It
    could even be flat or sorta parabolic. For most diodes operated at
    moderate current, it will slope down. I ain't gonna redraw it.


    John
     
  12. John  Larkin

    John Larkin Guest

    oops, Tja ^^^^^


    John
     
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