lm2576 troubles

Discussion in 'The Projects Forum' started by scudza, Jun 19, 2013.

  1. scudza

    Thread Starter New Member

    Jun 19, 2013
    4
    0
    Hi everyone
    I recently started messing around with electronics, i wanted to design a power supply using the lm2576t-5. I built one of the example circuits in the datasheet but the Vout was not 5V, it was approximately Vin-4v.

    So i decided to change a component, i swapped out the 1000uF 65V Cout cap for a 100uF and sure enough I got my Vout to stay at 5V. So my first question is why was this the case?

    [​IMG]

    In terms of power dissipation how is this calculated? in the datasheet there is a formula for Pd, when i use it with my values it gives Pd of 0.38Watts.

    Pd = (20V*0.009A)+0.25*0.4A*2V

    [​IMG]

    However when i touch the heatsink it is very hot while I'm supplying 20V input and outputting 5V with load pulling 0.4A.
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,061
    3,833
    Are you sure your diode D1 is the right direction (cathode AWAY from ground),

    Be careful with inductor selection, frequency and load are important. Make sure your inductor is rated correctly. It may become magnetically saturated if not rated for this.

    Finally, note the ground plane or ground point wiring suggestion on p11 of DATASHEET (Texas instruments).

    If everything else is connected correctly, I can't provide more than suggesting you check the basics. Also, an SMD chip will get hot to the touch with 0.4 watts of power. Add a heat sink.
     
    Last edited: Jun 19, 2013
  3. scudza

    Thread Starter New Member

    Jun 19, 2013
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    hi, I am using the TO-220 package with a heatsink. Is the 100uF cap suitable? it didnt work with the 1000uF cap.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    As GopherT mentioned:
    What is the part number of your inductor?
     
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    You should follow the simple switcher software and use the correct cap. That 1000uF might be wrong for the loop compensation. Use the recommended inductor.
     
  6. scudza

    Thread Starter New Member

    Jun 19, 2013
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    um i'm not sure about the part number, its just a regular inductor i got from the lab.

    gold brown black brown
    [​IMG]
     
  7. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    With 2W going to the load, assume 80% efficiency. That means the switcher dissipates roughly 0.4W. For a T0-220 device with no heatsink, it's about 65C/W so yours should be about 50C. That's hot but not terrible.
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    You can't just stuff random parts in and expect it to work. The output cap has to be the right value range and ESR. The inductor has to be the right value and have the correct current rating.
     
  9. absf

    Senior Member

    Dec 29, 2010
    1,493
    372
    You should be looking for some inductors like these....

    Allen
     
  10. scudza

    Thread Starter New Member

    Jun 19, 2013
    4
    0
    So i bought an inductor coil and a 1000uF 50V low ESR cap, connected it all up and it doesn't get hot anymore when i'm pulling the same load as before!

    Thanks everyone for your input :)
     
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