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Switch Mode DC-DC step-down PSU design troubleshooting

  1. Mar 15, 2017 #1
    I am working on a switch mode psu in step down configuration with the key components being:
    LM2592 (adj) http://www.ti.com/lit/ds/symlink/lm2592hv.pdf
    530-PC-24-1000 http://www.mouser.com/ds/2/643/PC-594199.pdf
    GSIB-25 http://www.vishay.com/docs/88646/gsib25xx.pdf

    Using a slightly modified version of the example circuit where:
    R2 = 23.82 KOhm
    L1 = 100 mH
    The diode on the left is not included but CFF is included as vin does not exceed 40vdc.
    nf0ec4.jpg

    I am using /HALF/ of the transformer and ending up with 20vdc at the output of the rectifier. (reading into this, it appears higher loads and adjustments of the filter capacitor should drop this down closer to 12vdc.) initially with both windings of the transformer attached, the rectifier was outputting a stunning ~40 vdc.

    ^this is not my issue however... (at least i dont think)

    At the moment the power supply is outputting ~18.6 vdc... (to a 5watt load of LEDs)
    With Vin reading 19.5 vdc and feedback to ground reading 0.75 vdc.

    I am not sure why i am getting practically no step-down?

    Adjusting R2 seems to have minimal affect on the output voltage.

    It's also important to note that the LM2592 produces little to no heat... slightly disturbing..

    Does anyone have a clue of some methods to troubleshoot this issue?
     
  2. jcsd
  3. Mar 16, 2017 #2
    Well... For first run I would try to start it with resistive load and a regulated lab PSU for input. I think it'll be the inadequate amount of input filtering and low output load.
    Also, you defined L1 as 100mH, but is the component OK for the current?


    By the way, are you sure it's a good idea to drive LEDs from medium grade fixed voltage source?
     
  4. Mar 16, 2017 #3

    jim hardy

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    I second Rive's observations.

    My guess would be your 470 uf input capacitor is a bit skinny .
    Pulling an amp out of a 470 uf drops its voltage 2.1 volts per millisecond,
    ΔV/ΔT = I/C and there's 8 milliseconds between line cycle peaks.

    Try tacking a 1000uf in parallel and see what happens?

    And i hope by 100mh for your inductor you meant 100 Micro-henries not Milli-henries. Got a datasheet for it?
     
  5. Mar 17, 2017 #4

    Baluncore

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    If the two secondary windings on the transformer are identical and isolated, then you might consider running them in parallel to reduce the resistive heat losses in the transformer secondary.

    LEDs may just confuse things. You need to use a better defined resistive load during testing. I would start with a couple of 12 ohm, 10 watt, wire wound resistors in series.

    It might help if you posted a photo of your circuit board with the components installed.
     
  6. Mar 17, 2017 #5

    jim hardy

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    You made a great first step right here:


    My high school electronics teacher lectured us boys "When all else fails , read the directions."
    From that datasheet:
    upload_2017-3-17_6-2-52.png

    Feedback pin 4 wants 1.23 volts and is getting but 0.75, you say ? Good troubleshooting !
    It'll stay saturated trying to raise feedback pin to 1.23 volts. So no stepdown and no heat. I like your powers of observation.

    What's R1 ? 1K as in drawing ?
    Could it be as simple as:
    1.23 = Vout X R1 / (R1+R2) = Vout / 24.82
    to get 1.23 volts from a voltage divider of 1K and 23.8K will require 30.5 volts at output ?

    Try adjusting ratio of R2/R1 to around (10/1.23) -1 ?
     
    Last edited: Mar 17, 2017
  7. Mar 17, 2017 #6

    Baluncore

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    TG_MechE. Have you tied pin 5, (off\on) to ground, or left it floating?
     
  8. Mar 19, 2017 #7
    Jim, you are 100% correct!
    I had put the incorrect voltage divider in!
    Thank you so much for your help (and to everyone else who commented.)

    R1 was 1K, I changed R2 to 10k to achieve roughly 13vdc (my end goal) and everything is working perfectly now.

    I have included a picture for those who were interested.


    2ebczz5.jpg



     
  9. Mar 19, 2017 #8
    Ahh good point! The inductor claims to be good up to 1amp. I plan to draw roughly 0.7-0.5 amps at 13volts in the final application. As for the LED, it is purely for testing as it was available and capable of handling the power.
     
  10. Mar 19, 2017 #9
    Pin 5 is straight to ground for instant-on. As this is going to be used to drive and amplifier IC, a time delay on may be better?
     
  11. Mar 19, 2017 #10

    jim hardy

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    Thanks for the feedback, and congratulations on your success .

    I really like that regulator IC, copied the datasheet to my "HandyThings to Know " folder.
    Will get a few on next parts order.Thanks for the introduction to it !
     
  12. Mar 19, 2017 #11
    Good luck! There is another version of this IC with 2 more pins which provides error reporting as well. Now onto the arduous process of eliminating 120hz hum.
     
  13. Mar 19, 2017 #12

    Baluncore

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    Assembled adjustable DC-DC buck converter modules, (using LM2596), cost less than $2 each on ebay.
     
  14. Mar 19, 2017 #13

    jim hardy

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    Amazing times we live in. Thanks, Balun !
     
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