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polyfuse and dc/dc converter

Discussion in 'General Electronics Discussion' started by johnny_cash, Aug 13, 2013.

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  1. johnny_cash

    johnny_cash

    39
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    Aug 13, 2013
    Hello,

    Im actually designing a power supply. This power supply will need the use of a polyfuse to protect lower stages (mainly logic, and speakers).

    I use LM2575 ( ds here: http://www.onsemi.com/pub_link/Collateral/LM2575-D.PDF ) based on typical usage page 2. For me, Vin = 12v; Vout = 5v, Iout = 1A max.

    I have tested successfully a polyfuse 750mA on my PSU, directly. It works as expected, and open as soon as i go over 800mA (the polyfuse is hold current 750, break current 1.5A)

    But if i push the polyfuse after the inductor on LM2575 (330uH) nothing happens. The inductor gets damn hot (and even melt the soldering...) but the polyfuse is not triggering. It seems current is limited to 1A continuously.

    How can i do to have my polyfuse working as expected ?

    Thank you,

    Johnny C.
     
    Last edited: Aug 13, 2013
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    You'll have to show us a schematic and where you're putting it.

    If you are placing it inside the feedback loop the power supply may simply raise turn on harder to maintain the output voltage despite the polyfuse's efforts to reduce it.
     
  3. johnny_cash

    johnny_cash

    39
    0
    Aug 13, 2013
    Here it comes. Excuse i did it manually right now with paint. The led shows me if my PSU is working.

    I plan to attach another LDO regulator on the output to make 3.3v (needed to my logic). But i'm not actually testing with this 2nd regulator attached - but perhaps it is important to make you know.

    Thank you Steve.
     

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  4. johnny_cash

    johnny_cash

    39
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    Aug 13, 2013
    Power supply: 5v and 3.3v from the same regulator

    Hello,

    I have a regulator Vout = 5v DC (Vin = 7 to 40v DC...). Max 1A
    I need a second supply, 3.3v for logic devices.

    I think about a tension divider with resistor, but i fear:
    - U stability
    - current (i need about 300mA in 3.3v line)

    I think about a 5v -> 3.3VDC but it is hard to get (generally, Vin must be 5.5v at least)

    Can you give me your opinion on theses ideas (or another idea if suitable).

    thank you !

    JC
     
  5. johnny_cash

    johnny_cash

    39
    0
    Aug 13, 2013
    Definitively, i can't trigger the fuse. How strange. It seems i'm out of the regulation loop. Can someone confirm i am ? :)
     
  6. davenn

    davenn Moderator

    13,837
    1,952
    Sep 5, 2009
    OK show us a pic of your actual construction
    just to make sure you havent done a wiring mistake

    EDIT: OK a couple of questions
    1) the 12VDC supply to the regulator ... I assume its capable of at least 1 - 2 amps ?
    2) I really want to see that pic of your construction as I want to see what sort of inductor you
    are using for L1 ( the 330uH)
    Im wondering if it is capable of at least 1 - 2 amps of current ??


    Dave
     
    Last edited: Aug 13, 2013
  7. Harald Kapp

    Harald Kapp Moderator Moderator

    11,522
    2,654
    Nov 17, 2011
    Use a 3.3V low drop regulator, e.g. LM3940
    A bit of Googling shows a selection from other manufacturers, too.

    Don't forget the bypasss capacitors.
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

    11,522
    2,654
    Nov 17, 2011
    Here's the catch: The LM2575 has a built-in current limit of 1A. If the output current rises above 1A, the regulator will reduce the output voltage.
    Your fuse has a break current of 1.5A, which is never reached because of the limit function of the regulator. Therefore the fuse never comes into action and the regulator and coil become hot.
    Try a fuse with a break rating of <1A. You may also try increasing the output capacitor to e.g. 1000µF so that in case of a short circuit the current is drawn from the capacitor (hopefully) activating the fuse. However, the capacitor may not store enough energy and you can't increase its size indefinitely because it is part of the control loop of the regulator.
     
  9. davenn

    davenn Moderator

    13,837
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    Sep 5, 2009
    Johnny

    I have moved your other thread posts to this thread as its part of the same project
    you didnt need to start another thread :)

    Dave
     
  10. johnny_cash

    johnny_cash

    39
    0
    Aug 13, 2013
    Thank you Davenn, i was wondering what happened :) Effectively, this is the same project. Basically, i'll have roughly 200mA load on 3.3v, and 200 mA on 5v.

    Thank you for the precisions aout the PTC fuse. At least i think it is correctly wired.

    1) Can you tell me if it is more suitable to put the fuse on general Vin (7/30VDC) rather on Vout ?

    2) Can you confirm i can safely serialize the regulators, i mean the bypass capacitor (output/LM2575) will not interfere with (input/LM3940) ?

    I appreciate the help, excellent place i just discovered. Kudos :)

    Thank you for the help, i'll try with a lower polyfuse.
     
  11. Harald Kapp

    Harald Kapp Moderator Moderator

    11,522
    2,654
    Nov 17, 2011
    You could also ry and thermally couple the fuse and the inductor. Polyfuses work by thermal activation. The additional heat from the coil may help.

    Putting the fuse in front of the regulator will not chnage your problem. Current is still limited to 1A. If you use a fuse with lower rating in front of the regulator, it will protect the complete circuit, including the chip.
    However, it will not be able to distinguish between a fault on the 5V lien or n the 3.3V line. If you want this, put the fuse after the regulator.
     
  12. johnny_cash

    johnny_cash

    39
    0
    Aug 13, 2013
    TY.

    I have ordered a 450mA hold current / 800mA current max fuse. I think i'll use it to protect the DC/DCs, as my max intensity should never go to 300mA total amount.
     
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