Switching power supply, buck/boost converter design

Discussion in 'The Projects Forum' started by danielb33, Aug 21, 2012.

  1. danielb33

    Thread Starter Member

    Aug 20, 2012
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    0
    I am completely new to this stuff. Have general circuits completed, am at an internship designing a switching power supply (specs below) to control a 24VDC coil activated relay with a 30Amp relay. I want to use a buck/boost converter to do the trick. I am unsure about how to pick which size fixed cap to use in the converters??? Or any cap sizes for anything. See the tag for schematic, note the SW1 OFF schematic. I figure after asking a few times I will start getting a feel for what might work and what probably won't. If you learned a different way, please let me know where I can look for resources. Searching online I don't find concrete answers, I find intuition based answers.

    Also note that my goal is simply to smooth out the switching frequency to deliver a constant voltage to the load, not actually add power. I may need to do that for the buck booster, but as seen by the schematic, I am working on the buck converter now.

    Thanks for helping the newbs.

    http://www.ecircuitcenter.com/Circuits/smps_buck/smps_buck.htm
     
  2. danielb33

    Thread Starter Member

    Aug 20, 2012
    105
    0
    Sorry, I forgot to note that the Vin is between 10 and 40 volts, Vout is a constant 24V.
     
  3. crutschow

    Expert

    Mar 14, 2008
    12,988
    3,226
    Such a design is not trivial. I would suggest using a buck-boost controller such as this, with output transistors sufficient to efficiently handle the 30A.
     
  4. Papabravo

    Expert

    Feb 24, 2006
    10,136
    1,786
    The capacitors are the least of your problems. Generally speaking you try to find the capacitance you want, twice the working voltage of anything the capacitor is likely to see, and as low of an ESR (Equivalent Series Resistance) as you can find.

    Your bigger problem will be finding inductors with core materials that will not saturate at twice the peak currents that they are ever likely to see. After that will be semiconductors that can survive the harsh environment of big current spikes, shorts, and inductive flyback.

    There is both art and science in such a design. You're going to have to smoke a few parts before you learn the difference.
     
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    How much load current does the 24V line have to supply? Is it just driving the coil of a relay? What is the resistance of the coil?
     
  6. danielb33

    Thread Starter Member

    Aug 20, 2012
    105
    0
    Thanks for the responses, and sorry for the late reply. I have been out.

    The 24V out only need to supply a MAX of 1AMP. Sorry if I did not make that clear. I asked about the capacitor because I was getting the impression that was the only thing I could not calculate, although I am not sure how to do other calculations yet.

    bountyhunter-I have not yet picked a coil. I have picked 0 parts.
     
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