Flyback converter

Discussion in 'General Electronics Chat' started by ashwini, Apr 22, 2008.

  1. ashwini

    Thread Starter New Member

    Apr 22, 2008
    3
    0
    Hi All,

    I am trying to build a 100V to 12V DC-DC flyback converter.
    Can anyone please help me in choosing the values for the Components???
    Suppose if the switching frequency is 80KHz.

    what should be the transformers turns ratio and magnetizing inductance??
    what should be the value of output capacitor??
    What kind of diode should i use???

    please do reply.......

    Thanks
    Ashwini
     
  2. S_lannan

    Active Member

    Jun 20, 2007
    247
    2
    If you read a book on power supply design you will find all the formulas you need to answer those questions.

    'Power supply cookbook' by marty brown is a good one to read.
     
  3. ashwini

    Thread Starter New Member

    Apr 22, 2008
    3
    0
    Thanks lannan
    But can u suggest some online stuff??
     
  4. Caveman

    Active Member

    Apr 15, 2008
    471
    0
    Go to magnetics inc. website. Look for their ferrite catalog. It has tons of info on magnetics.

    But frankly, buy Brown's book. It will save you a ton of time.
    The problem with switch mode power supplies is that they if they don't work, they often destroy themselves.

    Good luck.
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,148
    1,791
    That sounds more like a buck converter. Why are you using the flyback topology to lower the DC voltage?

    IMHO you have no chance of succeeding using online resources. Buy the book and be done with it.
     
  6. Caveman

    Active Member

    Apr 15, 2008
    471
    0
    Just FYI, the flyback topology is used quite a bit with offline power supplies to step down the voltage, so this may be the right topology to use with this big of a step down.

    Ashwini, you have to realize that there are people who spend their entire careers on this stuff. SMPS put semiconductors in situations where the assumptions (most of which you don't even know you are making) don't work.

    Let me just give you one example. In a buck converter, the diode does conduct current in the reverse direction during every cycle. No it's not a special diode. In fact, normal diodes like 1n4001's are the worst offenders. Get the book. Trust me on this one.
     
  7. Papabravo

    Expert

    Feb 24, 2006
    10,148
    1,791
    I've been doing SMPS for some time and I was unaware of this feature of the flyback topology. Maybe I should go buy the book. Hmmmm.....
     
  8. Caveman

    Active Member

    Apr 15, 2008
    471
    0
    It's just a method of reducing the requirements of the switch voltage swing. The transformer turns ratio handles it. The other common reason is for isolation purposes.
     
  9. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,699
    905
    I wanted to let you know I bought THE book by Marty Brown, $38 USD new and including shipping.

    I have only had a chance to look up a few things and skim others, but it is well worth the money. I particularly like the stepwise instructions for design, notes about potential potholes (e.g, the differences between snubbers for power supplies versus SCRs, page 146), and clear, instructive language. Thanks for the lead. John
     
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