Help with flyback converter design

Discussion in 'General Electronics Chat' started by Veracohr, Dec 31, 2013.

  1. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
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    I need to design a ±12V supply for my project. In the interest of size and efficiency I thought I'd go for a wall-wart AC-DC converter, then converted and regulated on the board with a switching regulator. I don't know much about switching regulators and I'm having a little trouble.

    I need to supply about 350mA per supply (actually less for the negative supply), so I've been using 500mA in my calculations (for margin). Most of the switching regulators I've looked at that can do voltage inverting can't seem to supply that much current in inverting configurations, and it also seems very inefficient.

    I found this regulator: LTC1372 which shows a method for dual-output flyback design (text page 7, schematic page 11).

    Some questions:

    1. It mentions line regulation for the reference voltage. Would this also correspond to line regulation of the output?

    2. I had to look up what flyback converters are all about, and they all include the turns ratio of the transformer/inductor in the output voltage calculation. However, the transformer specified in the example (page 11) doesn't have a turns ratio specified in its datasheet. Should I then assume it's 1:1?

    3. Also regarding the transformer, in the schematic the primary side has pins "2,3" on one end and "6,7" on the other. Does that mean "2 or 3" and "6 or 7", or does it mean "connect 2 & 3" and "connnect 6 & 7", or something else?

    4. Output ripple is a concern for me. A formula on this application note (page 35) includes C, ESR, and switching frequency. The LT1372 runs at 500kHz and says to use 22μF-500μF solid tantalum capacitors for low ESR. However, it seems capacitors of this type seem to have resonance around 100kHz (including the very one they recommend), and become inductive above that. How does one get a capacitor of at least 22uF when the rated capacitance is at something like 120Hz, and goes way down up at 500kHz?

    And lastly: is there a better way of going about this than the above? Tight regulation and low ripple are what I'm after. I thought about using linear regulators, but I'd still have to do a voltage inversion for the negative supply in that case, and it seemed like it would be quite inefficient for 12V output at 500mA.
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    How about you buy a 12 volt 3 or 4 amp wall wort and build the inverting supply?
    If you sign up for TI's web bench there are lots of designs complete. The one I looked at was the LM25576.
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Most tants do conk out at several hundred kHz. Above that, you pretty much have to use the ceramics. Check out taiyo-Yuden for large value ceramics. There may be some higher frequency Tants around, make sure the resonant frequency is several hundred kHz above the switching frequency.
     
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Since your load current is only a few hundred mA, I would use a switcher to make +/-15V and then use linear regs like 78XX and 79XX types to down regulate to +/- 12V.

    I would also go to ebay and buy the switcher. If you are not up on the subject, you will burn a ton of time and money trying to build it.
     
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
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  6. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
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    That's pretty much what I'm talking about doing. Like I said, the switching regulators I've looked at that show designs for inverters show them as inefficient and not capable of supplying nearly as much current as they can in step-up or step-down configurations.

    I guess if you ignore the questions about the flyback design, my basic question is: what's the best and most efficient way to get the +/- 15V?
     
  7. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    The way I suggested: get two separate 15V switcher wall warts and use one as the negative rail by grounding the positive line.
     
  8. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
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    OK, thanks for the suggestion, but as this is for my senior project I'm hoping for a more elegant solution. Less MacGyver-y, more like something you'd see in an actual product.

    I did find one option, an integrated module that doesn't require any design, but if I can find a solution that's less than $15 I'd prefer that.
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Go to TI's website and you can get a simple switcher design from their software tool.
     
  10. huang

    New Member

    Jan 8, 2014
    10
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    just searching AC DC 6W there are many converters can meet you no matter the sealed or open frame power supply.
     
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