Isolated DCDC power supply design

Discussion in 'The Projects Forum' started by maltmstr, Jan 7, 2011.

  1. maltmstr

    Thread Starter New Member

    Jan 7, 2011
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    Hi all. I'm looking to design an isolated DCDC power supply for a small microcontroller. The requirement is to have 5V 500ma output. In my previous (unisolated) deisgn, I just used an LDO, LP38691DT-5.0, to generate the regulated 5V from 6V coming it. This has worked very well. It has been a stable an inexpensive solution.

    However for my next design, I would like to have more flexible input voltage range, say 9-30VDC as well as DC voltage isolation to protect the uC and ADC's so that they can be used in an industrial environment. After doing some research, it looks like you can buy off-the-shelf isolated DCDC converters, like this one, however they are quite expensive, around $15 each.

    My guess is they contain high efficiency switching power supplies, and other things that I don't really need (or understand :)). Since efficiency really isn't a requirement, is there any reason that I can't just use a 1:1 transformer like this for isolation, followed by a linear regulator with a wider input range, such as this one MC78M05?

    What are the tradeoffs for using linear regulators in this application? Even though it specs a maximum input of 35V, is there a problem running with such a high in/out differential, say 24V stepping down to 5V? Why are the off the shelf isolated low current DCDC converters so expensive? Will the transformer give me the protection that I need in an industrial environment, or should I be looking at more complex regulators?

    Thanks in advance for your help.
     
  2. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    Much to learn!
    First, you can't just use a 1:1 transformer because transformers don't work with DC. You have to build an oscillator to change your DC to AC, then run it through the transformer, then turn it back into DC.

    And this is where I disagree with you. I don't think having someone else do all that design and assembly work for $15 is a high price compared to the time I (you) would have to spend.

    and, will a transformer give you the protection you need? I don't know. Fortunately, there ar smarter people that I, just waiting to help you.
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    If you are starting with a DC input, you will have to use some kind of switched-mode design in order to use a transformer for isolation.

    Next think about the 9.5W of power that you would need to dissipate in dropping 24V to 5V at 500mA. That's horribly inefficient, and would make the whole thing hot and bulky, with the big heat sink you would need.

    Probably you would be better off buying something off the shelf. It might also be easier to be sure of complying with safety requirements this way.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,090
    3,027
    That's putting it mildly. $15 for a purpose-built, off the shelf, "perfect" solution is a no-brainer. If you need 100 of them and think you can do better rolling your own, go for it. But maybe it's better to spend the $15 today and resolve to revisit your own design later if the rest of your project works out.
     
  5. maltmstr

    Thread Starter New Member

    Jan 7, 2011
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    0
    Thanks for the replies. Admittedly I need to dust off my EE201 textbooks. Looking at the datasheet application example where I got my transformer followed by regulator idea, I see that I mistook a DCDC isolator symbol for a transformer, oops. I knew that it couldn't be that easy, lol.

    And I agree that $15 is cheap for a few, but I would like to make a lot of these. If I can learn something in the process, thats even better.

    Searching more I found this one, which is only $6, although it needs 12V input. Is the large input voltage range requirement what makes them expensive? I would like to find one that would work from either 12 or 24 volts.
     
  6. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    Go look at the National Semiconductor website. They have design calculators for just what you want. Another way to say this is, if your books have dust on them, you are about to awaken to a whole new world.
     
  7. maltmstr

    Thread Starter New Member

    Jan 7, 2011
    4
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    ok, that website is way cool! thanks for the direction!
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    How about this one:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=945-1378-ND
    Supposedly isolated, 5v, 2.5W, that will work on from 6.5v to 36v input, for $6.02/ea, or $5.31/ea if you buy 100?

    No datasheet, and I'm honestly a bit skeptical if it's actually isolated - because I see an inductor and not a transformer. Might be worth trying one or two for a sample though.
     
  9. maltmstr

    Thread Starter New Member

    Jan 7, 2011
    4
    0
    SgtWookie, I saw that one too. The link to the datasheet on the manufacturer's website is dead, so I emailed them. It turns out it is not isolated as you suspected. I ordered one before I had the datasheet, and have it in hand but I doubt that I'll use it for anything.

    From the datasheet:
    [​IMG]
     
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