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Multiple output isolated DC/DC supply

Discussion in 'Electronic Design' started by dt, Feb 4, 2007.

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  1. dt

    dt Guest

    Hi, I need to charge lithium batteries (3 to 12 cell packs) and
    prevent individual cells from exceeding the desired voltage (4.2v
    each). This is most commonly done in hobby circles using a charger
    that takes the overall pack up to the desired total voltage (eg: 12.6v
    for 3 cells) and a separate balancer circuit to try to keep each cell
    the same.

    What I would prefer is a charger than can charge each cell
    individually to 4.2v and eliminate the need for the balancer circuit.
    Single cell lithium charging chips are readily available, and if I
    were to supply each of these from separate 5v mains power supplies I
    could daisy chain any number together and achieve my objectives. An
    example of what I mean is shown in this post if I'm not clear: http://
    www.rcgroups.com/forums/showpost.php?p=6049417&postcount=4

    However, I would like to do this from a single 12v car battery source.
    So I need a step-down design with up to 12 isolated outputs (~5V
    each). I need at least 3A each, preferably more. Does anyone know of a
    elegant way of achieving this please?
    Thanks, David.
     
  2. linnix

    linnix Guest

    Usually, that for isolating AC. Very difficult to isolate DC in this
    situation.
    Alternative is to switch cells in/out of charger or to switch by-pass
    loads.
     
  3. Yzordderrex

    Yzordderrex Guest

    Off the top of my head I think thats 180w. You could probably get
    that with one of those Topswithc GX controllers from Power
    Integrations. 12 secondary windings would track reasonably well.

    Although I think you said a car battery for the input.

    Then I would probably just use a homebrew open loop half-bridge to get
    the isolated supplies.

    then drive some battery charger chips. I think TI has benchmark or
    use a simple switcher and homebrew the output side too.

    many ways to skin this cat.
     
  4. me

    me Guest

    why use separate supplies? just use one high current source....
     
  5. dt

    dt Guest

    The 12 cell pack is my biggest problem. Its not practical to parallel
    them for charging purposes.
    You've given me a few ideas to pursue.
    Thank you, David.
     
  6. Yzordderrex

    Yzordderrex Guest

    I'm fast becoming an expert on these chargers. I didn't know what a
    Li-ion battery pack was two weeks ago. This stuff is great fun. Have
    you tried those A123 systems batteries. I have wallpaper on my work
    machine of a guy doing a burnout on a 9 second electric bike powered
    by Li-ion.

    Check it out.

    http://www.electrifyingtimes.com/a123/KillaCycle.html

    regards,
    NEO
     
  7. dt

    dt Guest

    Yes I'm using A123's and LiPo's. The A123's are more forgiving but
    still need balancing if taken to their boundary voltages. Regards,
    David.
     
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