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Low voltage step up design?

Discussion in 'Electronic Design' started by BobG, May 31, 2006.

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  1. BobG

    BobG Guest

    I have a D cell boostcap... 350 farads, 2.7V. Takes several minutes to
    charge up on a 3A 2.5V bench supply. The volts tics up a 1/10v every
    second or so. I want to make a dc-dc conv that will chop the cap
    voltage and step it up to about 14v. I'd like it to work from 2.5v down
    to .5v or as low as we can get it. Would a flyback topology work? Or
    maybe the cap voltage supplies an h-bridge and a transformer primary is
    driven from the h-bridge? There are commercial wide input range dc to
    dc converters, but the lowest input range seems to be 4.5-18v. Anyone
    have any hints, tips, ideas, problems to look out for? The secondary
    could be a regular old bridge rectifier and a pwm regulator. The trick
    seems to be sniffing the cap right down to .5v or so before shutting
  2. Tim Williams

    Tim Williams Guest

    Old germanium might be the only thing that'll let you run that low.. I mean
    if you keep a higher voltage supply (1-5V) at all times, you could even use
    FETs and get better efficiency still, but silicon BJTs need 0.7V or more to
    turn on and more than 2V for MOSFETs. If it shuts down and you don't have
    anything to start it below the operating voltage, you'll have to charge it
    again before it can be used.

    Ge gets Vf ~= 0.3V, as compared to Si ~= 0.7V. It's usually slower, so
    you'll need an iron cored transformer for the inverter. Such circuits have
    been used for single solar cells.

    How much current do you need, anyway?

  3. John_H

    John_H Guest

    Search the converter sites for "single cell." There are a bunch. When
    you're cut off at 0.8V, keep in mind that it's only 10% of the energy
    you're leaving in the 2.5V cap, not 30%. You may be challenged to find
    the current you want if it's excessive. If you're fine with low
    current, you may need to boost with the single cell converter first and
    boost to 14V second. You could also use the single cell converter to
    bootstrap a better boost converter. Options abound.
  4. John_H wrote...
    With care a two-step converter isn't necessary. Bob is starting
    with a charged cap, so he can power the converter IC from the cap
    to start, and from the converter's output as it starts running.

    I think a flyback _transformer_ is the best configuration for Bob.
    E.g, a 1:6 step-up ratio would allow for a sensible switching duty
    cycle, especially at the low end of the input-voltage range. If
    he wishes, Bob can add a second winding to run the switcher IC at
    its sweet-spot voltage after it's operating.
  5. John  Larkin

    John Larkin Guest

    Hi, Win,

    There are lots of small, cheap, stocked 2-winding surface-mount
    toroidal transformers around. This can sometimes be handy...

    | |
    | |
    d c
    ---g |
    s |
    | gnd

    It's a sort of autotransformer bootstrapped onto the input voltage.
    You don't get a lot of choice on turns ratio, I guess.

  6. Joerg

    Joerg Guest

    Hello Tim,

    Ge is expensive and not well suited for any serious production. You are
    basically dealing with a limited stock, almost collector's items.

    Another option might be to add a nifty start-up circuit around a JFET or
    other depletion mode device.
  7. Tim Williams

    Tim Williams Guest

    I would've suggested tubes, but they typically don't go that low. <g>

    That's true, he didn't mention if this was production or what...

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