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high voltage boost DC/DC converters (5V->250V)

Discussion in 'Electronic Design' started by Michael, Nov 29, 2007.

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  1. Michael

    Michael Guest

    Hi there - at work I'm probably going to have to design a high voltage
    DC/DC converter sometime in the near future. It looks like it'll be
    something along the lines of 5V input and 250V output. Current will be
    under 10ma. Probably more like 1-2ma. The more regulated the better,
    but I will probably just regulate it down a bit with a FET to clean it

    My understanding is that once you start to get to something like an
    output voltage 10x or more of the input voltage the standard boost
    circuit (cap + diode + inductor) no longer really cuts it. Instead,
    I've read that step-up transformers are a better way to go. However,
    when I look through Digi-Key, all I see are step down transformers for
    stepping down wall power, as well as various specialty transformers.

    Any suggestions? Thanks!

  2. Guest

    Why would you say that? What's your switching frequency?

    I hacked something like this

    with a boost converter in place of the motor, and got up to 100V out:
    enough to light up a 110V night light with about 10V (eight NiMH AAs
    in series).

    The inductor I used was pretty beefy, though: from a throw-away
    blender. Thick gauge wire.

    - another Michael
  3. Nico Coesel

    Nico Coesel Guest

    Wurth ( has solutions for these sort of 'problems'.
    They have standard transformers which can be connected in several ways
    to make all kind of ratios. Getting some samples and support from
    these guys is never a problem.
  4. This always worked for me:
    c any PNP |
    | |
    | |
    ---------------------------------- B Vout
    | | _---------A | A---------------a D k---------------------------------------------------------- + 250V
    | R2 2k7 |( | diode 1kV | | B +5
    | | |(300turns | | R1 +5 | |
    | |-------- |( | | | | | R 2k7
    | | 1/2turn) |(------------ | | c c |
    | | -- |( 5turns C 10uF 300V |--- b b ----|
    | | | |------------- | | e e |
    | | | c | | | |______| |
    |+ |- ---- b any NPN C1 22nF poly | | | Z zener 3.3V
    C3 10uF C2 10uF e | | R 10k R |
    |- |+ | | | | | |
    /// /// /// /// /// /// /// ///
    small potcore 2x any NPN
    calculate R1
    for Vout
    Waveform is sine! select C1 for frequency (few kHz).
    connect A with A,m and also B with B.
    Note C2 is reverse biased.
    C1 must be poly (ceramics may not work right).
    C3 and C2 best be tantalum, the transistors Si.
    Typos reserved.
    Copyright Jan Panteltje 2007
    All right reserved.
    Usenet patent 0x754
    Released under the hardware GPL.

    The only thing is that you will have to wind 300 turns or so thin wire on a coil former.
    Impregnate it too.
  5. Small smps transformers are very easy to wind.

    Here's what I designed for a similar need about
    12 years ago. I used an LT1172 switcher IC in
    a miniDIP package.

    My technician wound the transformer bobbin.

    I used an RM8 core with 3B7 material, gapped
    (on one side) for A160 (A_L = 160nH/N^2).
    The primary was 10 turns of #24 magnet wire
    (for L = 25uH), then a layer of tape, and
    the secondary was 100 turns of #30 wire.
    So you can see it was easy to wind.
  6. Guest

    You might want to look at the articles in the "Circuits" section at for a few ways you can
    build your power supply.
  7. Joerg

    Joerg Guest

    I went well past 10:1 on bucks. But when the factor nears 20 it does get
    old. You need a ferrite transformer like those used in very small
    switcher wall warts. Except that you'd operate this transformer in
    reverse. The problem is the usual: Very difficult to obtain at
    reasonable prices when outside China. Sure, you could get the for a few
    dimes from China but only if you buy a whole pallet of them. Also, keep
    in mind that the flyback versions are air-gapped.
  8. Joerg

    Joerg Guest


    Aha! I always have to put lots of aloe creme on my index fingers after
    doing too much of this winding. Else they'll split open in cold weather.

    I mostly go non-gapped. Push pull or AC-coupled half-bridge. And don't
    mis-count. One-Mississippi, two-Mississippi, ...
  9. Tam/WB2TT

    Tam/WB2TT Guest

    If you really want to do it on the cheap, get a 60 Hz 120:6VCT transformer.
    Apply push pull drive to the 6V windings at about 100 Hz, and take the HV
    off the 120 winding with a full wave doubler circuit. You will want to take
    the drive off a Dff to insure 50% duty cycle, otherwise there will be a DC
    component in the transformer. There should be a reasonable looking square
    wave on the 120 winding.

  10. John Larkin

    John Larkin Guest

    You're boosting about 10:1. He needs 50:1. High boost ratios are
    tough. The energy in the inductor is 0.5*L*I^2. When the fet turns off
    and things fly up, the winding, fet, and diode capacitance have to be
    charged, and that energy is 0.5*C*V^2. As the boost voltage goes up,
    the energy lost in those capacitances goes up as V^2. At typical
    numbers like 50 or 100:1, there's just nothing left for the load. The
    "fix" is less L, which requires a beefier, higher-voltage fet, which
    adds drain capacitance, and the whole thing bites you in the butt.

    Using a transformer, or an autotransformer, helps a lot. Mouser and
    Digikey sell lots of dual-winding inductors, which can be configured
    as a center-tapped autotransformer: +5 on one end, fet drain in the
    middle, flyback from the other end.

    It could be that a spiffy bipolar, like one of the super-saturating
    Zetex parts maybe, would have a better ratio of saturation current to
    output capacitance than a fet.


  11. Along the lines of page 10 in the LT1172 datasheet, top image?

  12. I think a few people do standard magnetics for Power Integrations' Topswitch family - these may be
    Also look at transformers for photo flash/strobe charging - you may be able to find these off the
  13. Winfield

    Winfield Guest

    Close, that's a 20-30V to 5V step-down, whereas mine
    was a 5V to 220V step-up. BTW, I see they specify a
    1:3 transformer, whereas wouldn't turning it around,
    making 3:1, be more appropriate?

    I used a 1:10 ratio, so the 220V output corresponded
    to a 22+5 = 27V flyback swing, plus a little leakage
    inductance spike. Looking at my old RIS-177 drawing,
    I don't see a clamp or snubber. Hmm, seems to have
    worked well all these years, but I do know better.
    Note, they suggest a 25V zener plus a fast diode.
  14. Well, of course, modulo that. I need some 1kV/10uA, preferably from a
    single AA (for an insulation tester; I'm tired of lugging around the
    mains-powered one), but I'm so swamped with other things that I'll probably
    never get around to it.

  15. Winfield

    Winfield Guest

    1000:1 is a bit of a challenge for the transformer,
    all kinds of painful issues, like excess winding
    capacitance, etc., may come up, especially at low
    power. Not to say it can't be done, or hasn't been
    done, but I'd be inclined to first step up from 1V
    to 5 or 10V, then go the rest of the way with a
    flyback transformer.
  16. Michael

    Michael Guest

    Hi Winfield - so is the standard path to wind your own for prototypes,
    then have a custom transformer wound when you hit production? I had
    been hoping to find a readily available transformer (as my quantities
    will never be high enough to warrant a full custom run of
    transformers, but will be high enough to make me scared at the thought
    of hand winding all of the buggers). Thanks!

  17. There are hordes of custom transformer shops out
    there that'll make quantities as low as 5 pieces,
    at affordable prices, hoping you'll come back for
    more later. Who can say, your product may take off.
  18. Tam/WB2TT

    Tam/WB2TT Guest

    Electronic photo flash units commonly work off 3 or 6 volts, and deliver
    several hundred V DC. You should be able to find a schematic for such, or
    even fix a junked flash unit; maybe from a disposable single use camera.

  19. Joerg

    Joerg Guest

    A bit too pricey for my taste, and single-sourced. I'd rather roll my own.

    And those for CFL backlights since those have HV windings.
  20. Joerg

    Joerg Guest

    Whenever I looked I found them to be rather expensive, even at 1k qties.
    Then when my clients went into production and we sourced it all in China
    the cost of a custom made dropped to 10% or so. Why is that? Somewhere
    in between there must be somebody trying to make oodles of money. So I
    never buy more than a handful.

    Zetex makes really nice transistors. Unfortunately mostly
    single-sourced, but good.
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