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Very small, very high ratio, boost converters

Discussion in 'Electronic Design' started by Michael, May 23, 2008.

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  1. Michael

    Michael Guest

    Hi there - I need to build a very small, very high ratio boost
    converter. It needs to go on a PCB that can be no greater than 1cm in
    width and can be no greater than 1cm in height. Length is more
    flexible. It will have a 5VDC input and needs to have a 1KV, 1ma
    output. Regulation is not terribly important.

    First I thought about just using a normal DC/DC boost converter and
    using a HV fet on the output. However, it seems all HV FETs are
    massive. The smallest relatively HV FET I could find was the STMicro
    STN1NK80Z, which is an 800V (I could probably get away with just an
    800V output - but 1KV is my goal) 250mA FET in a SOT-223 (7x6.5mm)
    package. However, I noticed that RDSon is ~13 ohms. So since my boost
    ratio is about 200, that'd mean I'd have about 200ma on average (if
    I'm remembering my maths right) going through the FET, producing a
    2.6V drop. So I don't think this FET is even going to be able to
    handle my 1ma load. So I'll probably have to go up to a larger FET.
    There are more FETs in the D-PAK (AKA TO-252) package available. The
    Infineon SPD06N80C3T looks much better 0.9 RDSon. A quick calculation
    shows my boost ratio ending up at about 208 once the RDSon is factored
    in (assuming the resistance of the inductor is much smaller than
    RDSon). So I'd have about 208ma going through the inductor. Now, for
    choosing the inductor, I may be completely off in the way I am
    calculating its value. This is the equation I came up with:

    Einductor = .5LI^2 = output power * (1/ boost ratio) * (1/operating

    If that is right, if I used a 100uH Sumida CDFH6D28, I'd have to
    operate it at 462KHz. I could put a couple of those guys in series, or
    find a higher inductance inductor, but already 462KHz seems nearly
    reasonable already. That circuit is seeming almost doable, though
    driving that big HV FET at that speed will take a decent amount of

    However, I remember being told that boost converters with high boost
    ratios do not work very well for some reason that escapes me. So then
    I started thinking about transformers. However, I can't seem to find
    any that are this small. Heck I can't seem to find any reasonable
    sources for boost transformers (none of the big three - Digi-Key,
    Mouser, and Newark) seem to have any.

    So then I started thinking about PZTs (piezo transformers). PZTs are
    typically very small with very high ratios - however again I can't
    seem to find anybody that sells any.

    With all of this in mind - anybody have any recommendations as to a
    possible route for me to go? Are my calculations for the boost
    converter right? I am not typically a power guy - so I could
    definitely be messing some things up here.


  2. Wim Lewis

    Wim Lewis Guest

    How about a boost to a lower voltage (100-200V?) followed by a Cockroft-
    Walton multiplier?
  3. mike

    mike Guest

    Stray capacitance is a killer in this kind of applicaiton.
    Can you use that to advantage by resonating the whole structure.
    Tap the drive way down on the inductor. Don't need hv fet any more.

    Might also be interesting to look into the details of how they
    make stun-guns work. 9V in 250KV out.
  4. Tim Williams

    Tim Williams Guest

    Obviously, one step isn't going to cut it. The most that's practical off a
    single inductor is, oh, let's say 10:1. The duty cycle is already awful
    (ca. 90%). You can get peaks in the 20-50x range, at nil output current
    and insane loads to the driving transistor (since it has to handle all that
    voltage AND current).

    The obvious solution is a transformer, which ends up having the same size
    as a single winding (an inductor), more or less. Put your ratio there and
    you get much happier duty cycle, current and voltage capacity.

    You could probably tinker with a camera photoflash charger. Those are
    around 1cc. I don't know how much current they do, and 1.5 to 3V stepped
    up to 200-300V is quite a bit lower than 5V going to 1kV, so you'd probably
    need a new transformer at least. But sure, a self resonant sort of
    inverter, just what you need.

  5. Guest

    Maybe a CCFL backlight power supply? They're usually small and output
  6. MooseFET

    MooseFET Guest

    You don't say how important but I'm going to guess 10% or so.

    When the duty cycle is large, the upper harmonics are large. You need
    an inductor that is not too lossy at all the frequencies that have
    much energy in them.

    The pass transistor has to handle a large current when on and a high
    voltage when off. The same applies to the output diode.
    You may need to abuse a transformer intended for audio if you want to
    get it from them. I'm thinking of something like a MET-23. I don't
    like this idea but I'll put it out there.

    The turns ratio is sqrt(1600/3.2)= 22.4 which is a bit low for turning
    5V into 1000, but lets push on with thinking about it.

    The inductance on the 3.2V side will be more than 3.2 Ohms at 300Hz

    3.2 / (2 * pi * 300) = 1.7mH Lets use 3mH below.

    We can assume that the inductance is more than this. You didn't say
    if you were making many of these or just one but I will assume several
    so we do need to work with whatever the inductance of any one. At
    this point you'd need to but one and measure it to see if the rough
    guesses are right.

    I'm going to suggest a half bridge series resonant design to make the
    secondary voltage hit about 250V or 330V so that only trippling or x4
    is needed in the rectifier. Unfortunately, you will end up with quite
    a large AC current in the 3.2 Ohm winding when you do this. This
    really messes with the desire to keep things small because it pushes
    you up into bigger transistors.

    You also need a 0.1uF capacitor that can take the high current.

    All in all, I'd rather get a transformer intended for the purpose.

    Here we hit the "use a PIC" suggestion. A micro can drive the
    switching devices and produce the needed timing. A voltage divider
    off the 1KV can do the feedback. The micro can also tune the
    switching rate onto resonance.

    The transformer is rated at 65mW at 300Hz but if we run at, lets say
    10KHz, we won't burn it out at a higher power level.
  7. Paul

    Paul Guest

    ....snip! there goes a lot of interesting design stuff!.....
    Why build when you can buy? This sounds like an "off-the-shelf"
    device, for example: . If you acan
    build one cheaper and better, then you can put these companies out of
    business or work for 'em!
    My point is, custom electronics is great for those applications
    where nothing exists to satisfy cutomers, but if there are a host of
    products already out there, why go through the misery of design,
    testing, electrocution, and customer unhappiness?
    I used to work in a research outfit, and it used to drive us
    electronics guys nuts when labs would want to build stuff (wasting
    weeks and months of time), when they could buy a far superior thing
    from an electronics supplier. Why would you want to waste your
    precious time on something that highly skilled people have already
    perfected, and reduced the costs by economies of scale?
    If your "thing" had to withstand high vaccuum, or radiation, or
    other weird "one-of" situations, then go for your own design!

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