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quick n dirty boost

Discussion in 'Electronic Basics' started by kell, Feb 15, 2006.

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  1. kell

    kell Guest

    I'd like to boost 12 volts to 24 volts at a few milliamps using minimal
    circuit board space. Regulation is not important, so I thought of
    using this
    12v
    |
    +------,
    | |
    | )||
    | )||
    | )||
    ,------,--,------+ )||
    | | | | |
    1K R R 1K +->|-+---,
    | | | | __| | |
    | | | | | | |
    +--||--+ +--||--+--||nmos --- |
    | \/ | |__ --- |
    \c /\ c/ | | |
    |----' '----| | | load
    /e e\ | | |
    | | | | |
    '----------------+------'----'---'
    |
    gnd

    with equivalent values for the timing caps and resistors giving 50%
    duty cycle for a nominal 24 volts out.
    Any other ideas or suggestions...?
     
  2. That circuit does not produce a regulated output voltage.

    At a few milliamperes output, a charge pump might be a simpler open
    loop supply:
    http://ww1.microchip.com/downloads/en/DeviceDoc/21468b.pdf
    See figure 4-6.
     
  3. Tim Williams

    Tim Williams Guest

    LC oscillator. Single transistor.

    Tim
     
  4. kell

    kell Guest


    Do you have a link or something I can see?
     
  5. Tim Williams

    Tim Williams Guest

    Bottom of:
    http://webpages.charter.net/dawill/Images/Induction_Heater_Draft1.gif
    Part of "Floating Supply Inverter".

    Q14, C27 (L2 optional), C28, R77, C28, erm, huh, I put two C28's on that
    schematic, and T4.

    Q14 was a ZTX651 but for your current requirement, a 2N3904, 2222, 4401 or
    anything else rated for the voltage will work.

    On the transformer, instead of a secondary winding, you can use a highspeed
    diode from the transistor collector to get the doubled voltage. In this
    schematic, I needed an isolated supply, so I needed a secondary.

    The primary winding (labelled 20uH) should come to around 20-40 turns of
    fine wire on a ferrite transformer core. The capacitor across it should be
    smaller, more like 0.01uF. The feedback winding (going to the transistor
    base) should be 1-5 turns, depending on supply voltage and primary turns.

    It's pretty fool-proof.

    Tim
     
  6. Bob Monsen

    Bob Monsen Guest

    You can add regulation by using a TL431 as a feedback element. it should
    shunt the gate of the nmos when the output reaches the appropriate
    voltage... use a voltage divider to feed the R input. You could also use a
    zener and an NPN transistor to do this.

    JP mentioned the TC7662, which is also a nice solution, will probably
    require fewer parts, and will be less likely to stall or fail. For a small
    current requirement, it works well (although it won't give you 2x voltage,
    due to the diode drops required.)

    --
    Regards,
    Bob Monsen

    The unleashed power of the atom has changed everything save our modes
    of thinking and we thus drift toward unparalleled catastrophe.
    Albert Einstein, Telegram, 24 May 1946
     
  7. kell

    kell Guest

    Okay, I think I will try it. Instead of winding my own inductor, I'll
    just take one of those little chokes that looks like a two-watt
    resistor and wrap a few turns of fine wire around it; if it doesn't
    work I'll wrap it in the other direction.
     
  8. Don Foreman

    Don Foreman Guest

    Depending on load and inductor value, the output voltage of that
    circuit could climb until something (MOSFET, diode or capacitor)
    breaks down.
     
  9. Bob Monsen

    Bob Monsen Guest

    I suggested using a feedback element to regulate it. He can do it like this:


    12v
    |
    +-------,
    | |
    | )||
    | )||
    | )||
    ,------,--,------+ )||
    | | | | |
    1K R R 1K +->|-+---+----OUT
    | | | | __| | |
    | | | | | | |
    +--||--+ +--||--+-o-||nmos --- |
    | \/ | | |__ --- |
    \c /\ c/ | | | |
    |----' '----| B | | load
    /e e\ | | |
    | | | | |
    '----------------+-------'----'---'
    |
    gnd


    B OUT
    | |
    | [150k]
    --- |
    /x\------- +
    --- |
    | [17.4k]
    | |
    GND-+---------'

    x is a TL431A

    He can also do this:

    B
    | /
    c |\|
    b---| |--[100]---OUT
    e |/|
    | /
    | 23V Zener
    GND-'

    However, just using the TC7662 is the easiest, and it probably won't fail
    to start in 2 years due to a decreased beta in Q1 and smoke...

    --
    Regards,
    Bob Monsen

    Science is a wonderful thing if one does not have to earn one's living
    at it.
    Albert Einstein (1879 - 1955)
     
  10. Don Foreman

    Don Foreman Guest

    Right. Probably less board space as well.
     
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