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Creating a 12Vdc + 1Vpp 1MHz ac 20W power supply?

Discussion in 'Electronic Design' started by I Throw Thumbers, Apr 20, 2007.

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  1. Hello s.e.d,

    Here's a puzzle I've been thinking about for a few days. I am testing
    a bunch of LCD montiors that draw around 19W or 20W of power on a
    12Vdc input. One of the tests is a ripple test wherein I supply the
    monitor(s) with 12Vdc supply that has an AC ripple on it of 1V peak-to-
    peak at a frequency of 1MHz and verify functionality.

    The basic question is: how do I do create this kind of supply?

    I've been thinking about op-amps, but the ones I had laying around
    can't source 1.6ish amps. I've been thinking about amplifying the AC
    portion through a simple 4-resistor BJT bias circuit, but the
    transistors had too low of a saturation voltage. I've been thinking
    that in a few weeks I'll have to do the same thing but for monitors
    that draw 30W and even 60W. I've been thinking about equipment that
    already exists that will do this, but I don't know it. Shouldn't this
    be an easy enough circuit?

    -drewbob
     
  2. Rich Grise

    Rich Grise Guest

    Yes, it should be almost trivial. Is this a homework question?

    How about a current-boosted LM317 with your reference modulated?

    Good Luck!
    Rich
     
  3. Tam/WB2TT

    Tam/WB2TT Guest

    I don't think the LM317 will come close to having enough bandwidth. The most
    straight forward way I can think of is to put the secondary of an RF
    transformer in series with the DC output, and apply the RF signal to the
    primary. This might take some doing if the monitor input is well bypassed,
    and requires driving 1 V p-p of 1 MHz into almost a short circuit (probably
    dozens of .1 uF capacitors) , and take gobs of power. Or suppose the supply
    was switched at high level between 12 and 13 Volts; now, the thing is, after
    you connect the monitor you might see that the voltage is only switching
    between 12.49 and 12.51 V if there are no series inductors in the monitor DC
    input.

    How sure are you that you really want to do this at 1 MHz?

    Tam
     
  4. Not homework, just part of my job that I'm no expert at :(. Maybe
    that's even worse on this forum...
    Ah. And a good place/vendor to look for an RF transformer?
    I did diode-or 11.5V supplies and 12.5V supplies at 1MHz, and the
    switching time on that FET switch I used was far too fast for the
    monitor.
    I will have to double check in the morning what the input capacitances
    are. I suppose that at 1MHz it's unreasonable to expect a simple
    circuit that will be able to drive the input capacitances. There is
    an inductor (ferrite) in series.
    I'm not. I may be able to convince the powers-that-be that 1MHz is
    unreasonable for my feeble brain/abilities and that something in the
    low kHz may have to do.

    thanks, db
     
  5. Small transformer secondary in series.

    On its primary for example a DL92 tube amp or a trans-sister.
     
  6. Tam/WB2TT

    Tam/WB2TT Guest

    I would think that you worry about 60 Hz on the DC feed, and RF pickup on
    the signal cable. Seems like your systems engineers should tell you what you
    need to do. For starters, pass UL and FCC part 15.

    Tam
     
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