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ATX second power supply

Discussion in 'Electronic Design' started by Jamie Morken, Apr 14, 2006.

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  1. Jamie Morken

    Jamie Morken Guest


    I am building a circuit using an ATX power supply's second power supply
    transformer in the self resonant configuration. I want to use this for
    generating 5V/1A+ from 120VAC.

    Here is some documentation on the circuit:

    overall ATX schematic:
    (from this site: "")

    ATX second power supply schematics:

    eagle cad schematic:
    " supply.sch"

    eagle cad board file:
    " supply.brd"

    Any comments on this circuit? I know it is a self resonant
    design for ~30kHz+ but I don't really understand the whole circuit very

    I have tried simulating the circuit but I couldn't figure out how
    to make the transformer with two primary coils and a reversed secondary.
    Anyone know how to do this in Texas Instruments Tina Pro or Pspice?

  2. switch mode power supplies are quite difficult to analyse due to the mutliple
    feedback paths. I will at a push try to debug any circuit but I give up with

    I'd take a look at the TL494 data sheet, as clearly that is central to this. You
    will probably find this follows closely an example circuit in the data sheet.

    It looks like pins 8 and 11 of that will be driven in antiphase and switch Q3
    and Q4 which will put power into the primary of T2 (primary being the bit with 2
    colis on it, not 3). The secondry then switches the Q1 and Q2 which drives the
    main transformer T3.

    I'm a bit surprised there is no independant regulation on the +/- 5 and +/- 12
    V. Only the +3.3 has any form of regulation itself.

    T6 must run all the time and provide a small amount of power to sence the
    power-good signal and so fire up the IC1 if needed.

    I think for T6, which is the bit you are interested in, you can ignore the
    secondry in any simulations. It looks like the top left of T6 is the real
    'primary' and the bottom left of T6 is in effect a secondry, with the output
    from that driving the base of Q12 as an oscillator. Once that oscillates and so
    Q12 switches on/off, the right of T6 obviusly produces a voltage. But that is
    completely sepparate. I would try to simulate just the left of T6 and consider
    the top the primary and the bottom the secondry.

    I think personally I'd just buy one and build something more interesting.

    Dave K MCSE.

    MCSE = Minefield Consultant and Solitaire Expert.

    Please note my email address changes periodically to avoid spam.
    It is always of the form: [email protected] Hitting reply will work
    for a couple of months only. Later set it manually.
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