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Variable Bench Switch Mode Power Supply

Discussion in 'Electronic Design' started by Scrim, Jun 23, 2008.

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  1. Scrim

    Scrim Guest

    Have any designs for widely variable switch mode power supplies suitable for
    use as bench power supplies been published? I've been looking at some
    comercially available supplies but they're still way too pricey. Ideally I'd
    like a supply that does 0-100V at 20A or 2KW with a constant current mode.
    Going right down to 0V isn't essential and a maximum of 60V and 1KW would
    still be great.

  2. Scrim

    Scrim Guest

    That'll be a 'no' then, I guess!
    What about less ambitious but still widely variable switch mode designs?

  3. legg

    legg Guest

    By the time you get involved in designing at the 2KW power level, you
    should have enough sense to realize that it's a lot easier, and makes
    more sense, to address the specific requirement. At that power level,
    the power supply itself is a major portion of the design.

    There's nothing to stop you hacking into the control circuitry of
    fixed-output designs, to produce output variations that are either
    voltage or current regulated. You will discover the limitations of the
    devices involved, when you do.

    These limitations are sometimes addressed commercially through
    modularization - where for a specific application, modules with
    varying characteristics are selected and applied in series or
    parallel. For example, flyback converters give good voltage range
    compliance, forward converters give good current range control and
    linear variations can provide noise-critical performance.

    Switchers are generally not good at regulating around zero voltage or
    zero current, unless they are configured for bipolar output
    voltage/current or can dissipate/store returned load energy.

  4. Scrim

    Scrim Guest

    Thanks for that. I'm really after a variable supply though for general lab
    use where I need from milliamps to tens of amps. Usually an amp or two
    suffices but recently I wanted to test several Peltier devices and needed a
    supply of unknown high current capability at between 8 and 28 volts. Mainly
    I was wondering if any of the hobby magazines had published a switch mode
    bench supply design.

  5. legg

    legg Guest

    That sounds more like 300W, with a voltage compliance that is within
    the range of conventional integrated circuitry, driving external power
    devices - cost shouldn't be an issue, if you stick to the immediate
    application requirement. The bulk source of isolated DC may already be
    available to you.

    Something like an LT1339 or LT3740, running off a simple bulk 48Vdc
    source with a 12V control supply rail, should do it. LTC offers app
    notes, pre-assembled evaluation printed circuit boards and even free
    simulator software for these parts. They are intended to control
    external mosfets for up to ~20A of buck-regulated current. As
    synchronous switch controllers, they are capable of discharging load
    capacitance under transient conditions.

    Such information is regularly presented in publications like EDN,
    EETimes or ECNews

  6. Scrim

    Scrim Guest

    Thanks again. Lowering my power output expectations somewhat, here's an
    example of the type of thing I'd really like, and for cost reasons, to build

    I expect that would find a permanent place in my equipment cupboard.

  7. JosephKK

    JosephKK Guest

    Nice. BTW i used some supplies very like that some 25 to 30 years
    ago, they were made HP back when they still made stuff. Single
    output, up to 40 V, up to 8 A, about 140 W max, GPIB.
  8. Scrim

    Scrim Guest

    HP Used to be one of my favorite companies. I particularly liked the how the
    few products from them I had included circuit diagrams and lots of info to
    help you calibrate and repair stuff. And encouragingly the manuals included
    errata rather than them pretending everything's perfect.You certainly paid
    for it though!

    I think I'm going to try the idea of hacking an ostensively fixed voltage
    switch mode PSU. Breaking into the voltage control loop shouldn't be too
    hard if it's a fairly dumb supply, but I suspect I'll have to build my own
    constant current circuit externally. Would be well worth it if it worked

  9. JosephKK

    JosephKK Guest

    And you may be able to get the original schematics from HP for that
    old thing if you can find the model number.
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