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Four-quadrant power supply

Discussion in 'Electronic Design' started by Christoph Loew, Sep 17, 2004.

  1. Hi everyone,

    I've been looking for a circuit for a 15 V, 5 A
    four-quadrant power supply powered from 110 V AC.

    So far I've found :
    http://www.reed-electronics.com/ednmag/article/CA371090
    the schematic :
    http://www.reed-electronics.com/ednmag/contents/images/371090f1.pdf
    which looks OK to me except for the current rating (fixable)
    and that weird LT1970 for which I can't find the Spice model.

    Any thought regarding this circuit ?
    Any other ideas ?

    Since for me the educational effect is more important than
    e.g commercial viability or efficiency I'd prefer a design based on
    off-the-shelf (more likely out-of-the-parts-bin-except-for-
    the-power-transistors) op-amps and tansistors.

    Any pointers appreciated.

    (And no, this is *not* a homework assignment)

    Chris
     
  2. Tim Shoppa

    Tim Shoppa Guest

    And you need the Spice model for.... ???
    Not a big deal. The voltage regulator part is an op-amp with a power
    booster (for low or medium power you may prefer bipolar transistors
    in the booster stage). The current regulator is where it gets tricky,
    you have several choices for where to put the sense resistor and
    you have to budget for the voltage drop across the resistor somewhere
    (depends on how much headroom you have in the unregulated supply and
    whether input and output have to share a common ground and op-amps
    with inputs operated near a rail... or not, depending on how you do it.)

    If you haven't done two-quadrant power supplies before and are
    really gung-ho about SPICE, start simple (op-amp with power booster) and
    build up the current regulation on top of that with a sense resistor
    and comparator for the limit current.

    Most folks don't model their linear power supplies in SPICE, and in fact I've
    never seen a LM723 SPICE model, but you could build one up based on the
    internal schematic I'm sure.

    Tim.
     
  3. CFoley1064

    CFoley1064 Guest

    Subject: Four-quadrant power supply
    Hi, Chris. Building your own tools is the best education.

    Generally, what you're referring to is called an "operational" power supply, as
    in op (operational) amp. The easiest way to make one of these is with a power
    op amp. I had to make one of these once, and chose the National Semiconductor
    LM12.

    http://cache.national.com/ds/LM/LM12CL.pdf

    Look at the circuit on p. 9 of the datasheet. That circuit has independent
    current limit controls for + and - current. The operational power supply I
    cobbled together was similar to the one in the datasheet. (I made a couple of
    mods, but nothing critical - the circuit on the datasheet works.)

    The LM12 has a bw of 700kHz, and the LT1970 has a bw of 3.6MHz. Your supply
    will be faster, and that's one of the rubs of the situation. Layout on both
    circuits is _extremely_ important. These things want to be bad if you let
    them. A tight layout and good power supply bypassing is a must on either one.
    Since the LT1970 is a 20-pin tssop with copper bottom, you're going to
    _require_ a PCB for your circuit, where I was able to do the LM12 circuit on a
    perfboard next to the heatsink, since it's a TO3-type package. Also, your
    article suggests the power input is a regulated bulk +/-17V power supply. If
    you use an unregulated supply as input, you're going to have an increased power
    dissipation issue with the LT1970. Lay out your copper heat sink carefully,
    and use at least 1 oz and preferrably 2 oz. copper on the board.

    If this is a "home use" lab supply, you also might want to consider that an
    LM12 will be easier to replace than an LT1970, meaning your power supply will
    be easier to service. However, I don't think you're going to be able to get
    quite to 5A because of power dissipation considerations. If you're willing to
    settle for 4A and a lower bandwidth, the LM12 circuit might be an alternative
    choice.

    Good luck
    Chris
     
  4. Thanks for the help so far.

    I was unfamiliar with the term "operational power supply"
    and the LM12CL pointed in the right direction. I'm now
    starting on a design with general-purpose op-amps and
    bipolar power transistors based on the LM12CL application
    schematic.

    Again, thanks for the quick replies.

    Chris
     
  5. legg

    legg Guest

    One of the important features of a 4-quadrant supply is it's ability
    to absorb power.

    The magazine schematic is of a 2-quadrant supply with absorption
    capability limited to that of its internal dc supply.

    RL
     
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