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Paralleling Switching Power Supplies? Can it be done.

Discussion in 'Electronic Design' started by Brian M., Apr 21, 2004.

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  1. Brian M.

    Brian M. Guest

    I have two (identical) 24v 4.5A switching power supplies. Can they be
    safely paralleled together to give me 24v at 9A.

  2. Tim Wescott

    Tim Wescott Guest

    They have to be made specifically for the job. Otherwise one will
    inevitably have a slightly higher setpoint than the other and will do
    all the work.

    Supplies that are designed for paralleling have some sort of interface
    to gang the two supplies together and to configure one as master. When
    they're ganged the master regulates the voltage, the slave just delivers
    as much current as the master asks for.

    It's up to you to spelunk through your documentation to see what you've got.
  3. mook johnson

    mook johnson Guest

    If you diode couple them (not very efficient with 3 watts loss each), when
    one supply hit its current limit and the voltage drops below the other, the
    second one will then supply the additional current above the CL of the first
    supply. you will lose about a volt in the process.

    that is about all this can be expected from two separate power supplies.
  4. Its possible to parallel two (or more) supplies that have remote sense
    inputs. You'll have to cobble together a circuit that will bias the
    sense inputs to achieve load sharing. Designing such a circuit to ensure
    system stability isn't trivial, but it can be done.
  5. Brian,

    As almost always: It depends. Some power supplies have the provision to be
    paralleled. Just read the manual. Some cannot be paralleled unless you want
    to start the fireworks. Most of the times it can be done when the settings
    are as equal as you can get them *and* the load is significant more then one
    supply can handle.

  6. James Meyer

    James Meyer Guest

    Malarkey (polite way of saying bullshit).

    The one with the higher setpoint will do all the work, but only until
    the load current causes its voltage to drop a little. The two supplies won't
    divide the load current exactly in half at all load currents, but that is not
    really necessary. There is no reason why two nearly identical switching
    supplies can't be paralleled with impunity.

  7. James Meyer

    James Meyer Guest

    Diodes are unnecessary. You can expect the output impedance of each
    supply, including the resistance of the wires connecting the supplies, to
    perform the same action as diodes and without the voltage regulation and
    efficiency losses.

  8. James Meyer

    James Meyer Guest

    Spoken like a true PE, Paul. Take a trivial "problem" that isn't really
    a problem and make it so complicated that only a PE can come up with a solution.

  9. In particular, ones with synchronous rectification shouldn't be
    paralleled, since they can both sink and source current and will
    "fight" unless a load sharing circuit such as the LCT4350 has been

    '4350 data sheet:

  10. Tam/WB2TT

    Tam/WB2TT Guest

    What if he puts an inductor in series with each supply? Problems I see are :
    If the two supplies are not adjusted close enough in voltage, the first one
    might shut down before the second one supplies any current; if the load is
    variable, as in an SSB transmitter, hard to tell how the dynamics work out.

  11. Brian M.

    Brian M. Guest

    Please correct me if I'm wrong but I'm guessing by using diodes you
    mean something like this

    Both supplies are 24v 4.5a

    +------------ +24v @~9A (minus diode losses)

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