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Need low cost ±15 VDC power supply design

Discussion in 'Electronic Design' started by Tom, May 20, 2006.

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  1. Tom

    Tom Guest

    I am looking for a low cost dual output DC power supply design with the
    following requirements:

    -input voltage 4.75 vdc to 30 vdc (must be able to work throughout the
    -output voltage +15 vdc and -15 vdc
    -output current requirement 25 mA maximum (anything over this is overkill)
    -relatively noise free output (ie. ideally less than 10 mV

    To date we have looked at using a DC to DC converter for this application
    but the startup current, although brief, is way beyond the 25 mA maximum
    which makes it unacceptable.

    We are also looking at a design that uses a Linear Technology LTC1871 chip.
    This device shows promise but we are experiencing severe noise problems.

    A complete design or any suggestions you might have on using the LTC1871
    chip to do this job would be most appreciated.
  2. If this chip is satisfactory other than the noise, you may try
    adding some filtering to the output. Put a capacitor, an inductor,
    and another capacitor in series (T filter). I would probably use 4.7
    uF caps with a 220 uH inductor, but you may want to try several values
    and look at the results on the scope. Also, if you only have one (or
    a few) components that are sensitive to the noise, but others which
    are not, just put the filter on the supply for the sensitive
    component. You may also be getting some noise from other components
    in the design, such as microcontrollers (if any are present).
  3. Let's get your requirements nailed down a bit.

    Do you need isolation between the supply voltage and the output voltages?

    How much average variation from +-15 volts can you accept?

    Is efficiency important?

    Is 25 mA the maximum input current available, or the load current on
    the +- 15 outputs?
  4. Tom

    Tom Guest

    The ground is common between the supply voltage and the output voltages.
    Some variation from the 15 volt requirement is acceptable--probably a volt
    but on the underside. ie. the outputs could as low as +14, -14 vdc.
    The 25mA is the maximum load current on the +15 and -15 vdc outputs. In
    practice, it will likely be about 10mA per leg.

    Efficiency is important.
  5. If the converter must start at 4.75VDC then the starting current must be
    greater than:
    Is= (2*15*0.025)/(0.85*4.75) = 186 mA which is greater than 25mA.
    Does this design require any laws of physics to be compromised?
    A flyback topology can completely control it's start up current profile.
  6. I think the LTC 1871 could work, if you have the correct magnetics,
    and probably a second stage of LC filter on each output (after the
    feedback point).,C1,C1003,C1042,C1031,C1115,P2118,D5240

    But this one might do it with fewer parts and maybe a bit less waste.

    Can you point us toward a schematic of what you have built, for us to
    criticize and make suggestions?
  7. legg

    legg Guest

    Voltage controlled circuits can also be configured to limit inrush
    through tailoring of the output rise-time.

    As the LTC1871 has only a crude peak input limit, and no slow start,
    then using the voltage control loop might be the only way to do this.

    The OP seems a little confused about why an unspecified DC-DC
    converter was 'unacceptable'.

    He also didn't really specify what the 'noise problems' were with the
    LTC1871 - emissions, interference, unpredictable regulation or

    Still no word on isolation requirements.

    The 4.75-30VDC input requirement suggests that he doesn't have a
    practical application in mind, as this doesn't describe any typical
    industrial source. He may simply be reading control chip specs -
    always a bad sign - because it means you likely can't do it with the
    chip described in the data sheet he's reading. The 1871 is ruled out -
    5V6 typical start-up.

  8. Fred Bloggs

    Fred Bloggs Guest

  9. Tom

    Tom Guest

    Okay, let me clarify:

    The application is real and it is industrial. The +15, -15 VDC outputs will
    be used to drive an instrumentation amplifier. The 25 mA requirement on
    each leg previously mentioned is likely not necessary. An input current
    (from the excitation voltage) in the running condition of perhaps 10 to 15
    mA is all that will likely be needed. The reason for the wide ranging input
    (4.75 to 30 vdc) is to accommodate typical industrial requirements when
    mating the signal conditioner to a PLC, data acquisition unit or meter.
    These typically run at 5, 10, 12, 15, or 24 VDC.

    The design using the LTC 1871 currently has filtering on the input and
    output including inductors on the output. What we are seeing on the output
    is a ratty 7kHz signal with peak to peak amplitude of about 600 mV. The
    frequency increases as input voltage increases from 5 VDC until things
    eventually go unstable and blow out the 1871 and the accompanying mosfet.

    Regarding the earlier design with DC to DC converter, the killer was the
    huge startup current. We were using a Burr Brown (now TI) DC to DC
    converter which demanded about a half amp or more of short duration start-up
    current--an unacceptable condition given that this new design is to replace
    an earlier design that has a soft start.

    I've tried to attach a copy of the schematic to this response but I guess
    that doesn't work.

    As you may have guessed by now, I'm not an electronics engineer. I'm trying
    to get some suggestions I can give my electronics engineer who seems to be
    at wit's end on this project.
  10. Hmmmm, why not let the electronics engineer post here?

  11. Tom

    Tom Guest

    Okay, I've posted a jpg of the schematic for the LTC1871 power supply to the
    alt.binaries.schematics.electronic newsgroup. I hope it comes thru.
    Outlook gave me some complaint about using HTML mode.

  12. It showed up just fine. I put my first comment there. You need to
    get out of the way (no disrespect intended), and get your designer on
    that group, so we can get him on to a practical approach.
  13. Genome

    Genome Guest


    I've been having a play......


    Not finished yet but it might help your electronics bloke out before he goes
    fruit bat like me.

  14. BobG

    BobG Guest

    I suppose the 4:1 input range (9-36V) off the shelf supplies from
    astrodyne don't fill the bill because they dont go down low enough?
    They also make a 4:1 switcher that does 4.5-18. That work better?
  15. Genome

    Genome Guest

    Don't know

    I just fried 16 bits of shit out of some nice sliced up sausages in soy
    sauce and then tried to do the same to potatoes and onions.

    This is for FAT bloke who KNOWS how to fry things.

    I gave up... Now it's in the fry pan with half a pint of milk and some nice
    herb and garlic sauce on a gentle simmer...

    Such is life....

  16. Genome

    Genome Guest

    Fortunately FAT bloke scoffed the lot.

  17. Genome

    Genome Guest

    Just updated the page and moved on to working out gains of things. Still not
    finished though.

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