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Thread: Modifying +/- output voltage on triple output SMPS

  1. #1

    Modifying +/- output voltage on triple output SMPS

    I have a Meanwell RPT-75 with +5/+15V/-15V outputs.
    I'm trying to increase the +/-15V outputs to +/-17V.
    You can see in the block diagram attached that like pretty much all the off-the-shelf multi-output SMPS it uses a common switching FET and single (multiwound) flyback transformer.
    The feedback is tapped off of the 5V output, and when I adjust the trimmer for the divider (the unit has a trimmer) I can get a decent range from the +5V output but not too much from the +/-15V outputs.

    1) Is is possible to modify the feedback to come from the +15V output instead of the +5V output?
    2) In general, is it reasonable to expect to be able to increase the +/-15V output voltages by up to 15%? (I don't care about changes in current rating there's tons of overhead)
    3) Are the output voltages very much tied to the specs (windings) of the flyback transformer?

    FYI, if it helps I traced the unit and it uses the following parts:
    - switching FET: 2SK3878
    - controller: NCP1203P60G
    - shunt reference: TLV431
    - optos: PC123
    Attached Images Attached Images

  2. #2
    Update:
    I changed out one of the resistors in the reference divider and removed a 5.1V zener that was hanging off the opto for overvoltage protection and I've been able to adjust the output voltages to a greater range.
    I was able to scale it to +6.3V/+17V/-17V which should work for my application. I haven't tested it under the actual load but I think it should work.
    Also, I didn't try but I don't see why I couldn't have also tried to tap the +15V output for the feedback but would have to calculate the resistor values and update any parts related to the over/under voltage sensing.

  3. #3
    Quote Originally Posted by azone View Post
    Update:
    I changed out one of the resistors in the reference divider and removed a 5.1V zener that was hanging off the opto for overvoltage protection and I've been able to adjust the output voltages to a greater range.
    I was able to scale it to +6.3V/+17V/-17V which should work for my application. I haven't tested it under the actual load but I think it should work.
    Also, I didn't try but I don't see why I couldn't have also tried to tap the +15V output for the feedback but would have to calculate the resistor values and update any parts related to the over/under voltage sensing.
    I am not sure if your modification will make it work the way you think, it is normal to get the higher voltage when adjusting those resistors, this happens with all power supplies.
    but the problem will be when you load it to get the higher voltage.

    Usually, regulated SMPS has 3 times the regulated voltage at its output.
    www.diysmps.com & www.Micro-Audio.com

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