Switch mode PSU voltage change help

Discussion in 'Power Electronics' started by Ian Lewis, Jul 4, 2016.

  1. Ian Lewis

    Thread Starter New Member

    Jul 4, 2016
    2
    0
    Hi all, I have browsed this forum go years but have never needed to post until now

    I am after some help with a switch mode PSU, this is 3x identical PSUs in one unit designed to supply 3 X independent outputs.

    The problem I have is its spec it's slightly wrong and it's over voltage, it's output is 26.57v, I need it to be 26.3v, that 0.2v is a problem as its a battery charger and the fuel gauge in the battery won't charg the packs above 26.3 and throws a battery error.

    I have stripped the PSU and looked over, it's a pretty standard SW PSU with a OB2269 to control the HV side and it's using a LM358 OP Amp to sort the feedback, the problem I have is in unable to to work out what I need to change to reduce the output.

    It looks like it may be in a bias current setup ?

    On the op Amp only one is used, the + appears to be the ref but it's not got any voltage at input, it's going through A few resistors and a 5v Zenner is there as well, on the - Input it appears to get tied to the op output via 1k resistor then into a cap, it then T off again through a 1k resistor then to ground.

    Attached are some pictures and I will try and draw out what I have been able to workout circuit wise shortly.

    Any hep with this will be very appreciated as I need to get the output down to 26.3v dead.
     
  2. dannyf

    Well-Known Member

    Sep 13, 2015
    1,811
    362
    if i were you, i would make sure that that 0.2v is really a problem first. it is rarely the case that a battery cares about being 0.2v off; and you want to make sure that the voltage is measured under the right load, etc.
     
  3. Ian Lewis

    Thread Starter New Member

    Jul 4, 2016
    2
    0
    thanks for the reply, it is, I can charge on variable PSU no problem at 26.3V go to 26.4v and it produced over charge error, these are smart LIPO packs with built in charge circuit and cell balancing, the pack handles the full charge control of the 6 cells, but its massively sensitive to over voltage in the final stages of CV charge, basically in the last 15 minutes of charge its in CV and expects no more than 26.3v, at that voltage current draw from the pack is about 60ma topping the cells off to balance them, when the voltage is 26.5v+ the current at that point hits over 100ma and over charge error from the pack occurs.

    I worked with these packs for a few years and they are massively sensitive.
     
  4. dannyf

    Well-Known Member

    Sep 13, 2015
    1,811
    362
    there is going to be a divider on the output, potentially with a voltage reference for higher gain, that will provide a feedback to the controll ic. Find that divider and try to lower the value of the lower resistor a little bit (paralleling something to it).
     
  5. siliconpower

    New Member

    Jul 8, 2016
    1
    0
    Find the IC chip TL431(maybe this model) and adjust the resistance which connected the REF pin.
     
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