LiteON PA-2911-1-LF Blown Components

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  • Kyleadonis
    Member
    • Oct 2020
    • 17
    • South Africa

    #1

    LiteON PA-2911-1-LF Blown Components

    I opened up a Cisco network switch that no longer powers on and the power supply has three blown components:

    (See Photos Attached)
    -Power switch IC
    -Optocoupler
    -Resistor

    There are no blown fuses, mosfets, diodes. I know its impossible to ask what could cause this without physically looking at the board but does anybody know
    what could have caused these components to blow?

    Also finding a replacement power IC seems to be a bit of a mission as the top blew off and I cannot see the model of the IC. Can anyone help?
    Attached Files
  • momaka
    master hoarder
    • May 2008
    • 12170
    • Bulgaria

    #2
    Re: LiteON PA-2911-1-LF Blown Components

    Hard to say exactly why that offline switch blew... but it's not uncommon.

    I see the PSU has really good input line filter, so I doubt a high voltage spike from lightning or similar could have done it... but still not impossible, I suppose.

    However, I do notice that this power supply appears to have an Active PFC circuit. Those are known for killing off-line switches and/or other primary-side components if the input capacitor(s) (in your case, the two big blue Hitachi HP3 450V 270 uF) have gone low in capacitance, high in ESR, or a combination of that... or open-circuit (APFC is well-known for murdering primary-side caps over time, too.)

    So perhaps start by removing the two blue caps and checking if they still show good ESR and capacitance.

    As for the bad off-line / power switch IC... I thought it might be a TOPSwitch IC, so I looked around at the various families. It appears that the TOPSwitch-HX family might be the one - i.e. TOP252, TOP253, TOP254.... and all the way to TOP262. See this datasheet:

    Then check the connections on your board.
    To me, it looks like you may have had a P or G package (see page 6 on the datasheet above, bottom left of the page.)

    The optocoupler is probably something similar to a PC817, but just in SMD version.

    As for the resistor... I can't quite tell from the pictures.

    But judging by the black spots on one corner of the bottom side of the PCB, I suspect you will find components related to the APFC circuit blown as well.
    Let us know what you find.
    Last edited by momaka; 10-06-2021, 07:36 PM.

    Comment

    • momaka
      master hoarder
      • May 2008
      • 12170
      • Bulgaria

      #3
      Re: LiteON PA-2911-1-LF Blown Components

      Actually, forget what I said above about the TOPSwitch. I think TinySwitch-III family might be what was used here, judging by the connection of the small SMD ceramic cap between pin 2 and primary-side ground (pins 5 thru 8). See this datasheet:


      So that blown switch IC is probably a TNY274, TNY275, TNY276... or TNY280. If you know how much output power the standby section has to provide, that should give you a good clue as to which one of these it is... or just go with a TNY280 and not worry, since that's the highest-rated one. All other parameters are pretty much the same for these TinySwitches.

      Comment

      • Kyleadonis
        Member
        • Oct 2020
        • 17
        • South Africa

        #4
        Re: LiteON PA-2911-1-LF Blown Components

        Hi Momaka


        Thanks for your reply, I will definitely try those things you mentioned.

        I will keep you in the loop on the repair.

        Comment

        • Kyleadonis
          Member
          • Oct 2020
          • 17
          • South Africa

          #5
          Re: LiteON PA-2911-1-LF Blown Components

          Okay so I am still busy with this power supply I've decided to go with the TNY208 IC. I tested the caps and did find some with a high ESR value. I also missed the one smd cap that's connects to the bypass pin on the power switch IC. The cap is definitely blown. I should be okay with a 100nF ceramic cap right?
          Attached Files

          Comment

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