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Original question:

UCC28950: UCC28950

UCC28950: PSFB

Rajasekaran S1
Prodigy 175 points
Part Number: UCC28950
Other Parts Discussed in Thread: PMP8740

Hi,

I am using UCC28950 based solution and taken reference design of PMP8740.

Problem is output voltage - okay at no load but when we starts loading even 2A also, synchronus rectifier mosfet is getting failed after 2-3 minutes, I increased voltage rating and current rating of mosfet, but still the same issue, any one mosfet will get failed.

Suggest suitable solutions.

Thanks

S.Rajasekaran

  • 0
    TI__Guru 65965 points

    Hello,

    Sorry that you are having an issue with the reference design.  

    If you built it yourself did you use Gerber Files that were provided for the layout  and build it with the bill of materials that was provided?

    Did you populate R7 and R18 on the design?  This programs the DCM comparator that will disable SR drives when the load is lite.  This prevents negative current from building up in the secondary windings of the transformer.  If these resistors are not populated and negative current develops in the secondary of the transformer this could lead to SR failing due to over voltage.  I would double check this?

    Regards,

  • 0 in reply to Mike O'
    Prodigy 175 points

    Hi,

    Thanks  for reply.

    We build our gerber files as per reference design and the bill of materials are same.

    We mounted the same value R7 and R18 in the board.

    Thanks & Regards,

    S.Rajasekaran

  • 0 in reply to Rajasekaran S1
    TI__Guru 65965 points

    Hello,

    Do you have clamping diodes D7 and D10 installed in the design?  If they are not in place the difference in current in T7 and T4 when the device comes out of free wheeling could cause excessive voltage across the transformer and output rectifiers.

    Regards,

  • 0 in reply to Mike O'
    Prodigy 175 points

    Hi Mike,

    D7 and D10 are installed in the board.

    Thanks & Regards,

    S.Rajasekaran

  • 0 in reply to Rajasekaran S1
    TI__Guru 65965 points

    Hello,

    Did you substitute or change anything on the design?   I just want to double check before getting the designer of the reference design you are working with involved. 

    You might want to study the SR current and voltage bringing up the load slowly to see if you can determine why the FETs are failing.  It is either going to be over voltage or over current.

    The SR FETs generally fail due to negative current developed in the secondary of the transformer.  If you turn the SRs off with DCM pin before critical conduction this generally will prevent that.  When you study the drain current you will see if that is the case.

    The other issue could be transformer and output inductor saturation.  You may want to study the transformer current and inductor current as well to make sure the magnetics are not getting saturated.

    Regards,

  • 0 in reply to Mike O'
    Prodigy 175 points

    Hi Mike,

    Thanks for your support.

    Please share your mail id, we will share the relevant details of the design.

    Thanks & Regards,

    S.Rajasekaran

  • 0 in reply to Rajasekaran S1
    TI__Guru 65965 points

    Hello,

    If you cannot post the schematic on the e2e please contact your local Texas Instruments Field Application Engineer for direct support.

    Regards,