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Original question:

UCC28950: PSFB leading leg mosfet driving voltage occur 100%duty cycle and transformer saturate when startup with 150% full...

UCC28951: How to solve the issue of OUTC and OUTD occurring 100% duty cycle, can’t find in datasheet.

Poh Kok Keng
Genius 5512 points
Part Number: UCC28951
Other Parts Discussed in Thread: UCC28950

Tool/software:

hi Expert, 

About UCC28951, how to solve the issue of OUTC and OUTD occurring 100% duty cycle as per happened in UCC28950?

I can’t find the explanation in datasheet, do you have any material or app note to share?

Best rgds, kpk

  • 0
    TI__Guru* 85556 points

    Hello,

    Your inquiry is under review and I will get back to you shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 85556 points

    Hello,

    There is an application note that describes the issue that you are discussing in the UCC28950.  This application note also explains that that issue was resolved by switching to the UCC28951 in application that require more than 90% duty cycle.

    https://www.ti.com/lit/pdf/slua853

    In the UCC28950 if you want to avoid this issue, you can select the transformer turns ratio so at the minimum voltage the maximum duty cycle is less than 90%.  I would suggest sizing for a maximum duty cycle of 70 to 80%.  The following link will bring you to an application note that describes how to design a UCC28950 and UCC28951 in a phase shifted full bridge.  There is even a section on how to select the transformers turn ratio based on the minimum input voltage. 

    https://www.ti.com/lit/pdf/slua560

    Regards,