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UCC28780: UCC28780 Ropp function

user5781628
Intellectual 970 points
Part Number: UCC28780

Hi,

During UCC28780 debug, there is big difference about entering into AAM about the load point between 90Vac and 265Vac.

At 90Vac, it enters into AAM at 20Vout/1.1A. But at 265Vac, it enters into AAM at 20Vout/1.9A.

Could it be set to the same load point to enter into AAM at 90Vac and 265Vac?

In datasheet, it says "The OPP function uses IVSL as a line feed-forward signal to vary VCST(OPP) depending on VBULK, in order to make the OPP trigger point constant over a wide line voltage range."

I want to know that the Ropp compesation is only active for OPP, right?

This Ropp has any effect of Vcs for burst threshold?

Thanks!

BR

  • 0
    TI__Guru* 86266 points

    Hello Eleven,

    IVSL is developed when the main switch is on and the flyback transformer is being energised.  This current is always comming out of the CS pin when the low side switch is on.  IVSL = Vin*Na/Ns)/RVS1

    So if the AAM range is not being triggered at the same point there could be a few reasons.

    1. Ropp may not have been selected correctly.  You can double check this with the excel or MathCAD tool.
    2. Your converter is less efficent at high line causing the design to enter the AAM at a different power level.

    Regards,

    Mike

  • 0 in reply to Mike O'
    Intellectual 970 points
    Hi Mike,
    I have tried to adjust the Ropp value, it would be helpful for ABM trigger point.
    It means that through adjust Ropp value, the ABM trigger point at high line and low line will be similar.
    But the AAM trigger point is at high line and low line is still highly different.

    Also I have checked the converter efficiency that there is no big difference.
    Do you know if there is other solution to resolve it?
    Thanks!

    BR
  • 0 in reply to user5781628
    TI__Guru* 86266 points
    Hello Eleven,

    The more I think abou this the ABM is controlled more by the output ripple voltage than the Ropp resistor this might be why it is tripping at different points.

    If for some reason the output ripple voltage changes it could trigger ABM. I would studdy this behaivor to see if the ripple voltage drastically changes with input line voltage changes.

    ABM, IFB ≈ 25 uA to 35 uA

    The LPM and ABM boundary is roughly IFB 35 uA.

    The ABM and TM bound is roughly 25 uA.

    Regards,

    Mike

    --------------------------------------------------------------------------------------------

    Hello Eleven,

    Current coming out the reference (IREF) is a mistake in the data sheet. The FB current IFB adjusts the modes of operation.

    The controller adjusts operation modes by pulling FB current out of the UCC28780's FB pin. Please refer to figure 23 sheet in the data sheet and the following current will explain which mode the UCC28780 controller is operating in based on the figure.

    At Io max, IFB ≈ 0uA

    AAM, IFB 25 uA to 0uA

    ABM, IFB ≈ 25 uA to 35 uA

    LPM, IFB ≈ 48 uA to 35 uA

    SBP, IFB ≈ 75 uA to 48 uA

    Io = 0A, IFB ≈ 75 uA

    Regards,

    Mike
  • 0 in reply to Mike O'
    Intellectual 970 points

    Hi Mike,

    Yes, I know the IREF is another parameter which is used for mode control.

    How is your study progress you mentioned?

    Thanks.

    BR

  • 0 in reply to user5781628
    TI__Guru* 86266 points
    Hello Elevn,

    I searched the email thread and don't see mention of myself conducting a study. I think there might be some confusion on a reply made to you. Could you link me to the response in question so I can look into for you.

    Regards,

    Mike