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UCC28600: UCC28600 Rcs and Rpl Equations Mismatch in Datasheet

Anas Metwally
Prodigy 10 points
Part Number: UCC28600

Tool/software:

Hi!

I have two questions:

  • in the datasheet a factor of 0.925 is used with tsw, I can understand the entire equation but this specific value, but I can't see the rationale behind it.

  • also in the datasheet, there is a mismatch between equation for Rcs and Rpl on pages 9 and 26, how are these equations derived exactly?
  • 0
    TI__Guru* 86286 points

    Hello,

    Your inquiry has been recieved and is under review.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 86286 points

    Hello,

     

    Not sure how the systems engineer derived that equation.  This is an older device and this engineer no longer works at TI; So, I cannot aske them how this was derived.

    However, I do know the UCC28600 is quasi resonant flyback converter.  Based on energy, Power Efficiency (eff) switching period (tsw) you can calculate the primary peak current as follows.

    E = (Pout/Eff)*tsw = (1/2)*Lp*(Ip(1))^2

    Ip(1) =(2*( Pout/Eff)*tsw)/Lp)^0.5

     

    When it comes to the equations in the data sheet on page 9 and page 26.  I agree that these don’t match and they may not be correct.

     

    I reviewed the data sheet and the RPL resistor adds an offset to lower the peak current as the line voltage increases. I reviewed the block diagram and the offset is created by (Iline/2)*RLP.

     

    Iline = Vbulk*Ns/(Na*ROVP1)

     

    ROVP1 and ROVP2 are selected based on OVP and startup conditions, so there is no adjustment for these resistors.

     

    The power limit occurs at CS of 1.2 V.  A good target is size Rcs target would be to set power limit to trip at 125% nominal input power.

     

    To make things easier you can size Rcs based on the following equation.

     

    Iline = VBULKmin*(Na/(Np*ROVP1))

     

    Rcs = ((1.2V-(Iline/2))*RPL)/(Ip(1)*1.25)

     

    Regards,