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UCC28951: UCC28951 datasheet issues

Bill Xu
Intellectual 1520 points

Part Number: UCC28951

Dear PL AE

my customer design in UCC28951 in their project and they have several issues as below associated with datasheet of UCC28951 need consult.

  1. in figure simplified appliaction(page 1),  figure 7-18(page 35), 8-3(page 40), the ADEL pin connected to CS pin by RAHI. this meets all descriptive in the datasheet. but in figure 8-5(page 56), seems ADEL pin actually connected to VREF. is this correct and what is the reason?
  2. in page 46, what is part number for infineon's MOSFET? I checked IPP65R190CFD and several Infineon's 650V/20A CoolMOSFET. the output capacitance is in range 23~100pF and measured in 400V or 100V. the datasheet said the output capacitance is 780pF@25V. also, there has several output capacitance such as time related capacitance, energy related capacitance. so my question is: which cpacitance should be used to calculate tank frequency in Equation 89?
  3. Does TI has any document to explain equation 49 & 80?
  4. Does TI has any document to explain equation 90(page 52)? also, seems the equation 90 and equation 131(page 60) is not exactly same. which should I used to decide the delay time?
  5. in equation 134(page 61), what is voltage of CS? the KA=0.0405, according equation 134, I can deduce that voltage of CS=3.461V. this is over CS range: 0~2V. Do I have any mistake or misunderstanding?
  6. for equation 143(page 65), in my application, output voltage is vary from 0~500V. how should I calculate Me and then Rsum, RDCHMI? 

big thanks for your supporting.

regards,

Bill

  • 0
    TI__Intellectual 1170 points

    Hi Bill,

    See my response below.

    1. ADEL pin need to be connected to CS pin by RAHI if there is a requirement in the application to adjust the deadtime based on the load which is done on first figures you have referred to. If this feature is not required, this pin can be connected to a constant voltage and it will translate to a fixed deadtime instead of adjustable based on the load this is done in the last figure you mentioned (figure 8-5).
    2. You can choose a certain datapoint for Coss at a certain voltage in datasheet and calculate the avg using equation 49
    3. These equations are derived by curve fitting the Coss vs Vds plot 
    4.  Please follow equation 90, equation 131 is based on empirical data
    5. The equation is for calculating RAB for a given TABSET and CS voltage. In other words, for a certain load, the required delay is TABSET and a corresponding CS voltage is generated with this data you can calculate RAB
    6. You can consider using max output voltage for calculation.

    Regards,
    Deepak K

  • 0 in reply to Deepak K
    TI__Intellectual 1520 points

    Hi, Deepak

    big thanks for great supporting! another question.

    In my design, I have not use a current transformer to measure bus current. I use a current sensing resistor to sensing bus current. should I make CTRAT=1 in equation 12(page 28)?

    thanks.

    regards,

    Bill

  • +1 in reply to Bill Xu
    TI__Intellectual 1170 points

    Hi Bill,

    Yes, since entire primary current is flowing through the shunt resistor you can keep CTRAT=1.

    Regards,
    Deepak K