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UCC2897A: SLUA535A

Jason Ciou
Expert 3600 points
Part Number: UCC2897A

Hi team,

My customer use the ACF topology and he wants to know the power dissipation on each components, and I refer this document to calculate, but I have some questions about this document.

Here are my questions,

  1. On page 17, how to get the Vds_max?  I do not know why is 5V. and when I put Vds_max to equation(23), I cannot get the 509.231mW.
  2. On Page 17, when we calculated the switching loss, whe do not need to multiply current? I do not know why eq(23) can use to calculate the switching loss.
  3. On page 25, how to get the equation (54)? how to know the value of CGDEE Coss_QMAIN_100V Coss_QAUX_100V.
  4. Is Vcl reset voltage?
  5. How to get Vclamp voltage, which formula I can use.
  • +1
    TI__Mastermind 33115 points

    Hi Jason,

    1. On page 17, how to get the Vds_max?  I do not know why is 5V. and when I put Vds_max to equation(23), I cannot get the 509.231mW.
      1. Since the self-drive SR are connected directly to the transformer secondary, VDS for each SR is varying by the input voltage range, transferred to the secondary through the transformer turns ratio. the VDS of one SR is the VGS of the opposite SR. The correct VDS and VGS values for QF and QR are shown in Table 2.
    2. On Page 17, when we calculated the switching loss, whe do not need to multiply current? I do not know why eq(23) can use to calculate the switching loss
      1. I agree. Even when eq.22 is substituted into eq 23, the dimensions are incorrectly given as P=I*R. There is a peak current term missing and not shown in the equation. The peak current is given by equation 18 and should be multiplied into equation 23.
    3. On page 25, how to get the equation (54)? how to know the value of CGDEE Coss_QMAIN_100V Coss_QAUX_100V.
      1. These capacitance are the estimated individual capacitances that make up the total node capacitance that must be discharged for ZVS to occur. The CGDEFF term is determined from the manufacturers data sheet for the primary clamp MOSFET and main primary MOSFET. QGC (total gate charge for the clamp MOSFET), QMAIN (total gate charge for the main MOSFET), COSS is taken from each data sheet as a function of the applied VDS (100 V in this example)
    4. Is Vcl reset voltage?
      1. Correct
    5. How to get Vclamp voltage, which formula I can use.
      1. Equation 43

    Regards,

    Steve M

  • 0 in reply to Steven Mappus1
    TI__Expert 3600 points

    Hi Steve, 

    Thanks for your kindly help.

    I still have some questions,
    1.) For QF Vgs range, I can get the value from  Vin_min / N and Vin_max /N, the calculate result will be 5.798V / 11.577V, I thought that is why Table show 6V < Vgs < 12V.   but how to get QR Vgs range 8V < Vgs <5V, could you tell me which formula I can use to get these value.

    2.) I try to use equation 18  multiplied into equation 23 but the result is not the same, I have a concern about equation 18, if we wants to get the peak value of inductor current, why we need to add 2A in the equation. Io + 1/2 delta IL can get the peak value right? could you check the equation 24 result is correct or not?

    3.) According to equation (54), what does Vcl120 mean, is 120V?

    4.) Could you tell me where I can find the rms current formula in TI's material? Because I am curious about how to get this equation.
     


    Jason

  • +1 in reply to Jason Ciou
    TI__Guru 74115 points

    Hi,

    1. It is self-driven SR, so VGS and VDS are based on the transformer turns ratios and input voltage range. As N is about 6, so for QR, you would need to calculate the primary side voltage during clamping at low line 36V, then VGS (QR) = (Vclamp - Vin)/N -Vo, where

    Vclamp = VCL = Vin/(1-D), Vin = 36V, if D = 0.65, VGS (QR) is about 8V

    Similarly, you can calculate Vin 75V, VGS (QR) is about 5V, then you have 5V < VGS < 8V, with D = 0.4

    2. the primary peak current also includes magnetizing current, 2A is assumed, the calculation is about 1.5A, the 2A was more or less from a real measurement.

    3. VCL = Vin/(1-D), at 75V, and D = 0.4, VCL = 125V

    4. These are assumed from RMS definition and assumed waveform shape.

  • 0 in reply to Hong Huang
    TI__Expert 3600 points

    Hi Hong,

    OK, Thanks.