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UCC28220: UCC28220 RCHG and RDISCHG value calculation

Bharath Gilla69
Prodigy 70 points
Part Number: UCC28220


Hii 

I was trying to use the UCC28220 IC for interleaved non-isolated boost converter. My input voltage is 470V and I required output 850V. I was trying to calculate the RCHG and RDISCHG values. As my max duty will be 0.447 and switching frquency is 78khz. Im getting the RCHG as negative as per the eq(1) to eq(3) in page 12. Please let me know how to calculate the values as per my value....??

  • 0
    TI__Genius 13660 points
    Hi Bharath,

    I am working on it.

    Regards
    Teng
  • 0 in reply to Teng Feng
    TI__Genius 13660 points
    Hi Bharath,

    To determine where it went wrong with your calculation, could you share your calculation details?

    Regards
    Teng
  • 0 in reply to Teng Feng
    Prodigy 70 points

    Hi Mr. Teng
    Thank you for reply. Please find the calculation

    specification
    Vin=470V,Vout=850V
    Dmax = 0.447-----------------------(Vout-Vin/Vout)

    As per datasheet page 12


    Kosc=20400000000 ohm/s
    Fout = 78000 Hz
    Fosc =156000 ------------- (2xFout)
    Dmax= 0.447
    Dmax(osc)= -0.106 -----------------(1-2x(1-Dmax))
    RCHG = -13861.53846 ------------------------(Kosc x Dmax(osc)/Fosc)
    RDISCHG =144630.7692------------------------((Kosc x(1- Dmax(osc))/Fosc)

    since Dmax is <0.5 than Dmax(osc) will be negative. Is calculation is write or wrong

    Regards
    Bharath Gilla

  • 0 in reply to Bharath Gilla69
    TI__Genius 13660 points

    Hi Bharath,

    The maximum duty cycle clamp should be set to a value between 60% and 90% duty cycle per channel.  Please find it in the datasheet.

    Regards,

    Teng

  • 0 in reply to Teng Feng
    Prodigy 70 points
    Hii Teng
    So if i want to operate at 78khz please let me know the value of Rchg and Rdischg resistance value for an output of 850V with input of 470V.
    Regards
    Bharath
  • 0 in reply to Bharath Gilla69
    TI__Genius 13660 points
    Hi Bharath,

    The maximum duty cycle is not decided by power input/output parameters. It is defined by yourself considering the inductance design, request of protection and etc.
    So in your case, the normal operation duty cycle is 0.447, you can define the maximum duty cycle to be a any value between 60% and 90% according to the power request. Anyway, the maximum duty cycle must be set bigger than the normal operation duty cycle of power.
    So once the maximum duty cycle is selected, then calculate Dmax(osc), RCHG, RDISCHG later.

    Regards,
    Teng