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400- to 690-V AC Input 50-W Flyback Isolated Power Supply Reference Design for Motor Drives

seungpyo kang
Prodigy 210 points
Other Parts Discussed in Thread: UCC28711

Hi,

I wonder that how bleow equation is calculated. I mean that I cant unerstood of equation.

1. Here, td calculated 1/(6*50) which Equation 4. How come number 6, 50?

 

2. Below picture, the RLC calculated equation 14. How com this equation?

  • 0
    TI__Mastermind 24375 points

    Hi,
    Thank you for the query on UCC28711.
    1. 50 is the line frequency. I would encourage you look at datasheet equation 11 and calculate the capacitance.
    The designer is no longer with TI so I cannot comment on the choice of number 6. Vbulk(min) is generally chosen at
    0.7 times the minimum input voltage. I would encourage you to look at the following link page -13.
    www.ti.com/.../slup302.pdf

    2. I will give an idea of this is done. You can do a formal derivation yourself.

    VCST = (Ids+Ics)Rcs + Rlc*Ics

    Ics = Ivsl/Klc (Klc is an internal current gain factor)

    Ics = Vin/(Klc*Rs1*Npa) according to the current in the aux winding

    (Vcst-Ids*Rcs-Ics*Rcs) = Rlc*Ics

    The left hand side of the above equation can be approximated as deltaI

    deltaI*Rcs = Rlc*Vin/(Klc*Rs1*Npa)

    deltaI/Vin = td/Lp

    Rlc = Klc*Rs1*Npa*Rcs*td/Lp

    Regards,

    Harish