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Turns ratio of flyback transformer calculation

Nguyen Xuan Phu Son
Prodigy 30 points

Dear TI E2E,

I am designing the flyback adaptor with UCC28600. Some parameters: Vin_min = 80VDC, D_max = 0.47, Vo' = Vout + Vf = 19 + 1 = 20VDC.

But i wonder how to right calculate n = Npri/Nsec.

Following slup127.pdf TI's document, n = Vin_min/Vo'*D_max/(1 - D_max) = 3.54

Following SLVC104I.xls, n = 7.2 is recommended and n = Vxfmr/Vo' and Vxfmr = 142V is recommended

So could you help me to understand what is Vxfmr and how to calculate it? Vxfmr = Vin_min + n*Vo' is it true?

Which document can i use to calculate the turns ratio of transformer, n?

Thank you,

Son.

  • 0
    Intellectual 930 points

    Hi Son.

    First of all, in order to have a little bit of dynamic, I find that D=0.47 is too high.

    Usually, 0.4 is the selected value for the design @Vin_min.

    After this you have to consider losses on your primary side (switch, current sense) and diode on secondary side.

    For secondary side, in order to guarantee DCM also during transitory, usually D2=0.45 is selected.

    So your turn ratio will be evaluated as:

    [(Vin_min-Vsw-Vcs)*0.4]/[(Vo+Vf)*0.45]

    Bye

    Paolo