Other Parts Discussed in Thread: TIDA-00173, , TIDA-00315, UCC28700
In my new design including the UCC28711 in a flyback power supply, I have some calculation questions:
I checked next documents:
a) TI reference design "400 to 690 VAC Input, 50-W Flyback Isolated power supply: TIDA-00173
b) TI reference design "100-V/200-VAC input 30W Flyback Isolated power supply: TIDA -00315
c) TI UCC28711EVM-160. Calculations checked according to document SLUA653B (UCC2870x)
My questions are related to calculations. I checked formulas in the three cases, and found some doubts/error/mistakes:
1) Power spec to Vauxiliary:
In case a) power is 15w, and in case b) is 6w. In case c) is estimated 50mW. Why is required so much power for Vaux, in case a and b?. What load is it calculated for?
2) DMAX duty cycle calculation:
In case a) used formula is : Dmax=(12*DMag(Vout+Vdg) )/ (Vin min-Vaon-VRCS)
In case b and c the formula is: Dmax= 1-DMAG- Fmax * Tr/2 . Which formula is correct? both?
3) NA/NS Turns ratio:
The formula is the same in the three cases, but Vddmin value is 16v in case a and 8v in case b and c. Is a mistake case a)?? If this is so, next calculated values may be wrong
4) Iaux peak (auxiliary voltage peak current)
In case a and b, the used formula is the same: Iauxpk = (Pout * 2) / (Vout*Dmag), but in case c the formula is Iauxpk = ((Pout * 2) / ((Vout + Vde)*a2*Dmag).
I think the a2 parameter (Na/Ns) ratio in case c, is a mistake. Please confirm it. If this is so, next calculated values may be wrong
Thank you for you prompt answer. I look forward it to continue my design