WEBENCH® Tools/UCC28911: Transformer Turns Ratio

Hi Shinichi,

Thanks for using WEBENCH.
As Mike mentioned in the previous reply, yes WEBENCH calculations are bit different.
However the design should still work properly even with WEBENCH turns ratio.

1. Nps = 0.48*(75.1)/((24+0.45)*0.413) = 3.56 is close to 63/19(3.31), there is always some margin taken from the actual calculated value just to give some headroom so that to enable selection of different off the shelf transformers for the design, if it is possible for that operating condition
So we should be good here, you can use these turns ratio for the design, it should be fine.

2. Nas = (20+0.45)/(24+0.45) = 0.83 is close to 16/19(0.84)
now for this we have assumed Vaux of 20 volt in the design for the Vout of 24 volts. the maximum Vdd clamp voltage is ~ 26 volts.

In the datasheet, Vaux is taken as 7 volts which is the max Vdd turn off threshold for a Vout of 2volts(when the actual Vout for that design is 5 volts), this is because to support CC(constant current) region for battery applications when battery voltage drops to 2volts also, the IC should still keep running and Vdd>Vdd_UVLO_off
and when the battery voltage rises to 5 volts, then Vaux will also increase close to 17 volts(17.45*Vsec/5.17)

In WEBENCH we are designing for Vaux of 20 volts for Vout of 24 volts, hence turns ratio are different.
So in this case, even when your Vout drops from 24 volts to lower, Vaux will correspondingly fall down from 20 volts as well but should still remain above turn off threshold.

We can always design for low or high Vaux by adusting (Nas)turns ratio. Full load or light load efficiency might be affected though accordingly.

I guess design should be ok to work. 

let me know if any question/issue

Thanks
sahil