Transformer selection

[dot4]

Hello,

I am selecting a transformer and I found several that would suit my application. But I have a difficulty to understand the datasheet (it is attached). Why is the primary voltage given in a range? I want to use it for lower secondary voltage than stated in the datasheet (hundreds of milivolts) and I need to know the conversion ratio.
Thanks for advice.

 

[FvM]

If you look sharp, you'll find the turns ratio in the datasheet.

 

[dot4]

Yes, I see, but when I apply this ratio to voltages on the picture, it doesn't fit.

 

[FvM]

It's a matter how a flyback converter is designed. For wide range off-mains converters the transformer is often wound for about 50% primary duty cycle at lowest input voltage. The said transformer is dedicated to be used with LT3798/99, which are implementing active PFC. Thus even lower duty cycles are reasonable. You can review the respective section in LT3798 datasheet. An output voltage of "several 100 mV" suggests a different windings ratio, however. Or you'll operate the transformer and primary switch considerably below it's rated power.