How to design for existing transformer?

You need to download PI expert   http://www.powerint.com/en/pi-expert-download

Then for this project you  use PI XLS program (imbeded in PI expert)

Enter your system variables  (Vin, Vout, IOUT, Efficiency. Enter voltage DCV drop in your output rectifier)

Then enter your number of seconday turns (NS).  

Then play with VOR until you get your primary number of tunrs (NP) 

Play with bias voltage until you get your number of turns in your bias winding (NB)

Play with KP and KI to adjust your primary inductance (LP) and your peak flux desnity.

At this point you should have a desing that match your transformer's specs and you know what device you can use to run the power supply 

"Then enter your number of seconday turns (NS).  "

unfortunately the transformer datasheet does not give this value so i do not know what it is.  all i have is the turn ratio

Ok I understand

Unfortunately you need to know the number of turns on each winding because you need to know the flux density of the core.  Bp = (VL * Ton)/(Np*A) . The turn ratio is not enough. 

If you have more than one transformer you can take it apart and check for NP, NS and NB. Or if you just can know one winding number of turns, then the others windings can be calculated with the turn ratio.

i agree it is possible to determine Np. i could even call the manufacturer and ask, probably they know.  but i am really interested in learning if there is a way to use TOPswitch with a COTS transformer using only the information that is commonly provided.  i suspect there are MANY more people interested in this as well.

so we don't have the flux density.. but what we do have from transformer datasheet is the saturation current, which is relatable.

the way i was approaching this design challenge was to modify Krp until i match Lp given in transformer datasheet (application note 16, step 12). 

also check Ip and make sure it is less than saturation current of transformer (application note 16 step 7).

one thing i wonder.. if i check Ip vs. transformer saturation current and it is less, that is good.  should i also check the secondary current against the SAME saturation current?  or is it a primary current check only?

Bp = (VL * Ton)/(Np*A)

this formula shows that BP is directly proportional to VL.  In the flyback case VL is equal to Vin  during the ON time of the controller.  The product VL * Ton is what determines how large will be the flux density.  

You need to know NP