i need to design fly back
switching freq =100khz
output power=100w
voltage out put=48v
voltage input=120v
so how can i do design for the transformer ,what can of core to used that will not saturate

 

Maybe we need to know the power source as well... ac, dc, 120 V, etc. Are you making a dc-dc flyback converter (switching power up-converter)?

 

1kW is too much for a flyback design.
Look at push-pull, half-H-bridge, full-H-bridge designs, resonant with zero-volt switching preferred.

 

Maybe we need to know the power source as well... ac, dc, 120 V, etc. Are you making a dc-dc flyback converter (switching power up-converter)?

i just i need to know how to choose the core that will not saturate

 

Here are some application notes/articles for you:
http://www.ti.com/lit/ml/slup124/slup124.pdf
http://www.fairchildsemi.com/an/AN/AN-4140.pdf
http://www.gotopower.net/article.asp?art_id=65
http://nptel.iitm.ac.in/courses/Web... Electronics/PDF/L-22(DP)(PE) ((EE)NPTEL).pdf
Here is Magnetics Inc. technical documentation page:
http://www.mag-inc.com/design/technical-documents
Click the "+" to the right of "General Information"
There are a number of documents of interest in there.