A stupid question

[MaXiZ]

SMPS... Mosfet...
Gate go to SG3525 or any other chip...
Drain go to earth
Source go to tranformer..
OR
Gate go to SG3525 or any other chip...
Drain go to tranformer
Source go to earth
??
Thanx in advance

 

[peranders]

I'll guess you haven't read the datasheet?

http://www.st.com/stonline/books/pdf/docs/4286.pdf

Check figure 10, page 12