Its a flyback converter.I saw that, but I don't understand how you can have "2.7 – 5.5 V input; 350 V output" if the ratio is only 1:15.
Can someone explain?
The transistor switches on to pass current through the primary to build up magnetic flux. When the transistor lets go, the current flow stops and the flux collapses abruptly - that produces a back emf several times the magnitude of the original applied voltage - that back emf is what's multiplied by the turns ratio.