Output Voltage is not coming

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I an designing a SMPS 12V using a TOP260EN IC. I got design through PI expert online. But the internal mosfet is not turning on. What may be reason ?
I am getting 290V across primary winding but the output voltage is not coming
Can you please let me know in detail how it works

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TOPSwitch-HX_PIDesign2.pdf2.41 MB

When you say you are getting 290V across the primary winding, are you monitoring the drain pin of the TOPSwitch? You cannot get voltage across the primary if the mosfet is not switching. If you verify that there is voltage at the drain, the next step is to check the voltage at the TOPSwitch control pin. It should be ~5.8V - if not, look for shorts or a reversed electrolytic cap (C5) on the control pin. Capacitor C4 could also be shorted. Also, check the V-pin resistors (R9-10) to make sure they are the right value and actually connected to B+.
The operation of the IC is discussed in great detail in the data sheet.

Thanks for your support. Now it is getting On but the output voltage is increasing from 40 to 70V linearly with input. Sometimes the TOP IC is getting burnt. What may be the isuues ?
where i have done wrong ?
I have kept R15 = 40K, R16 = 10K, R6=R7= 10M and R8 = 8.2K and output inductor 6.8uH

TOPSwitch-HX_PIDesign19.pdf424.54 KB

It sounds very much like you have a transformer with the wrong phase. Try swapping the primary connection..