I have made some changes in the schematic posted.The bootstrap diodes and capacitor are changed to MUR160 and 1uf tantalum capacitor.

Added TLP250 optocoupler between 8051 and IR2111.MOSFET IRFP460 (500V 20A)is replaced by IRFPBC30 (600V 3.6A).

I connected the digital GND to -ve terminal of DC power rail( I am not sure whether this is required)

I first tested circuit with 24V DC input to DC power rail and connected three 24V lamps(star connection with start point without any connection) to output.The lamps glows and the circuit works properly.When 325V Dc supply was given to DC rail after line voltage rectification the MOSFET blows out instantly...

Is this due to voltage rating of MOSFET IRFBC30 is low, may be 1000V Vdd is required or any other reason.Do I use diodes across the mosfet if yes than which are they?

PS:i am sorry to start new thread as my old thread
(http://www.8052.com/forum/read/157326)
was blocked.Can this thread be moved to original post.