Topswitch damaged

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we have designed 50W power supply using top switch, during testing i observed that TOP switch get damaged i.e. burned completly but fuse (2A, slow blow) is found ok. I didnt getting how fuse is not damaged?? also have movs for surge protection in circuit. Can anyone suggest what is the problem??

You can damage the topswitch for either too much DRAIN current or too much DRAIN to SOURCE voltage.   The first thing to do is to review your design.  Where did you get the transformer from?  If you designed your own transformer,  did you use PI expert to design it?    Please post your schematic and your transformer design in PI expert format.