500 watt smps based inverter

[DeepOne]

not able to find that

this because they are absent ) but then there is TVS (supressors) and chains through which voltage spikes are connected on a + rail.
Certainly, it is possible to use snubbers or that anything else, if You want.
But without oscilloscope or/and detailed scheme possible else to singe much more fets until find reason of the origin to faults.

 

[Tahmid]

For snubber, you can start with a set of values and then adjust the values by observing the waveforms using oscilloscope. Trial and error method.

Here's a snubber configuration commonly used (even in commercial inverters):

Hope this helps.
Tahmid.