plastic welding machine; generator

[homraj reddy]

i am making plastic welding generator where i am using 20 khz signal to for welding(BY using inverter with IGBT), this 20 khz electrical signal is given to piezo electric device, this 20 khz signal is converted into 20 khz machanical vibrations, and this vibratios are given to boosters and finally given to horn for pastic welding,
i have one problem regarding it, that is when horn is get heated then frequency get changed, as the freqeucny is always increasing ad so igbt get burn out, why this happen?
i am using 4046, 3525, pic18F452, and with some op-amps.( i am not usig any phase comp. of 4046)

 

[FvM]

BY using inverter with IGBT (...)
i am using 4046, 3525, pic18F452, and with some op-amps.( i am not usig any phase comp. of 4046)

I presume something is wrong with the design.