Why does the output 12V square wave always burn Q2?

The upper and lower tubes may be turned on at the same time.

It is caused by the heavy load, and it is necessary to not replace the mid-power transistor.

The transistor is burnt out due to insufficient drive current. The transistor drive resistance is 10K. Assuming that the drive voltage is 12V at the initial resistance, 12V/10K=1.2mA, assuming that the transistor magnification is 100 times (not achieved under power conditions), that is 120mA, in the case of insufficient drive, the transistor is in a linear amplification state, and the power consumption is large. When the control frequency is high, the MOS also requires current and burns out due to heat.