I was working on a simple flyback driver that uses an astable oscillator to give a signal to the MOSFET which would drive the flyback. Even though the signal generator looks extremely fine and could generate a frequency from 10k to 0.1±Mhz, upto 5.7V with variable duty cycle, as soon as the drain of the GaN FET is connected to the primary coil it gives s spark and kills the timer.
My attempt was to isolate the timer from the GaN (as shown in the circuit) with a BJT which was failure because the BJT was working properly even after the timer is fried.
I also tried adding a resistor to the gate of the FET which worsend the instantaneous spark.
The thing that freaked me out was a circuit that uses the same logic (astable oscillator using transistor instead of 555 timer) worked great and I was able to increase the voltage through the gate by placing an inductor series to R4.
What should I do to prevent this?