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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?

  • 2
    \$\begingroup\$ You need a base resistor for Q1. \$\endgroup\$
    – user173271
    Commented Jul 2, 2022 at 16:39
  • 2
    \$\begingroup\$ You probably also need a flyback diode for the transformer primary or something else is going to die. \$\endgroup\$
    – DKNguyen
    Commented Jul 2, 2022 at 16:50
  • \$\begingroup\$ Without snubber diode it's possible for large induced drain voltage to couple back to the gate (and 555's output) via gate-drain capacitance. \$\endgroup\$
    – user173271
    Commented Jul 2, 2022 at 16:53
  • 1
    \$\begingroup\$ Let's add some decoupling capacitors to the shopping list, that transformer is going to kick VCC into orbit \$\endgroup\$
    – bobflux
    Commented Jul 2, 2022 at 16:53
  • \$\begingroup\$ @James sorry i forgot to include it there but I have already added it in my original circuit (I have edited the circuit) \$\endgroup\$ Commented Jul 2, 2022 at 17:07

2 Answers 2


-Define specs to ensure Flyback Xfmr is not saturating and specify DCR, L, I max.

common layout issues:

  • Avoid loop current crosstalk by using twisted pair.
  • Decrease current loop area of driver so that LM555 supply is not affected and does not share power/ground path to source. Twisted pairs preferred.
  • use avalanche protect FET or add TVS across Vds and not to shared 12V, then RC filter to 555 Vcc may be necessary.
  • add 15V TVS to Vgs.
  • also compute resonant frequency from Coss=700 pF with Lp and DCR/X(f) and Q for full cycle and pulse width in choose f.

It is possible your duty cycle was too high and Flyback Tfmr saturated, overheated then arced when released.

In the future start with lower supply voltage and zero duty cycle then measure margins to failure. (saturation, temp rise, peak voltage)


I first thing I see is that you have no capacitor on pin5, when there's large current involved you need that bypass on the internal divider chain to keep the 555 stable. add 100nF MLCC between control voltage and ground (pin5 and pin1)

Make sure all of your grounds are well connected, this is not a not a circuit to build on solderless breadboard, especially everything right of R7, needs to be soldered connections an connected directly to the supply and ground, not through solderless breadboard.


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