I made a flyback driver which just uses an astable 555 circuit to drive the flyback. The 4 and the 8 pins were connected to the positive rail. 7 pin was connected to the positive rail via a 1K resistor and a 10K pot. The 6 pin was connected to the 7 via another 1K resistor and pot. A 10nF cap was between 2 and the negative rail, which was connected to the 6 pin. I used an IRF540 MOSFET directly connected to the 555 to drive it. I was able to get atleast a centimeter long arcs out my flyback (FKD15A001). As excpected, the MOSFET gets hot, I had already attached a heatsink to it so it was no problem. But, the chip also gets very hot for some reason. I have already lost 3 of my chips because of this. Is there anyway to prevent this? Why is this happening? I am using a 12V power supply.

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    \$\begingroup\$ You can insert a schematic using the schematic button above the editor. That would be easier (for everyone) than describing each connection in detail. \$\endgroup\$
    – JYelton
    Mar 17, 2014 at 17:22
  • \$\begingroup\$ Also, what is the exact part number of the chip? \$\endgroup\$ Mar 17, 2014 at 17:49
  • \$\begingroup\$ NE555 from texas instruments \$\endgroup\$
    – AvZ
    Mar 18, 2014 at 0:51
  • \$\begingroup\$ Never mind, all my chips are dead... I am using another driver now... \$\endgroup\$
    – AvZ
    Mar 22, 2014 at 5:04

1 Answer 1


You should consider interfacing the 555 indirectly to the MOSFET for a couple of reasons: -

  • The gate capacitance of the MOSFET (circa 1nF) likely means that the waveform output from the 555 is being turned from a nice looking squarewave into a more triangular shape and this will cause the 555 some stress and be not ideal for driving a FET used in a flyback transformer circuit.
  • The output capacitance of the FET can couple significant energy from the transformer primary to the gate when switching and this may also be causing the 555 a lot of stress. Assuming you are running the circuit from 12V, the primary winding will likely be seeing 24Vp-p (as per a regular flyback design) and if you imaging this couple via (say) 100pF to the 555's output you might understand what I'm hinting at.

There may be other things wrong with your design but I would definitely consider using a high current logic driver to feed the gate. The gate driver should be able to deliver about an amp into the gate in order to charge and discharge the gate's capacitance up quickly.

With a poor driver you might also be getting an instability when the FET switches. When it turns off (gate attempts to go low), the output rises quickly and with the inter-lead capacitance this rising voltage can couple to the gate and cause it to momentarily switch off partially. Again it will warm up the FET and if it happens on the rising edge it will likely happen on the falling edge of the drain voltage.

  • \$\begingroup\$ That said, I have a Chinese novelty neon sign that uses almost this exact circuit. \$\endgroup\$ Mar 17, 2014 at 18:52
  • \$\begingroup\$ @SpehroPefhany - maybe the OP isn't running it at high enough supply voltage. I think, when it comes to TV LOPTYs, I'd use a BJT!! \$\endgroup\$
    – Andy aka
    Mar 17, 2014 at 18:54
  • \$\begingroup\$ First time I've seen a 555 in a production design in... forever. A CMOS one. I'll have to dig it out.. the circuit didn't fail, the glass got broken. \$\endgroup\$ Mar 17, 2014 at 19:07
  • \$\begingroup\$ Can you provide a schematic please? It will help a lot. \$\endgroup\$
    – AvZ
    Mar 18, 2014 at 0:54
  • \$\begingroup\$ Can I use a power NPN (BD679 or TIP127) to switch the MOSFET? \$\endgroup\$
    – AvZ
    Mar 18, 2014 at 7:10

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