MOSFETS do not switch off properly

[Roff]

My question is, and pardon me for shouting, but this is the third time I have asked this question, WHY ARE YOU NOT USING THE OUTPUT OF THE 555 TO DRIVE THE MOSFETS DIRECTLY?
Back EMF is what gives you the high voltage, so clamping the flyback voltage would seem to defeat the purpose. Have a look at **broken link removed**. Pay particular attention to the discussion about the capacitor. I think you might benefit from one.

 

[Hayato]

The IRF740 is rated for 400V so I don't think back EMF is your problem. I meant to replace the resistor with the ferrite bead.

However, the last circuit looks quite good to me.

But what scared me is that Q2 was instantly burned at the first turn on, maybe not back emf, but a leaking spark may have killed it.

Ron,
Thanks for the text.
I don't want to defet the back emf effects on the secondary winding, but its effects on the primary/some kind of high voltage leaking from the secondary to the primary/some kind of high voltage spike on the primary that causes the mosfet destruction.

That's why I'll add a MOV between the drain and source.

I don't wanna drive it directly from the 555 because, in some cases, I'll need to use it w/o the 555. And it's better to burn a PNP transistor than a whole IC, in case of some malfunction, isn't it?

 

[Hayato]

Hello,
I added a ~285V Varistor in parallel with the IRF740s. The circuit worked 100%, I ran it for 3 hours and there where no problems.