Look at the class B push-pull amplifier and compare it to how you have your transistors hooked up. - Do you think the PNP will ever turn on?
http://en.wikipedia.org/wiki/Electronic_amplifier

If you say a the mosfets are heating up with "no signal", does that mean the gates are unconnected, or is the pulldown 1K resistor still connected?

 

Ron, Nigel, James,

Sorry, I hooked the transistors right way, but when I drew the schematic I drew it wrong.

In the circuit I've built it is hooked right.

This is the correct schematic:


James,
When I say no signal is, that I'm not applying signal on Q5's base (BC337).

This is what happens, I apply some signal there, then I disconnect the Q5's base, the MOSFET keeps heating.

 

Your zener will never conduct unless your supply goes above about 13.4V. If it does, either Q3, or the zener, or both will overheat and possibly fail.
What about my other comment?
Why do you need push-pull emitter followers? Are you using a CMOS 555? A bipolar 555 will source and sink over 200mA, IIRC.

__________________
Ron

 

Ron, the main problem is that my mosfets are not turning off.

Well, the main purpose of the push-pull is to "pull down" the mosfet gate when no tension or low level is applied to Q5 base.
I think that a transistor conducting to the ground do a better job than a "pull down" resistor.

Well, the circuit is working "nice" now, look what I've done a few hours ago:
I connected the circuit in a tweeter with an ammeter, I've notice that it were flowing about 0.5A of current and Q2 (IRF740) were getting a little bit hot.
I removed it (letting Q1 only) and changed Q4 (PNP one).

The circuit worked fine, but when a spark, from the ignition coil HT terminal, leaked and hit the negative terminal...the remaining mosfet were destroyed.

Well, I have to discover what caused the Q2 to fail, with an ohmmeter I measured the resistance between all the terminals (G-S, G-D, D-S...), for my surprise it was about 50 ohms (!!!).
Q2 gate were leaking causing Q1 to conduct as well, causing malfunction in the whole circuit, and probably that was the cause of Q4 burning as well.

But I have no idea what caused Q2 to fail, maybe the it was first affected by the coil back-emf (this is my only guess).

Later I'll try to add some zener or varistor between the drains and sources to protect the mosts against back-emf.

 

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.

__________________

I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.

Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

 

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 this discussion of the Kettering ignition. Pay particular attention to the discussion about the capacitor. I think you might benefit from one.

__________________
Ron

 

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?

 

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.