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MOSFET problem with 555 timer flyback driver

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Dr. Dark Current

Fri Aug 20 2010, 12:00PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

IRF540 with only 100Vds is not enough for 18V. Use an IRFP260.

teravolt

Fri Aug 20 2010, 06:43PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

hi Plasmana if I were you I would use a irfp450 and add a diode like a mur1660 in series with your drain. It should work good if the primary sees a high impedance when your flyback goes into its negitve half of the cycle. the mazzilli circuit conducts on boath halfs. I also noticed that your trimpot is ganged in your drawing. If I were you I would set up your frequency and pulse width seperatly insted of a 50/50 duti. You may only need 200us or so to excite your primary tank

Plasmana

Fri Aug 20 2010, 11:50PM

Registered Member #3108 Joined: Thu Aug 12 2010, 05:37PM
Location: Worthing, England
Posts: 72

Teravolt and Kilovolt, thank yall very much for the advice! Ill improve the driver tomorrow as i got other projects going on. Also, why put a diode in series with the mosfet's drain?

Antonio

Sat Aug 21 2010, 12:33AM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

If the mosfet starts to get hot, stop the circuit immediately. Something that is too hot to touch IS too hot. When you increase the power supply voltage the voltage pulse over the mosfet when it stops conducting increases in proportion. You can increase the value of the capacitor to compensate, but this will also limit the output voltage. To obtain more output voltage without increasing the voltage over the transistor, use less turns in the primary coil.

Proud Mary

Sat Aug 21 2010, 01:03AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

You might also think about a snubber network in the drain circuit to clip excessive spikes.

teravolt

Sat Aug 21 2010, 03:32AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

Plasmana wrote ...

Teravolt and Kilovolt, thank yall very much for the advice! Ill improve the driver tomorrow as i got other projects going on. Also, why put a diode in series with the mosfet's drain?

the reason why I think you need a diode in series with the drain is that your circuit is basically a class C amp powered by your 555. your primary tank on the flyback will swing back and forth in a AC wave form. The body diode of the mosfet is conducting in the negative half cycle. The mosfet should conduct in the positive half and be high impedance in the negative cycle. think of it like pushing a person on a swing or a piston and a crank. You push away only half the time. a mazzilli circuit works by pushing on each half cycle. The primary being a parallel tank works like a flywheel storing energy at the resonant frequency and that is connected to the crank so every time the fet turns on it adds energy to each cycle. In a class c amp the fet is only on for 15 to 50 percent of the time so adjusting your pulse width should have an effect on your power out.

Another possibility is that your fet is not being turned on at the resonant frequency so the flyback is out of sync especially if you have a load. the flybacks resonsnt frequency will go down while the 555 has a fixed freq causing the fet to turn on at the wrong time. if you repalce the 555 with another winding from the core you can make a armstrong oscillator like a VTTC. the winding may have to be biased. In that way the fets frequency will be governed by the flyback

hope that makes some sence

Plasmana

Sun Aug 22 2010, 10:11PM

Registered Member #3108 Joined: Thu Aug 12 2010, 05:37PM
Location: Worthing, England
Posts: 72

Harry, I have added an snubber network, the arcs are even bigger now!

Teravolt, that is some really interesting piece of information, knowing how my driver work. I have added an diode similar to what you suggested, I have not noticed any real changes other than the arc output is slightly shorter. And the diode stayed relatively cool after 30 sec of operation.

EDIT:
I have also built another improvised driver, Ill post pics and schematics when it is all done and dusted. :)

Dr. Dark Current

Sun Aug 22 2010, 10:34PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Teravolt: this is closer to class-E than C, and it doesnt need to run at the resonant frequency (actually it runs below it). For class C you should have a feedback directly from the tank circuit, otherwise tuning away from the resonant frequency might blow the transistor. Also for class C you need low duty cycle, maybe 20 percent or less.

Myke

Sun Aug 22 2010, 11:55PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Would it be closer to push-pull class B because the conduction angle is pi? Class C is a conduction angle less than pi. Class E requires V=0 and dV/dt = 0 when switching from off to on and so needs a pulse shaping network.

Plasmana

Sat Aug 28 2010, 05:07PM

Registered Member #3108 Joined: Thu Aug 12 2010, 05:37PM
Location: Worthing, England
Posts: 72

Sorry for the short delay, I was a bit carried away with my other projects.

Anyway, here it is!

Video:

If you don't like the music, my apologies, I have no idea how to chose a music that will fit well with the video.

Pics:

This effect had an special name for it, but I forgot..

Schematic!

At the moment, I am still using the IRF540 MOSFET as I got no IRFP's available. It works great as long as I don't above 12v

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