MOSFET problem with 555 timer flyback driver
Plasmana, Tue Aug 17 2010, 11:48PMHi all,
I have fairly recently redesigned my 555 timer flyback driver using Dr. Kilovolt's idea of putting a resonance capacitor in parallel with the primary of flyback transformer, it had dramatically improved the performance of my flyback driver. I am amazed how a single component can have such a dramatic effect on a circuit! My driver now makes some very impressive long arcs that can be stretched up to 6cm and yet, the MOSFET still remains cold..!
However, I still have the same problem with the MOSFET's popping... The highest voltage i can run this driver without the MOSFET popping in seconds is 12v. The MOSFET sometimes even pops while running the driver at 6v!
I am thinking it might be due to overvoltage but I read somewhere that the MOSFET must be 3 or 4x the supply voltage, the IRF540 is rated max 100v, so I assume it is suitable..? So is there another flaw in my design that I can't see? If so, can you point it out?
Schematic:
Just two things to point out in the schematic:
I like to power 555 oscillator and the driver separately, to prevent the oscillator getting interfered.
The 100K failsafe resistor is soldered directly onto the gate and source of the MOSFET to prevent it blowing up if the signal wire gets disconnect accidentally or something.
Any help with be greatly appreciated!
Re: MOSFET problem with 555 timer flyback driver
william L, Wed Aug 18 2010, 02:39AM
I think It might be a result of the diode recovery time. It says that the mosfet needs to be rated for ~ 100v, right? (the back emf could even reach this at low input) well, at 100 volts, the diode can take up to 700ns to recover (my understanding of the data-sheet) Try putting a fast-recover diode rated for the circuit across the mosfet. Hope this helps!
william L, Wed Aug 18 2010, 02:39AM
I think It might be a result of the diode recovery time. It says that the mosfet needs to be rated for ~ 100v, right? (the back emf could even reach this at low input) well, at 100 volts, the diode can take up to 700ns to recover (my understanding of the data-sheet) Try putting a fast-recover diode rated for the circuit across the mosfet. Hope this helps!
Re: MOSFET problem with 555 timer flyback driver
Plasmana, Wed Aug 18 2010, 09:45PM
I have tried that, still same results and MOSFET's popping.
The only problem I can really think of is overvoltage, Ill try using a different MOSFET and see what happens.
Plasmana, Wed Aug 18 2010, 09:45PM
I have tried that, still same results and MOSFET's popping.
The only problem I can really think of is overvoltage, Ill try using a different MOSFET and see what happens.
Re: MOSFET problem with 555 timer flyback driver
AleSeg, Wed Aug 18 2010, 10:07PM
How about the voltage Vcc for the 555 ? I think is better to use 12 Vcc, and a good ceramic 0.1 uF decoupling capacitor between power pins in the 555. With low volts in the gate the MOSFET can enter in linear mode and get destroyed.
See you
AleSeg, Wed Aug 18 2010, 10:07PM
How about the voltage Vcc for the 555 ? I think is better to use 12 Vcc, and a good ceramic 0.1 uF decoupling capacitor between power pins in the 555. With low volts in the gate the MOSFET can enter in linear mode and get destroyed.
See you
Re: MOSFET problem with 555 timer flyback driver
Goodchild, Wed Aug 18 2010, 10:36PM
As AleSeg said I would really try driving that MOSFET at a higher voltage try at least 12V and 15v would be even better.
What could be happening is this, when you turn the gate on at only 9v you are only partly turning the gate on, witch means that on resistance would have been high witch would result in heating of the device.
The only way you can really know is to scope the circuit to see what kinda of voltage are present in that circuit.
Goodchild, Wed Aug 18 2010, 10:36PM
As AleSeg said I would really try driving that MOSFET at a higher voltage try at least 12V and 15v would be even better.
What could be happening is this, when you turn the gate on at only 9v you are only partly turning the gate on, witch means that on resistance would have been high witch would result in heating of the device.
The only way you can really know is to scope the circuit to see what kinda of voltage are present in that circuit.
Re: MOSFET problem with 555 timer flyback driver
Plasmana, Wed Aug 18 2010, 11:06PM
That could be the problem, I did not think of that! (and answers my question why the MOSFET heats up so much on my previous experiments months ago)
However, the MOSFET dies cold, it never got hot after I added the resonance capacitor.
Plasmana, Wed Aug 18 2010, 11:06PM
That could be the problem, I did not think of that! (and answers my question why the MOSFET heats up so much on my previous experiments months ago)
However, the MOSFET dies cold, it never got hot after I added the resonance capacitor.
Re: MOSFET problem with 555 timer flyback driver
Antonio, Thu Aug 19 2010, 12:23AM
This circuit needs a fast diode across the mosfet, as already said. It's also a good idea to really ground the 0V line, or the device will produce positive HV at the flyback output and negative HV at the line that should be ground. If the circuit touches something that is grounded, as you, a spark can cause damage to it.
Antonio, Thu Aug 19 2010, 12:23AM
This circuit needs a fast diode across the mosfet, as already said. It's also a good idea to really ground the 0V line, or the device will produce positive HV at the flyback output and negative HV at the line that should be ground. If the circuit touches something that is grounded, as you, a spark can cause damage to it.
Re: MOSFET problem with 555 timer flyback driver
Plasmana, Thu Aug 19 2010, 02:11PM
Thanks for the feedback guys!
I have improved the 555 timer's input voltage from 9v to 15v. I put an ultra-fast diode between the drain and source of the MOSFET. The performance is still the same but the MOSFET lasts much longer!
However, there is one thing I am still quite baffled with, the driver works well up to 12v without the MOSFET even getting warm, but when I push 18v into the driver, it made some pretty huge arcs then the MOSFET blows seconds later...
I am still using the IRF540 MOSFET and my power source is the SLA batteries.
Plasmana, Thu Aug 19 2010, 02:11PM
Thanks for the feedback guys!
I have improved the 555 timer's input voltage from 9v to 15v. I put an ultra-fast diode between the drain and source of the MOSFET. The performance is still the same but the MOSFET lasts much longer!
However, there is one thing I am still quite baffled with, the driver works well up to 12v without the MOSFET even getting warm, but when I push 18v into the driver, it made some pretty huge arcs then the MOSFET blows seconds later...
I am still using the IRF540 MOSFET and my power source is the SLA batteries.
Re: MOSFET problem with 555 timer flyback driver
Dr. Dark Current, Thu Aug 19 2010, 02:56PM
I think your MOSFET could be popping because of overcurrent when the capacitor discharges to it at turn-on. Replace the 100ohm resistor with a 680ohm one, and put an ultrafast diode (1N4148/UF4007 etc.) in parallel to the resistor, anode on Gate cathode to 555.
Dr. Dark Current, Thu Aug 19 2010, 02:56PM
I think your MOSFET could be popping because of overcurrent when the capacitor discharges to it at turn-on. Replace the 100ohm resistor with a 680ohm one, and put an ultrafast diode (1N4148/UF4007 etc.) in parallel to the resistor, anode on Gate cathode to 555.
Re: MOSFET problem with 555 timer flyback driver
Plasmana, Thu Aug 19 2010, 06:35PM
Kilovolt, I have done what you suggested. The performance of the flyback driver had increased and when I try running it at 18v, the MOSFET survives a few seconds longer then it erupted into a heck a lot of smoke!
Should I start try using IRFP450 or similar? The only thing I am worried about is destroying them when I run the driver at 18v and above..
Plasmana, Thu Aug 19 2010, 06:35PM
Kilovolt, I have done what you suggested. The performance of the flyback driver had increased and when I try running it at 18v, the MOSFET survives a few seconds longer then it erupted into a heck a lot of smoke!
Should I start try using IRFP450 or similar? The only thing I am worried about is destroying them when I run the driver at 18v and above..
Re: MOSFET problem with 555 timer flyback driver
Dr. Dark Current, Fri Aug 20 2010, 12:00PM
IRF540 with only 100Vds is not enough for 18V. Use an IRFP260.
Dr. Dark Current, Fri Aug 20 2010, 12:00PM
IRF540 with only 100Vds is not enough for 18V. Use an IRFP260.
Re: MOSFET problem with 555 timer flyback driver
teravolt, Fri Aug 20 2010, 06:43PM
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
teravolt, Fri Aug 20 2010, 06:43PM
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
Re: MOSFET problem with 555 timer flyback driver
Plasmana, Fri Aug 20 2010, 11:50PM
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?
Plasmana, Fri Aug 20 2010, 11:50PM
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?
Re: MOSFET problem with 555 timer flyback driver
Antonio, Sat Aug 21 2010, 12:33AM
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.
Antonio, Sat Aug 21 2010, 12:33AM
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.
Re: MOSFET problem with 555 timer flyback driver
Proud Mary, Sat Aug 21 2010, 01:03AM
You might also think about a snubber network in the drain circuit to clip excessive spikes.
Proud Mary, Sat Aug 21 2010, 01:03AM
You might also think about a snubber network in the drain circuit to clip excessive spikes.
Re: MOSFET problem with 555 timer flyback driver
teravolt, Sat Aug 21 2010, 03:32AM
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
teravolt, Sat Aug 21 2010, 03:32AM
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?
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
Re: MOSFET problem with 555 timer flyback driver
Plasmana, Sun Aug 22 2010, 10:11PM
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. :)
Plasmana, Sun Aug 22 2010, 10:11PM
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. :)
Re: MOSFET problem with 555 timer flyback driver
Dr. Dark Current, Sun Aug 22 2010, 10:34PM
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.
Dr. Dark Current, Sun Aug 22 2010, 10:34PM
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.
Re: MOSFET problem with 555 timer flyback driver
Myke, Sun Aug 22 2010, 11:55PM
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.
Myke, Sun Aug 22 2010, 11:55PM
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.
Re: MOSFET problem with 555 timer flyback driver
Plasmana, Sat Aug 28 2010, 05:07PM
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
Plasmana, Sat Aug 28 2010, 05:07PM
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
Re: MOSFET problem with 555 timer flyback driver
william L, Wed Sept 08 2010, 12:43AM
wow. That's amazing. It's only 12 volts!?
I'll have to try that with an irfp260.
william L, Wed Sept 08 2010, 12:43AM
wow. That's amazing. It's only 12 volts!?
I'll have to try that with an irfp260.
Print this page