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MOSFET problem with 555 timer flyback driver

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Plasmana

Tue Aug 17 2010, 11:48PM

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

Hi 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!

william L

Wed Aug 18 2010, 02:39AM

Registered Member #3093 Joined: Mon Aug 09 2010, 11:40PM
Location:
Posts: 68

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!

Plasmana

Wed Aug 18 2010, 09:45PM

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

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.

AleSeg

Wed Aug 18 2010, 10:07PM

Registered Member #2727 Joined: Tue Mar 09 2010, 02:39PM
Location: Montevideo - Uruguay
Posts: 33

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

Goodchild

Wed Aug 18 2010, 10:36PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

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.

Plasmana

Wed Aug 18 2010, 11:06PM

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

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.

Antonio

Thu Aug 19 2010, 12:23AM

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

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.

Plasmana

Thu Aug 19 2010, 02:11PM

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

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.

Dr. Dark Current

Thu Aug 19 2010, 02:56PM

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

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.

Plasmana

Thu Aug 19 2010, 06:35PM

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

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..

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