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Alex M

Thu Oct 06 2011, 02:19AM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Hello, I have recently remade my 555 flyback driver and can get pretty decent arcs for just 12v input, although the arcs are not hot flaming arcs they do stretch to around 3 inchs if I pull the electrodes apart steadily.

My problem is that my MOSFET (IRF540) gets very hot unless I use a very big heat sink.

How can I stop the MOSFET from heating up? as I have seen some videos on youtube of people with the 555 driver and their MOSFETs stay cool (according to them).

Here is my circuit (Note I am not using the 1nF cap from pin 5 to ground, but that should not make any difference in MOSFET heating).

Any advice on what to do in order to reduce MOSFET heating would be appreciated. I have been reading a couple of the archives and some people have mentioned using "clampers" to reduce heating, what does that mean?

As you can see the output is not bad for just 12v input, although the arcs are not hot by any means.

Patrick

Thu Oct 06 2011, 03:39AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Im looking at your irf540 transistor now.

]irf540n.pdf[/file]

Why the hell do you have pins 4 and 8 connected to a resistor? that will slow the turn on of the mosfet, causing your heating? change that, and change the R4 to 10 ohms and then tell us how hot it gets.

Alex M

Thu Oct 06 2011, 04:05AM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

:-(( Sorry, I did like that as it was like that on the schematic.

I have tried it without the resistor and the MOSFET still gets hot though.

Patrick

Thu Oct 06 2011, 04:18AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

so you swapped R4 so it was 10ohms, AND while the pins 4 and 8 were tied positive with no resistor... it still got hot?

Alex M

Thu Oct 06 2011, 05:56AM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Yes, after doing that my MOSFET died.

Any ideas why? And what modifications can I make to help protect the MOSFET?

Patrick

Thu Oct 06 2011, 06:18AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Alex1M6 wrote ...

Yes, after doing that my MOSFET died.

Any ideas why? And what modifications can I make to help protect the MOSFET?

Look a the TVS's (clamps) on my project thread here. ->

The TVS's protect the MOSFET from overvoltage damage. they behave like zeners. My 1000 Volt FET is clamped down to about 700 V max.

Alex M

Thu Oct 06 2011, 06:29AM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Sorry, I am still learning about all of this and am not quite sure what I am meant to be looking at.

Are TVS some sort of protection diode and if so how would they look in my circuit?

Again I am not trying to be a pain and am still learning about what killed the MOSFET.

Thanks.

Patrick

Thu Oct 06 2011, 06:34AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Alex1M6 wrote ...

Sorry, I am still learning about all of this and am not quite sure what I am meant to be looking at.

Dont worry everyone on this forum is STILL learning.

Alex1M6 wrote ...

Are TVS some sort of protection diode and if so how would they look in my circuit?

Yes, see my above reply.

Alex1M6 wrote ...

Again I am not trying to be a pain and am still learning about what killed the MOSFET.

Thanks.

Dont worry we'll figure it out, but those mosfets will get killed on the time. so get used to it and buy more then you need.

Here's a pic:

Power chopping circuit.

D1 and D2 are the TVS's they are in series to double their clamping voltage.

Alex M

Thu Oct 06 2011, 06:43AM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Thanks that makes sense now.

TVS stands for Transient voltage suppression diode?

Ok, thanks.

What voltage Transient voltage suppression diode should I use?

Patrick

Thu Oct 06 2011, 06:49AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

They arent zeners, there similar, but they are made differently have different silicon doping and a whole college semester of stuff i could tell you.

I choose my TVS's to fully conduct between 60% and 80% of the mosfet breakdown volatge.

So, for example using my preference a 1000 v Fet would be clamped at 600-800 v. Now rememeber the flybacks we use need a "recoil" (like a rifle) of the inductive back emf, and the collapse of the magnitizing current in the core to make the whole thing work. So if you clamp them at a very low voltage they wont work well.

you say youre putting 12v input, but youre transistor could be seeing 300 plus voltage.

TVS's can be uni-directional (like mine) or bidirectional.

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