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Forums
4hv.org :: Forums :: High Voltage
« Previous topic | Next topic »   

555 flyback transformer MOSFET gets hot.

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Alex M
Thu Oct 06 2011, 06:54AM
Alex M Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
 		Thanks, I have learnt something this morning thanks to you.

Well I am only using 12V input with this circuit and no more. My MOSFET's have a higher voltage to help protect them, which does not seem to help.

I have just looked and can get them at my local Maplin store for next to nothing, 600W and 1500W ones.

Would I need bi-directional or uni-directional for use with this circuit?
Thanks.
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Patrick
Thu Oct 06 2011, 07:01AM
Patrick Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
 		The IRF540N only handles 100V or less. The IrF540 is kinda low on the D-S rating. so id buy a 400-600 volt mosfet and the 1500W TVS's that will fully clamp at 80% of 400 v = 320V

So around 320v or so, there are a lot of specs related to when the clamping occurs. youll have to research the datasheets.
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Alex M
Thu Oct 06 2011, 07:06AM
Alex M Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
 		Thanks for your help. Should it be a uni-directional or bi-directional?

I only used the IRF540 as it was all Maplin had in stock but I will probably be ordering some "decent" components from Farnell soon.
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Patrick
Thu Oct 06 2011, 07:08AM
Patrick Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
 		well id go with which ever is cheaper, and buy several different voletage ones, incase you choose a wrong value.

I need some sleep, its 12:10AM here, ive got a LAB at 8am any last questions?
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Alex M
Thu Oct 06 2011, 07:09AM
Alex M Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
 		Thanks for you time.

Does it make a difference if I use bi or uni directional?
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Patrick
Thu Oct 06 2011, 07:15AM
Patrick Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
 		Im thinking....
...
...
...
...
... i think it does not matter in this application.
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buxtronix
Thu Oct 06 2011, 07:34AM
buxtronix Registered Member #4078 Joined: Tue Aug 30 2011, 12:53PM
Location: Sydney, Australia
Posts: 19
 		You probably also want to look at the duty cycle, if it's too high, the flyback might be saturating. The mosfet only needs to be on long enough to build up the magnetic field to just before saturation.
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Patrick
Thu Oct 06 2011, 07:51AM
Patrick Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
 		I also need to know if there is an internal diode which could explain the overheating too.

Whos the maker of your mosfet?
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Alex M
Thu Oct 06 2011, 08:00AM
Alex M Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
 		IRF (international rectifier) I think as there is a F on the front of the MOSFET.

Yes there is an internal reverse bias diode in this one.
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astralhighway
Thu Oct 06 2011, 09:11AM
astralhighway Registered Member #4107 Joined: Sun Sept 25 2011, 07:30PM
Location: London
Posts: 53
 		Change the primary turns (increase them.). Also have you thought of making this a resonant circuit?
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