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help needed for a buck converter

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haxor5354

Mon Jul 18 2011, 12:33AM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

its powered by an ATX psu so the ground terminal is acutally connected to the mains ground.
the gate is 10v relative to ground and source is 6v relative to ground.

when I power it with a 12v battery with the scope's ground probe connected to source and tip on gate it reads 4v peak to peak.

Patrick

Mon Jul 18 2011, 12:46AM

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

haxor5354 wrote ...

its powered by an ATX psu so the ground terminal is acutally connected to the mains ground.
the gate is 10v relative to ground and source is 6v relative to ground.

when I power it with a 12v battery with the scope's ground probe connected to source and tip on gate it reads 4v peak to peak.

Let me look at the datasheet....

Ok looks like the Vgs thereshold is 4V, im thinking if you can get it up another volt i think your heating problem will go away.
(With a 30 amp capability, at 0.075 ohms, heating should be minimal.)

haxor5354

Mon Jul 18 2011, 12:51AM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

opps just noticed i was using the 260N instead of the 250N, they pretty much the same but the 260N can handle bit more current
http://www.datasheetcatalog.org/datasheet/irf/irfp260n.pdf

Patrick

Mon Jul 18 2011, 01:05AM

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

Patrick wrote ...

Let me look at the datasheet....

Ok looks like the Vgs thereshold is 4V, im thinking if you can get it up another volt i think your heating problem will go away.
(With a 30 amp capability, at 0.075 ohms, heating should be minimal.)

Im thinking you need to increase your gate drive voltage.

haxor5354

Mon Jul 18 2011, 01:23AM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

how? use a 2n3904 to amplify it?

Patrick

Mon Jul 18 2011, 01:27AM

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

Im thinking a 1.5 volt battery in series with the gate and 555? (just temporarily) Or have I been drinking to much?

I dont think increasing the input voltage will work, nor a 2n3904, since the 555 is swinging the full input voltage anyway, so a 2n3904 wont help. if the batteryb idea is to crazy or doesnt work then i think youll need a gate trawnasformer.

if you were going tobe a GDT, it could be a 1:1, since your no longerr ground referenced, but source referenced, at 1:1 youd get 12V in from the 555 and 12Vout to the G-S, i think.

do you have a simulator, like MultiSIM 10?

haxor5354

Mon Jul 18 2011, 01:51AM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

Patrick wrote ...

Im thinking a 1.5 volt battery in series with the gate and 555? (just temporarily) Or have I been drinking to much?

best idea ever!!!
I think the 555 can handle 15v for VCC, that should do the trick.

Patrick

Mon Jul 18 2011, 01:55AM

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

I dont know if its the best idea ever... i think that honor goes to capatilism.

try it let me know, scope it too so we know for sure whether it works or not.

haxor5354

Mon Jul 18 2011, 02:41AM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

ok i got 2 AAA in-line with pin 3 to gate, positive connects pin 3 of 555 and positive connects gate. i though it would be the other way around, but my scope tells me if I do that I subtract 3v from the gate!

20110717222704392

here, I clearly have 6 volts between gate and source and it still heats up like mad!

could the "quick and dirty" buck converter spitting voltage back to the mosfet? because last time when I was driving a FBT with a 555 oscillator, with an IRFP250N as the switch, it heated up like crazy. back EMF flowing back into the mosfet heating it up?

Patrick

Mon Jul 18 2011, 02:49AM

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

haxor5354 wrote ...

ok i got 2 AAA in-line with pin 3 to gate, positive connects pin 3 of 555 and positive connects gate. i though it would be the other way around, but my scope tells me if I do that I subtract 3v from the gate!

so it worked! that was a totally crazy idea i just spit out!

haxor5354 wrote ...

could the "quick and dirty" buck converter spitting voltage back to the mosfet? because last time when I was driving a FBT with a 555 oscillator, with an IRFP250N as the switch, it heated up like crazy. back EMF flowing back into the mosfet heating it up?

ok, maybe, do you have any TVS'es, MOVS or snubbers?
well, the equivelent of V x I = Watts of heat, so maybe you should scope the curretn and Voltage in real time and see what product is heat? [ Current through, and voltage across your transistor--multiplied = watts of heat. ]

EDIT: Does the irfp260 have an internal diode? YES ! it does... keep that diode form working! I think that internal protection diode is whats heating up!!! lets see... what can we do about that?

Do you have a full schematic i can see?

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