Parallelling 2xIRFP240 and 2xIRFP9240

[Barre]

Hi,

Just before I do so I would like to know some opinions about paralelling 2xIRFP240 and 2xIRFP9240.

One pair IRFP240/IRFP9240 already works just fine in my amp, but since I have one pair left which I don't intend to use...
I'd like to parallel these things.
Mainly to see what the effects are.

So, IR gives some tips on this... separate source resistors for each HEXFET and a carefully designed PCB-layout should do it?

Any more hints,tricks,problems?

Thanks,
Bart

[sivan_and]

post the circuit...first

[richie00boy]

Make sure the VAS/driver stage can drive the increased capacitance/charge.

[Barre]

Oké, here it is!

It was the IGBT-amp, but since that amp wasn't stable at all, i started to use IRFP240 and IRFP9240 instead and it is working oké.

The reason for parallelling is that I have one pair of hexfets extra and...maby the amp will have better performance when fully driven.

I know the driverstage has too be able to handle the capacitances of the HEFFET's...but don't know where to start from... We don't see stuff like that at school, and I'd really like to learn.

Oké,now here's the circuit (only the IGBT's are replaced by HEXFET's):
it's on this site (picture was too big to post)
http://www.arky.ru/audio/shem/igbt/igbt.htm

[Workhorse]

Barre,

I have seen your circuit , there are little adjustments that are to be implemented for paralleling mosfets...
Gate resistors value should be raised to 39Ohms and source resistors should be around 0.25ohms 5W value.. except this everything seems perfectly okay....

cheers
Kanwar

[Barre]

Oké, thank you. Very kind of you.
...but..any chanse you could tel my how you get those numbers?
I like to learn you know.
But still, thanks :-)

Bart

Edit:

Oké, I found it here ;-)

<<<Here is the formula for the adequate driver bleeder resistor choosing.

"RDbleeder<=2*[Rgate/N]"

where n is total number of mosfets in parallel.>>>

[uli]

Originally posted by Barre
[B]
I know the driverstage has too be able to handle the capacitances of the HEFFET's...but don't know where to start from... We don't see stuff like that at school, and I'd really like to learn.

Use about 200R as gatestoppers and about 0R33 as source resistors.
With gatestoppers too high you form kind of lo pass filter.
Look at the passlabs section as those fets are used by many
pass circuits.

Uli

[Workhorse]

Hi Barre,

If you use too high values of Gate resistors than there may arise a possiblity of cross-conduction at high frequencies....

[Barre]

Hm...

Oké, i will do some breadboard- testing this summer. See what comer out of that.

Right now I have exams, and even though this interests me very much... I guess it's not the best time to experiment with that now

Thx again and!

Btw, are the IR- AN notes any good?

Bart