Lm3886 + irf540/irf940 (mosfet) ?

JediRemi · 4th September 2010, 05:40 PM

Hello all,
I have a few LM3886 and a very strong power supply.

I would like a few more Amp to be able to flow trough the output...
So I thought "what if I add some mosfet there?"

would that scheme work ?

Would another (similar) design work ?
what if I use a bipolar based design instead of mosfet ?

and would this thing would be able to deliver the 15A my regulator is able to supply ?

Sorry if those are very stupid questions...

gootee · 4th September 2010, 10:08 PM

Something similar should work. I didn't look at your circuit in detail but there are many similar "current booster" ideas that have been used successfully, especially with opamps in place of the chipamp.

There are some application notes with similar ideas and topologies, such as AN-272 at national.com and AN18 at linear.com. But none of the ones I saw used MOSFETS.

Maybe someone could try simulating something very similar in LTspice.

star882 · 4th September 2010, 10:46 PM

That's a very poor way to drive MOSFETs. Use a proper gate drive circuit. And PMOS generally doesn't perform as well as NMOS, so most modern designs use all NMOS output stages.

hitsware · 4th September 2010, 10:59 PM

I've used this approach and it works well :

abraxalito · 5th September 2010, 12:09 AM

Using a single forward biassed silicon diode in the drive circuit is an approach copied from a bipolar output circuit I'm guessing. These MOSFETs need around 4V gate bias, quite a bit more than the 0.7V that suits bipolars.

gootee · 5th September 2010, 12:15 AM

I would think, too, that you'd want to find some way to also use the output current from the LM3886. Otherwise, you might as well just have a little opamp, there.

theAnonymous1 · 5th September 2010, 12:28 AM

Or just use one of the LME498xx IC's from national designed to drive FETs and BJTs.

Sebastiaan · 5th September 2010, 12:30 AM

Quote:

Originally Posted by hitsware

I've used this approach and it works well :

But this one will be totally in class B. Which would give some crossover distortion.

See here some good bias circuit/alternative for the same concept.

http://electronicdesign.com/article/...class-ab-.aspx

With kind regards,
Bas

jackinnj · 5th September 2010, 12:54 AM

Quote:

Originally Posted by theanonymous1

or just use one of the lme498xx ic's from national designed to drive fets and bjts.

lme49830

JediRemi · 5th September 2010, 01:05 AM

Thanks a lot for all your advices,

you are very right that an opamp would work as well.

I tried to modify the design according to what you said and I have no clue if it would work or not.

I have also no clue about the right values for the resistors R6 to R8, nor how to calculate it.

4th September 2010, 10:46 PM	#3
star882 diyAudio Member Join Date: Mar 2007	That's a very poor way to drive MOSFETs. Use a proper gate drive circuit. And PMOS generally doesn't perform as well as NMOS, so most modern designs use all NMOS output stages. __________________ "Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!

5th September 2010, 12:09 AM	#5
abraxalito diyAudio Member Join Date: Sep 2007 Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though... Blog Entries: 46	Using a single forward biassed silicon diode in the drive circuit is an approach copied from a bipolar output circuit I'm guessing. These MOSFETs need around 4V gate bias, quite a bit more than the 0.7V that suits bipolars. __________________ I think ideas are what you want to get rid of. I don't really like songs with ideas. - Leonard Cohen

5th September 2010, 12:28 AM	#7
theAnonymous1 diyAudio Member Join Date: Feb 2004 Location: Anonymityville	Or just use one of the LME498xx IC's from national designed to drive FETs and BJTs. __________________ "If you don't like funerals don't kick sand in Ninja's face." - Ninja

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4th September 2010, 10:08 PM	#2
gootee diyAudio Member Join Date: Nov 2006 Location: Indiana Blog Entries: 1	Something similar should work. I didn't look at your circuit in detail but there are many similar "current booster" ideas that have been used successfully, especially with opamps in place of the chipamp. There are some application notes with similar ideas and topologies, such as AN-272 at national.com and AN18 at linear.com. But none of the ones I saw used MOSFETS. Maybe someone could try simulating something very similar in LTspice.

5th September 2010, 12:15 AM	#6
gootee diyAudio Member Join Date: Nov 2006 Location: Indiana Blog Entries: 1	I would think, too, that you'd want to find some way to also use the output current from the LM3886. Otherwise, you might as well just have a little opamp, there.

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