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 Post subject: MOSFET questions
PostPosted: 31 Dec 2008, 13:52 
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Joined: 26 Aug 2008, 20:57
Posts: 415
Location: Atlanta
Hey guys, how's it going?

I have a subwoofer amp that uses 4 IRFB31N20D in it. International Rectifier has an upgraded and improved MOSFET, the IRFB4227PBF which they say is a direct replacement.

They state that the new one will have better current handling capability.

Now, I am completely ignorant on MOSFET design and implementation so...

What will this do for me? Will I actually see an increase in output or will I need to modify the amplifier itself to reach this goal?

The plan is as follows:

Upgrade to the new MOSFETs.
Increase PSU Capacitance (currently 4 1500uf) on board.
Add heatsinks on MOSFETs and transformer
Here is a pic of the amp board, you can see the MOSFETS at the bottom, they are TO-220 jobbers and look just like a regulator.


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 Post subject: Re: MOSFET questions
PostPosted: 31 Dec 2008, 18:58 
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Joined: 28 May 2008, 21:53
Posts: 4580
Location: Winnipeg, CANADA
Hi Bill,
Really you would need to increase current through the mosfet for more power output.
Cheers

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 Post subject: Re: MOSFET questions
PostPosted: 31 Dec 2008, 20:47 
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Location: Atlanta
So how can I go about that?

:mrgreen:


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 Post subject: Re: MOSFET questions
PostPosted: 01 Jan 2009, 01:29 
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Joined: 03 Aug 2008, 20:22
Posts: 386
Location: Denver, Colorado
You may notice a very small increase in volume as the transconductance of the new Mosfet is 49 vs. 17 of the older one. That is the relationship of Drain current with relation to gate bias. But to double the volume you would have to multiply power 10 times, and as Gio stated you have to increase the current through the mosfet and that is a lot of power and heat. Is this a home theater amp or car audio?


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 Post subject: Re: MOSFET questions
PostPosted: 01 Jan 2009, 21:57 
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billnchristy wrote:
So how can I go about that?

You would have to set the bias. The problem with this will likely be that the power supply won't be able to supply much more current unless it was over designed.

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 Post subject: Re: MOSFET questions
PostPosted: 01 Jan 2009, 22:50 
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Joined: 26 Aug 2008, 20:57
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Location: Atlanta
It is a home theater sub.

If I can disassemble and provide a basic schematic do you think we can figure it out?

Also, would upping voltage a tad do anything?


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 Post subject: Re: MOSFET questions
PostPosted: 02 Jan 2009, 00:23 
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Location: Denver, Colorado
No,
There is only so much you can do with the design you have. The transformer is wound to provide only so much current. How about adding another sub on the other side of the room?


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 Post subject: Re: MOSFET questions
PostPosted: 02 Jan 2009, 11:11 
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It is technically feasible, but will almost certainly require a larger transformer and more capacitance which I suspect will cause space problems. Also, as mentioned the design may not be accommodating to changes.

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