Re: Paralleling FET's

• To: tesla-at-pupman-dot-com
• Subject: Re: Paralleling FET's
• From: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
• Date: Mon, 18 Oct 1999 04:20:43 -0600
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Hi Nick, Byran, all,

Comments interspersed.

> Original Poster: NickandSim-at-aol-dot-com
>
> Hi Bryan,
>logic level FETs are so closely matched that as long as they're
> all the same type to start off with you should be able to just wire
>them in parallel.  In a 2kW audio amp I have seen this was
>exactly how it was done.

Yes, but you shouldnīt forget that these are specially "matched sets".
In other words, the manufacturer (of the amplifier) sorts the
transistors into such sets that eq-Rīs arenīt necessary. This is the
only way to do it, otherwise youīll get all sorts of junk (=distortion)
in the audio output (which is one of the reasons why el cheapo stereos
sound so crappy).

> I can see where you are coming from but you have to remember that FETs
>have a negative temperature co-efficient.  The hotter they get the
>higher the on resistance.

This is true, but the problem is the *instant* turn on current. Letīs
say you have to switch 100A using an array of five 20A FETīs. If
one of these FETs turns on early, it will have to take the full 100A
load alone. The NTC effect wonīt help here, because the FET is
destroyed more or less instantly due to the peak current and not
due to run-away thermal effects. This is esp. important when
dealing with FETs in high power pulsed circuits.

Coiler greets from Germany,
Reinhard

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