Re: Paralleling FET's

Hi Jim, all,
                  FWIW, Motorola application notes on paralleling 
MOSFETs have a couple of key things to say about load sharing:

Linear/analogue:  source resistors are a must.  I know from 
experience that a single device can hog current in linear 

Switching: source resistors not necessary (and in fact might well be 
a nuisance if they are inductive). Motorola show good sharing on 
oscillograms for FETs which have reasonably well matched 


> Original Poster: "Jim Lux" <jimlux-at-jpl.nasa.gov> 
> Bryan's strategy would be important if you were running your FETs as a
> linear amplifier, as opposed to a switch. In most TC power oscillator
> applications, the FET is a switch, so all you worry about is making
> sure that they turn on and off at mostly the same time, and even this
> is non critical.
> A FET is a voltage controlled current source.  So let's look at what
> happens to a set of paralleled FETs as they turn on.  You put a
> healthy bias on all the gates (more than enough to turn them on, say
> 10V). The gate capacitances start to charge and the FET's start to
> turn on, all at different rates. As each one starts to turn on,
> current flows through it up to the amount of current determined by the
> Rds on characteristics. Each device is essentially independent of its
> siblings. The power dissipation is mostly determined by Rds on and the
> Vds. If one device has a lower Rds, more current will flow, but, you
> don't get the thermal runaway characteristic of bipolar devices, where
> as it gets hotter, the current increases, etc.

<snip of excellent post>