MOSFET arrays

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Matthew Smith by way of Terry Fritz <teslalist-at-qwest-dot-net>" <matt-at-kbc-dot-net.au>

Hi All

I'm just looking for a little general guidance on the use of multiple 
MOSFETs in SSTCs, OLTCs, flyback drivers, etc.

My main reason for asking is that I have just acquired a couple of hundred 
(four rails) IRF620's at a *very* good price.  Since multiple capacitor 
arrays and multiple diode strings seem to be standard practice in coiling, 
why not multiple MOSFETs?  It's more effort to build something with many 
elements, but when component cost is an issue rather than construction 
time/complexity - well, I don't even think that I need make this argument 
in the face of MMC's ;-)

1) Parallel operation.  I believe that the gate capacitance of a MOSFET 
device can cause slow "off" switching unless the current can be sunk nice 
and quickly, hence the use of a totem-pole driving stage.  I see these in 
use both as discrete components and also in the output stages of various 
PWM chips.

Given sufficient current sourcing/sinking of (probably bipolar) driver 
devices, what are the practical limits to the number of MOSFET devices that 
they can drive in parallel?  With our potentially high ambient 
temperatures, I'm working on a current handling capacity of only 3.5A per 
device with the IRF620; would eight in parallel to switch 28A be practical?

2) Gate drive transformers.  I can see that these would be required when:
	a) outputs are required out-of-phase (as in a bridge
	configuration).
	b) devices are employed in series for higher voltages
Are there other circumstances where these should be employed?

3) Series operation.  Since we only need a few (relative) volts on the 
gate, I guess that this is a job for a transformer (see above); since the 
secondaries will float, I would see the issue here of being one of 
winding/winding and winding/core insulation, especially when switching 
larger voltages.

I seem to remember Terry looking at using optical drive on a large IGBT 
array, which would give an alternative (with better isolation) to the gate 
drive transformer.

Again, what are the real practical limitations to the number of series'd 
elements that we can use?  Could I switch, for instance, the rectified 
output of an MOT, using a series array of, say, twenty IRF620's?  (200V per 
device max.)

All comments appreciated.

Cheers

M

-- 
Matthew Smith
IT Consultant - KBC, South Australia
KBC Web Site    http://www.kbc-dot-net.au
PGP Public Key  http://gpg.mss.cx