RE: MOSFET arrays

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

Original poster: "Mccauley, Daniel H by way of Terry Fritz <teslalist-at-qwest-dot-net>" <daniel.h.mccauley-at-lmco-dot-com>



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.

 >>>>Parallel operation is okay if you need lots of current.


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?

 >>>>>>>>Multiple (series) FETs in bridge configurations are very difficult
and not recommended.  Its better to use
a large cascade type switching topology or similar.  For large FET arrays in
high voltage switches I design, I typically
use a gate drive system which is based on the previous FET turning on.  You
basically control only the first FET and as this
FET turns on, it turns on the next FET, and so forth.  This method works
extremely well, although it does require a lot of resistors and capacitors
and diodes for each gate for proper tuning etc...  But I've design typical
switches up to 10,000V with rise / fall times of 50nS ! ! !


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.

 >>>>>For series arrays, get rid of the gate transformers.  You need to go to
a different gate topology.  (See above)

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.)

 >>>>>>>You really want to limit the number of FETs due to obvious reasons.
If you are doing that, then ditch the 200V FETs
and get some 1000V FETs.  I'm using (4) IXFN24N100 FETs for use in my MOT
switch i'm currently building.



All comments appreciated.

Cheers

M

-- 
Matthew Smith
IT Consultant - KBC, South Australia
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