Re: Paralleling FETS


"Hammy" <spamme@xxxxxxxxxxx> wrote in message
news:3vmbf4hj9sqv49u0eop8uhf1ma9c9mi21r@xxxxxxxxxx
On Wed, 15 Oct 2008 06:13:57 -0500, "Jon Slaughter"
<Jon_Slaughter@xxxxxxxxxxx> wrote:


Google is very easy to use:


http://www.sampson-jeff.com/tcrobowar/motor1/para.htm

http://www.irf.com/technical-info/appnotes/an-941.pdf

http://www.microsemi.com/micnotes/APT0402.pdf

http://www.powerdesigners.com/InfoWeb/design_center/articles/MOSFETs/mosfets.shtm

http://www.fairchildsemi.com/an/AN/AN-558.pdf



Yes I know I've already read all that and similar stuff in textbooks.

If you want to get anal about it every question posed here as well as
SED could be answered by reading a book are googleing.

I post here to ask for practical advice i.e someone who actually has
experience doing circuit design and could maybe share insight. A
category which excludes you.

There was a lengthy thread about parallel MOSFETs last December. I did an
LTspice simulation, which showed good current sharing among several
devices, and when Phil freaked out about it, I did an actual breadboard
experiment that proved it works for real devices:

http://groups.google.com/group/sci.electronics.design/msg/c64478c824397da8

Essentially, the results were as follows:

==========================================================================

So I did an experiment. I connected four MOSFETs, HUF75645P, 100V, 75A,
310W, 0.014 ohm. Sources to GND. Gates tied together, through 100 ohms to a
pot across 10 VDC supply. Each drain to a 100 ohm resistor and a red LED to
+10 VDC.

Vgs: 2.50 2.60 2.73 2.86 3.00 3.20 3.50


Vd(1) 8.80 8.75 8.60 8.48 7.95 3.43 0.016
Vd(2) 8.80 8.76 8.62 8.51 8.15 4.56 0.018
Vd(3) 8.76 8.72 8.58 8.45 7.93 2.95 0.015
Vd(4) 8.79 8.76 8.63 8.53 8.21 5.40 0.020


LEDs were all about equal brightness and were barely lit at 2.73V and
fairly bright at 3.00V to 3.20V.


============================================================================

When called to task about the low current used for this test, I redid the
experiment:

http://groups.google.com/group/sci.electronics.design/msg/da059ef9c84d05fa?dmode=source

============================================================================

OK. I changed the circuit to source follower. Devices 1, 2, and 3 are
connected through 12 ohm 10 watt resistors to GND, device 4 has a 100 ohm
to GND. All drains connected to a 10 VDC supply. Gates in parallel through
100 ohms to a pot across the supply. Results:


Vg: 5.00 6.00 7.00 7.50 8.00 9.00 10.0 7.50(cold)


Vs(1) 2.25 3.13 4.11 4.65 5.13 6.12 7.11 4.53
Vs(2) 2.31 3.23 4.09 4.73 5.20 6.16 7.18 4.60
Vs(3) 2.18 3.12 4.09 4.60 5.09 6.07 7.06 4.27
Vs(4) 2.26 3.20 4.17 4.67 5.03 6.16 7.16 4.66


Paul


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