Winfield Hill said:
Sadly, your experiment was nearly meaningless, because the power
dissipation was so low it didn't increase the junction temperature
sufficiently to get interesting. There's a smidgin of interesting
data when you compare the next-to-last two columns. The
Vgs = 3.00V column at about (10-1.2)/100 = 8mA shows
modestly-matched currents, but the next column with higher 34
to 58mA currents shows a 70% mismatch. However, these tiny
currents are silly: these are 75A 310W mosfets, for Pete's sake!
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
The threshold voltage is usually specified at very low current, such as 250
uA. I expected the greatest mismatch to occur here, at what I would call
the "knee" of the curve. The point is that a few tenths of a volt Vgs take
the device from just barely conducting to a full ON condition. Certainly I
would not recommend running in parallel without adequate source resistors,
which would equalize the currents due to negative feedback.
Device #3 drawing 58mA had only 0.17W of power dissipation,
and Intersil's datasheet tells us this will raise the FET's junction
temperature only 0.17*62 = 10C, which is not enough to learn
anything. I suggest you replace the LED+100 with 0.1-ohm
3-W current-sensing resistors, etc., to maintain the MOSFETs
at the same Vds. You can run them at currents of say 0.2 to
2A each, for 2 to 20W dissipation, without heatsinks, natch,
and measure the relative voltage drops across the 0.1 sense
resistors. Then perhaps you'll learn something interesting.
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
This seems to be a reasonable test within the lower normal limits of the
power supply being discussed. Current in each device varies from 0.18 A to
0.60 A. As a reference, device 4 is running at about 1/8 the current of the
others. It does have some thermal connection to the others, as I have them
mounted in free air next to each other (and probably touching). I started
measuring at 7.50 (labeled cold), and those values were drifting as the
devices heated up, so they are not very accurate, but do show the increased
current as temperature rises. Also, device 2 is probably the hottest since
it is sandwiched between 1 and 3, so with good heat sinking the current
sharing may be even better.
An exhaustive test would involve hours of work taking measurements or
setting up a data acquisition system to plot curves at many points. Also
more devices would be needed to achieve true statistical confidence. But I
would be comfortable using these devices for the OP's application, and I'd
expect current sharing to be within 20% or better. With the 0.1 ohm
resistors, it would be easy to try.
Right now there is a tube of 50 pieces on eBay item 170062103958, for $0.01
plus $8.00 shipping from Sweden, if you'd like to give it a Schottky.
Paul
