John Larkin said:
message
On Fri, 29 Dec 2006 01:40:10 -0500, "Paul E. Schoen"
This circuit might work well with logic level power MOSFETs, something
like
STP20NF06, which is only $0.60, compared to about $1.30 for 2N3055, if
you
can use the TO-220 package instead of TO-3. MOSFETs work very well for
current sharing, as their resistance increases by a factor of about 2
from
0 to 100C.
Actually, no. RdsON isn't relevant here, because they're not
saturated, and if they're on the same heatsink most local
thermoregulation would cancel out anyhow. Even "identical" fets have
to be hand-matched (and expect to throw some away) to get safe levels
of linear-mode sharing. Huge source resistors help the sharing at high
currents but it's hard to find a value that works over a decent load
range.
But it's easy to use paralleled mosfets: just use an opamp per fet as
closed-loop gate drivers, with feedback from small source resistors.
That will linearize and equalize them to microvolts precision.
Switching (saturated) mosfets can share a load reasonably well. [1]
John
[1] I use "saturated" in the bipolar sense, lots of gate drive and low
drain voltage, the ohmic region. Some people consider fet "saturation"
to be the opposite region, the high-voltage constant-current place.
I tried a simulation (LTSpice) using three very different MOSFETs, and
current sharing at high levels was reasonably good. At low levels, one or
more were essentially turned off, but that's not a problem. I have no way
to simulate device variations or temperature effects, however. I used a
20
volt raw source and 15 volts on the gates. The MOSFETs are IRF7811,
IRF7468, and IRF9410. Here's the results:
Rload Vload I(R1) I(R2) I(R3)
10k 13.19 51pA 1.3mA 49pA
1k 13.17 60pA 13.2mA 58pA
100 13.11 -360pA 131mA -346pA
10 12.90 253mA 870mA 167mA
1 12.25 4.31A 4.50A 3.44A
0.5 11.71 8.38A 8.10A 6.95A
0.1 8.91 31.8A 29.4A 27.9A
Paul
I assume you hooked them hard in parallel, with no source resistors.
My experience in building NMR power amps is that it's ugly to parallel
"identical" power fets even with the largest feasible source
resistors. In my situation, a class AB push-pull amp, quiescent
current ran about 10% of peak output current, and with a little bad
luck, one of the fets (out of 4 on each side) would wind up furnishing
most of Iq and getting a lot hotter than the others. We wound up
matching parts, a real nuisance. After that experience on our first
amp, we went to closed-loop control of each fet, which has a number of
side benefits.
If you don't care which fet does most of the work at low loads, and if
you can afford to drop a volt (or preferebly 2) in each source
resistor at full load, it can be made to work. But at 1 volt drop,
there could well be some intermediate load that's embarassing.
I just wouldn't trust those Spice numbers. The last couple of lines
have three different parts whose transfer curves spread less than 20%.
I don't think that's realistic.
John
I used the same 0.1 ohm source resistors as in the OP's schematic. The last
two lines are at and considerably above the rating of the supply. At 8 amps
each, the source resistors drop 0.8 volts, and there is a large difference
in transconductance over that range. At 90 amps out, the source resistors
are doing most of the work, dropping 3 volts each. Some intermediate
values:
1.5 12.46 2.85A 3.23A 2.22A
2 12.57 2.10A 2.57A 1.61A
3 12.69 1.34A 1.90A 0.99A
4 12.76 0.95A 1.55A 0.69A
6 12.83 0.56A 1.18A 0.39A
The current sharing gets crappy at low currents, but at that point it
doesn't matter very much. What I don't know is how the transconductance
varies with temperature. I don't do much with linear power anymore, now
that PWM is so easy to implement, but linear circuits are still useful. Any
real data, experimental or theoretical, that supports or refutes these
results, would be appreciated.
Just for fun I made a similar circuit with bipolar transistors: ZTX1048A,
2N3055, and FZT849. Results:
Rload Vload I(R1) I(R2) I(R3)
10 13.33 51mA 1.25A 31mA
6 13.23 289mA 1.69A 222mA
4 13.14 647mA 2.08A 560mA
3 13.05 1.02A 2.41A 925mA
2 12.89 1.77A 3.01A 1.67A
1.5 12.74 2.51A 3.55A 2.42A
1 12.42 3.98A 4.54A 3.90A
0.5 11.52 8.07A 6.95A 8.02A
That's pretty good current sharing for such a wide range of transistor
types. I would expect no more than 1 ampere difference between any two
devices at the nominal 4 amperes each for five, with 0.1 ohm resistors.
It is quite possible that an amplifier running at audio frequency may
require much better device matching. There are probably considerable
variations in gain for transistors used for that purpose.
Paul
