John Larkin said:
[...]
I had a serious rant here a few years ago about the way in which
manufacturers of VMOS devices perpetuated the myth that "MOSFETs
allow easy paralleling".
In fact nothing could be further from the truth.
For the VMOS, at fixed Vgs, dId/dT is positive. This means that if
any one MOSFET in a bank runs slightly warmer it will take more
current, getting hotter, so taking more current. And so on.
Thermal runaway.
Look at Fig.3 of the IRF1405. At Vgs= 4.5V, Id is 4.2A at Tj=25C,
rising to 28A at 175C. That's roughly +0.15A/C. For the IRF1405
dI/dT does not approach zero (and go negative) until Id is about
180A.
Hitachi realised this problem 20-odd years ago and they produced
power MOSFETs where dI/dT went negative at only about 100mA. ISTR
they were called lateral or long channel MOSFETs, an entirely
different process to the VMOS and it's derivatives. The negative
dI/dT did allow easy paralleling and those Hitachi MOSFETs were
widely used in successful high power audio amps.
Good info. Thanks, Tony.
What about the current example using source resistors? These provide
negative feedback, so Vgs is no longer fixed. How do we calculate
the minimum value needed to prevent runaway?
A positive drain-current tc doesn't guarantee runaway. If the "loop
gain" is less than 1, it won't run away. The math depends on the Id/t
slope, heat sinking, mutual heat sinking, stuff like that. Actually,
true thermal runaway in paralleled mosfets is rare, almost impossible.
The same datasheet shows threshold voltage as a function of temperature,
but at a drain current of only 250 uA. I think this is a worst case
scenario. So the change in threshold voltage is 3.3 to 2.7 from 25C to
100C. A 0.1 ohm source resistor will drop 0.4 volts at 4 amps (per the OT
application), so it should ensure reasonable current sharing. Device
temperature difference should not exceed 25C on a good heatsink, so
threshold should only change about 0.2 volts, or 2 amps with the existing
resistors.
Actually a lower current rated MOSFET would probably work better. The
IRF2903Z (75A,30V) shows threshold change from 4.0 to 3.6 volts from 25 to
100C, with drain current of 1 amp, and 3.4 to 2.7 at 1 mA.
The FQB30N06 (30A,60V) shows a current increase from 5A to 9A at fixed gate
voltage of about 4.8V from 25 to 175C, and no change at currents of 25A
with 6.2V Vgs, and negative change above that. Current varies with gate
voltage from 4.5A at 5.0Vgs to 11A at 5.5Vgs.
In that it reduces power supply headroom and efficiency at peak
output, all it can do is make heat and hurt. In a lot of situations,
there is no value of source resistor that gives decent idle-current
sharing but doesn't introduce huge peak-output losses. The old
nonlinear resistor trick (ie, parallel the emitter resistor with a
diode) often won't work for fets.
Given the 22 VDC input voltage (probably about 16 V at full load), and 20
amp 14 volt maximum output, efficiency is not really a factor. The
resistors provide current limiting of about 40 amps if any one of the
devices should short out, if the load is a battery. Also the resistors can
run hotter, and are less expensive than the semiconductor devices. You lose
only about 0.4 V of headroom. And current sharing at idle levels does not
really matter.
Looking at the specs for the bipolar 2N3055, the Vbe changes sharply as a
function of collector current at moderate levels. It is 0.7V at 0.5A, 0.8V
at 2A, 1.0V at 5A, and 1.5V at 10A. This shows that the 0.1 ohm resistors
are more than adequate to ensure safe current sharing at the rated output
of 20A. Much below that it is not so critical. There does appear to be a
need for current limiting or fusing to protect against severe overloads and
short circuits, however.
Another severe limitation with the 2N3055 is the sharp drop of hfe at high
currents. It takes about 1 ampere to drive the output to 10 amps, and 100
mA to drive it to the rated 4 amps per device. Thus you need a solid 2-15
volt 500 mA base drive circuit to get decent output regulation. For the
MOSFETs, you can use a simple voltage divider or pot.
Paul
