I didn't answer this, sorry. You can estimate relative
MOSFET dies sizes by comparing power dissipation ratings
and gate capacitance Ciss.
Wouldn't power dissipation ratings also be highly dependent on the
package that the FET was in?
The switching time of a cmos gate, typically under 10ns,
is basically zero compared to the several 3 to 5us delay
in the optoisolators, and the 10 to 15us time to charge
the MOSFET gates. So one before the other has no impact.
Generally one likes to draw a circuit including necessary
driving pieces, etc., so as not to make excessive demands
on the signa; source, etc. That's why an inverter would
be customarily shown at the input to start things off.
Ah - that makes perfect sense.
Well, even so, are you thinking well about the bigger
picture? I mean, is this just a bunch of batteries to
gang up and help run a motor? What's wrong with just
using a diode-OR on each battery? Is it necessary to
isolate the OFF batteries? If so, then you'll need two
MOSFETs back-back (source-to-source) to insure the ON
battery doesn't work to charge the discharged ones.
What about fuses, current-limiting, status indicators,
etc. Does the motor have a PWM modulator or something,
you don't just always give it the full juice, do you?
What about current monitoring, etc.?
So this board is a power distribution board. It carries power to a
number of other boards that run various motors through all sorts of
current loops. It also powers some digital systems that take in the
high voltage and step it down to something more usable.
The reason I want be able to switch on and off the power sources is
that I want to be able to protect them from undervoltage and
overcurrent situations. They are li-polys, so if I drain them too much
or if I pull too much current they die. If I just used diodes I would
lose that protection. I also want it high side switched as we have had
problems due to low side switching - with fun ground loop problems
like through external VGA cables and stuff like that. Also, the hope
was that using FETs would give me a bit more efficiency, though I
suppose that is not a huge gain. Note that there will be 3 power
sources - two batteries and wall power. The wall power input does not
need undervoltage protection, obviously (except for it being too low
to power anything - at about 9VDC).
I don't understand why I would need two mosfets, or even how that
would work... I mean once the FET is off (VGS < VT)current can't flow
through it, or at least that was my understanding. Errr wait -
intrnisic body diode... Argh. It has a 1.3V forward voltage according
to the IRF1407PbF datasheet. I see your point now. What happens to a
FET when it is reverse biased? It looks like the body diode turns on
at about -1.3V and then can handle a ton of current, but what about
between VDS = -1.3V - 0? I'm assuming it's just off. That's not really
relevant, but now I'm really curious as I've never really heard the
properties of N-FETs discussed with regards to VDS < 0. But why put
two FETs source to source? To me it'd make more sense to put a
schottky diode in series with the FET. I'd probably put it on the
drain side of the FET just so as to increase VGS a little. I mean - if
you put two FETs source to source wouldn't you see a 1.3V drop across
one of them?
As for current limiting and indicators and whatnot - I am planning on
putting a 5V buck converter that powers all the logic switching these
FETs, as well as a high side shunt resistor with a high side current
sense amplifier. A comparator will watch the output of the high side
current sense amplifier and drive an OR gate that is driving the FET
driver. I haven't figured out what all indicators are needed - but
that is planned. Probably one to show if a battery has been turned off
for undervoltage, and another to indicate which power source is being
used. Maybe a couple latched LEDs to show if there were any transient
problems (ie a surge in current).
All that said, if your batteries are all tied together
and share the ground system, there's a MUCH simpler
approach that can be used. We can dispense with all
the optically-isolated stuff. Like I said, generally
it's more fun on usenet s.e.d. to talk about whatever
interests us most at the time, as conversation goes,
rather than what may actually be best for a project.
The batteries all share a common ground. What simpler system were you
thinking of? I would really like to keep it high side switched, if
that was your idea.
Took me 2.5 hours to get home! But my Prius was an
ideal car for the task - it's great at stop and go
traffic - the gas engine shuts down so you can inch
forward on batteries. I can get up to 35mph before
the gas engine kicks in, if I'm accelerating slowly.
(And if the battery is well charged, and if the heater
is off, and if it's not so cold the software insists
on protecting the battery. That's a lot of ifs.)
Ah - I did better than you! Took me a mere 1:45 to get home
(typically ~25 minutes). I heard that the highways were completely
backed up so I went back roads the entire way, and it worked out
fairly well. It was clear sailing from Watertown to Cambridge - almost
no cars on the road whatsoever.
