Your mosfet has an internal schottky diode from source to drain, which, in
your picture, will conduct when the generator is on, regardless of whether
you have gate drive. It also enables you to get rid of the external diode.
However, the reason the mosfet is able to deal with 10A is that it has a
very low Rds(on). A diode won't have that, and so 10A will cause it to
dissipate more heat than you might expect.
Also, I'm not sure what the rest of the circuit does, so I'm not sure what
the mosfet drive is going to do. Are you going to charge the battery with
it? If so, you probably want the Source to be next to the battery. However,
that makes the substrate diode conduct when the generator is off, which you
say you don't want.
I wonder if you can go with a simpler circuit? Here is one, assuming I
understand your requirements:
generator--->|----(battery +)---->|---load
When the generator is on, it'll charge the battery and power the load. When
the generator is off, the battery will drain through the load. You may want
to control the charging current somehow, since dumping a huge current into a
lead acid battery is a mistake (it'll overheat and vent). For the cost of
another a power resistor, you can do that easily:
Gen o------>|----o----------------------------------.
| |
| |
| ___ |
'---|___|----o-------->|-----------o
1 Ohm 5W | |
--- .-.
- Battery | |
--- | | LOAD
- '-'
| |
GND o--------------------------o---------------------'
(created by AACircuit v1.28.6 beta 04/19/05www.tech-chat.de)
Here is a schottky diode that is about a buck in single quantity from
digikey:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=497-...
Here is a suitable resistor:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=1.0W...
It is only 5W, but you'll need to evaludate that, and may need a beefier
one.
Total circuit cost is about $2.50.
Regards,
Bob Monsen
PS: Vista mail doesn't like this message, so it wouldn't put the appropriate
'> ' in front of the replied text. Some formatting thing is screwing up its
parser. Please forgive the top posting.
---
Since there's no DC path through the gate, the only limit to the
value of the pullup will be how long it takes the gate to charge up
through it (and, therefore how much power the MOSFET will be
dissipating) until the MOSFET turns on hard.
---
---
They should all turn on unless they have ridiculously high gate
leakage currents, It's just a question of how long it takes for them
to turn on.
Since you're doing high-side driving, why can't you use a P channel
MOSFET, like this: (View in Courier)
.+15V>---+-----+----+----+
. | | | |
. [R1] | [3K] |
. | | | |
. +----|+\ | S
. | | >--+--G IRF4905
.IN>-----|----|-/ D
. | | |
. [R2] | [LOAD]
. | | |
.GND>----+-----+---------+
where, assuming IN is positive true, R1 and R2 are selected to
cause the voltage at the + input of the comparator to be about
halfway between the swing of IN?
Hey John, I'll explain what I'm doing here and why I think I need a
high-side drive.
In the interest of economy the sketch below leaves out most of the
circuit I'm building, but it shows the part involving `the mosfet I
asked about in the original post. It's a kind of
synchronous recitifier that turns on and off at relatively lengthy
intervals when the generator revs up or turns off. With the generator
turned off or idled, the mosfet acts as a blocking
rectifier to keep the battery from discharging into the generator
windings.
Vboost
|
,-----------------, ,-+-,
| | | |
| | Rp |
| ,--+-----|---|---,
| | | | |/| |
) V S|_ | /+|-'
) - _||--+--< |
) | D| \-|-,
) | | |\| |
) '--+---------|---'
) generator | |
) windings | |
) ___+ |
) _ |
) ___ |
| _ |
| ___ |
| _ - |
| | |
| | |
'-----------------+---------'
|
-----
---
-
The rationale: using a mosfet avoids expensive, bulky heatsinking a
conventional rectifier would require. The mosfet is also cheaper than
a rectifier for the same current. Less than $2 in small quantities
for the NTP90N02 from Digikey. It needs a charge pump for the drive,
but having the charge pump there also allows me to use a cheap
jellybean comparator on the high side.
Without the charge pump, to run 10 amps wihout a heatsink, using a p-
channel like an STP80PF55 would require two in parallel at $2.50 each,
and a more expensive rail-to-rail comparator like the LT1716, which
only has one comparator in it and costs $2.50 (in small quantities).
But I need four comparators altogether in this circuit, three of them
on the high side, and with an LM339 I get it all for about 50 cents.
So I come out ahead even though I have to spend an extra buck or two
on a charge pump. Two STP80PF55's and three LT1716's comes to more
than ten bucks.
