I have a situation where I need to use a pretty stiff pull-up resistor
on a mosfet gate because I'm going to use a comparator
to turn it off, and I plan to limit the current the comparator sinks
to about 5 mA.
The N-mosfet source will be sitting at 15 volts and the gate drive
comes from a voltage doubler.
That's a bit less than 30 volts so the pull-up resistor needs to be in
the 6k ballpark.
I'm going to use the NTP90N02:
http://www.onsemi.com/pub/Collateral/NTB90N02-D.PDF
Speed doesn't matter -- this is basically an on-off switch, so I have
only to consider the dc behavior of the mosfet.
The mosfet has to conduct ten amps.
My question is, how much resistance can you put in front of the gate
before it gets too weak to turn the mosfet on hard?
Mosfet models don't address this, so far as I can tell. I haven't
ordered the NTP90N02 yet so I can't simply experiment
on it, but I remember other mosfets having problems turning on when
the gate resistance gets into the tens of kilohms.

gearhead wrote:
> I have a situation where I need to use a pretty stiff pull-up resistor
> on a mosfet gate because I'm going to use a comparator
> to turn it off, and I plan to limit the current the comparator sinks
> to about 5 mA.
> The N-mosfet source will be sitting at 15 volts and the gate drive
> comes from a voltage doubler.
> That's a bit less than 30 volts so the pull-up resistor needs to be in
> the 6k ballpark.


Attention: Keep in mind that the abs max for Vgs is usually no more than
+/-20V, including the FET you have selected.


> I'm going to use the NTP90N02:
> http://www.onsemi.com/pub/Collateral/NTB90N02-D.PDF
> Speed doesn't matter -- this is basically an on-off switch, so I have
> only to consider the dc behavior of the mosfet.
> The mosfet has to conduct ten amps.
> My question is, how much resistance can you put in front of the gate
> before it gets too weak to turn the mosfet on hard?
> Mosfet models don't address this, so far as I can tell. I haven't
> ordered the NTP90N02 yet so I can't simply experiment
> on it, but I remember other mosfets having problems turning on when
> the gate resistance gets into the tens of kilohms.


It can be very high but two things can cause some grief:

a. Slow turn-on, on account of a high Cgs and Cdg. In your case Cgs is
over 2000pF so a 6K would slow that down to 10usec or more. During this
time the FET transitions through its linear range, briefly dissipating
lots of power and possibly going kablouie. Then there is Cgd which works
"against" turning it on, further slowing it down.

b. Depending on what is connected Cgd might cause a "dent" in the
turn-on phase and possibly oscillation around that point. With a stiff
load the latter is usually accompanied by a bang, molten solder
splattering about and so on.

--
Regards, Joerg

http://www.analogconsultants.com/

I design my own MOSFET amplifiers, up to 600 watts RMS.
I usually use a 330R in series with the gate.
The drive to the 330R is very low impedance.

On Mon, 14 Jan 2008 12:15:57 -0800 (PST), gearhead
<nospam@billburg.com> wrote:

>I have a situation where I need to use a pretty stiff pull-up resistor
>on a mosfet gate because I'm going to use a comparator
>to turn it off, and I plan to limit the current the comparator sinks
>to about 5 mA.
>The N-mosfet source will be sitting at 15 volts and the gate drive
>comes from a voltage doubler.
>That's a bit less than 30 volts so the pull-up resistor needs to be in
>the 6k ballpark.
>I'm going to use the NTP90N02:
>http://www.onsemi.com/pub/Collateral/NTB90N02-D.PDF
>Speed doesn't matter -- this is basically an on-off switch, so I have
>only to consider the dc behavior of the mosfet.
>The mosfet has to conduct ten amps.
>My question is, how much resistance can you put in front of the gate
>before it gets too weak to turn the mosfet on hard?

---
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.
---

>Mosfet models don't address this, so far as I can tell. I haven't
>ordered the NTP90N02 yet so I can't simply experiment
>on it, but I remember other mosfets having problems turning on when
>the gate resistance gets into the tens of kilohms.

---
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.

Question...

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?



--
JF

On Mon, 14 Jan 2008 14:49:02 -0800 (PST), Marra
<cresswellavenue@talktalk.net> wrote:

>I design my own MOSFET amplifiers, up to 600 watts RMS.

---
There is no such thing as "watts RMS".


--
JF

John Fields wrote:
> On Mon, 14 Jan 2008 14:49:02 -0800 (PST), Marra
> <cresswellavenue@talktalk.net> wrote:
>
>> I design my own MOSFET amplifiers, up to 600 watts RMS.
>
> ---
> There is no such thing as "watts RMS".
>

But according to Walmart there is Watts PMPO

--
Regards, Joerg

http://www.analogconsultants.com/

Marra wrote:
>>I design my own MOSFET amplifiers, up to 600 watts RMS.
>
John Fields wrote:
>There is no such thing as "watts RMS".

You're joking. Right?
Back in (what?) the early '70s, EIA decided that
instead of using the logical term "Watts, continuous"
they would call it "Watts RMS" for audio gear.

That is is similar to *root mean squared* makes this confusing
and is a sign of bad judgement on their part.

"JeffM" <jeffm_@email.com> wrote in message
news:5c7df474-ef92-401c-bede-7f8b17527e20@c4g2000hsg.googlegroups.com...
> Marra wrote:
>>>I design my own MOSFET amplifiers, up to 600 watts RMS.
>>
> John Fields wrote:
>>There is no such thing as "watts RMS".
>
> You're joking. Right?
> Back in (what?) the early '70s, EIA decided that
> instead of using the logical term "Watts, continuous"
> they would call it "Watts RMS" for audio gear.
>
> That is is similar to *root mean squared* makes this confusing
> and is a sign of bad judgement on their part.

It appears to have been the Institute of High Fidelity - a group of
manufacturer's
http://www.hifi-writer.com/he/misc/rmspower.htm
This, and other articles, seem to agree that it is, at best, confusing.

John Fields wrote:
>>>There is no such thing as "watts RMS".
>>>
JeffM wrote:
>>You're joking. Right?
>>Back in (what?) the early '70s, EIA decided that[...]
>>
Richard Seriani wrote:
>It appears to have been the Institute of High Fidelity[...]
>http://www.hifi-writer.com/he/misc/rmspower.htm
>This, and other articles, seem to agree that it is, at best, confusing.

Shows how out of touch *I* am. I didn't realize it was a quote.
http://www.google.com/search?q=There.is ... .watts.RMS

On Jan 14, 7:54=A0pm, "Richard Seriani" <richard_s...@cox.net> wrote:
> "JeffM" <jef...@email.com> wrote in message
>
> news:5c7df474-ef92-401c-bede-7f8b17527e20@c4g2000hsg.googlegroups.com...
>
> > Marra wrote:
> >>>I =A0 design my own MOSFET amplifiers, up to 600 watts RMS.
>
> > John Fields wrote:
> >>There is no such thing as "watts RMS".

It is indeed possible to calculate RMS power. However, the result is
a mathematical curiosity and is not useful. If you calculate RMS
power, the result will be 1.225 X the value you get by multiplying
(RMS voltage) X (RMS Current). See the article in JAES (Journal of
the Audio Engineering Society) "RMS Power: Fact or Fancy" by Eargle
and Locanth for the details. The latest IHF Standard for Amplifiers
was released in the late 60's. I does not make reference to "RMS
power". In its table of minimum amplifier specifications, it refers
to "Continuous output in watts per channel". Partially in response to
audio equipment manufacturers' abuse of power ratings ("RMS Power",
"Peak Power", "Music Power", etc), the FTC published its "Electronic
Code of Federal Regulations". Part 432 of this regulation specifies,
among other things, that power output shall be disclosed as "rated
minimum sine wave continuous average power output". In fact, the only
use of the term "RMS" in this regulation is with respect to the power
line voltage and current. "RMS Power" makes absolutely no sense.
Regards,
Jon

On Jan 14, 9:08 pm, Jon wrote:
>
> It is indeed possible to calculate RMS power. However, the result
> is a mathematical curiosity and is not useful. If you calculate RMS
> power, the result will be 1.225 X the value you get by multiplying
> (RMS voltage) X (RMS Current). See the article in JAES (Journal of
> the Audio Engineering Society) "RMS Power: Fact or Fancy" by Eargle
> and Locanth for the details.

That's a 35-year-old article, which makes it a bit hard
to get. But it's no doubt interesting. Could you put
up a scan?

On Jan 15, 7:42=A0am, Winfield <winfieldh...@yahoo.com> wrote:
> On Jan 14, 9:08 pm, Jon wrote:
>
>
>
>
> =A0That's a 35-year-old article, which makes it a bit hard
> =A0to get. =A0But it's no doubt interesting. =A0Could you put
> =A0up a scan?

The articel can be obtained by visting the AES website: http://www.AES.org
Regards,
Jon

On Jan 14, 3:20=A0pm, John Fields <jfie...@austininstruments.com> wrote:
> On Mon, 14 Jan 2008 12:15:57 -0800 (PST), gearhead
> <nos...@billburg.com> wrote:

> >I have a situation where I need to use a pretty stiff pull-up resistor
> >on a mosfet gate because I'm going to use a comparator
> >to turn it off, and I plan to limit the current the comparator sinks
> >to about 5 mA.
> >The N-mosfet source will be sitting at 15 volts and the gate drive
> >comes from a voltage doubler.
> >That's a bit less than 30 volts so the pull-up resistor needs to be in
> >the 6k ballpark.
> >I'm going to use the NTP90N02:
> >http://www.onsemi.com/pub/Collateral/NTB90N02-D.PDF
> >Speed doesn't matter -- this is basically an on-off switch, so I have
> >only to consider the dc behavior of the mosfet.
> >The mosfet has to conduct ten amps.
> >My question is, how much resistance can you put in front of the gate
> >before it gets too weak to turn the mosfet on hard?
>
> ---
> 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.
> ---
>
> >Mosfet models don't address this, so far as I can tell. =A0I haven't
> >ordered the NTP90N02 yet so I can't simply experiment
> >on it, but I remember other mosfets having problems turning on when
> >the gate resistance gets into the tens of kilohms.
>
> ---
> 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.
>
> Question...
>
> Since you're doing high-side driving, why can't you use a P channel
> MOSFET, like this: =A0(View in Courier)
>
> .+15V>---+-----+----+----+
> . =A0 =A0 =A0 =A0| =A0 =A0 | =A0 =A0| =A0 =A0|
> . =A0 =A0 =A0 [R1] =A0 | =A0[3K] =A0 | =A0
> . =A0 =A0 =A0 =A0| =A0 =A0 | =A0 =A0| =A0 =A0|
> . =A0 =A0 =A0 =A0+----|+\ =A0 | =A0 =A0S
> . =A0 =A0 =A0 =A0| =A0 =A0| =A0>--+--G IRF4905
> .IN>-----|----|-/ =A0 =A0 =A0 =A0D
> . =A0 =A0 =A0 =A0| =A0 =A0 | =A0 =A0 =A0 =A0 |
> . =A0 =A0 =A0 [R2] =A0 | =A0 =A0 =A0 [LOAD]
> . =A0 =A0 =A0 =A0| =A0 =A0 | =A0 =A0 =A0 =A0 |
> .GND>----+-----+---------+
>
> where, assuming IN is positive true, =A0R1 and R2 are selected to
> cause the voltage at the + input of the comparator to be about
> halfway between the swing of IN?
>
> --
> JF- Hide quoted text -
>
> - Show quoted text -


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.

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 04/19/05 http://www.tech-chat.de)

Here is a schottky diode that is about a buck in single quantity from
digikey:

http://search.digikey.com/scripts/DkSea ... -2749-5-ND

Here is a suitable resistor:

http://search.digikey.com/scripts/DkSea ... =1.0W-5-ND

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.

"gearhead" <nospam@billburg.com> wrote in message
news:43f4428d-486d-4c05-bfac-c1ebf080f1e1@j20g2000hsi.googlegroups.com...
On Jan 14, 3:20 pm, John Fields <jfie...@austininstruments.com> wrote:
> On Mon, 14 Jan 2008 12:15:57 -0800 (PST), gearhead
> <nos...@billburg.com> wrote:

> >I have a situation where I need to use a pretty stiff pull-up resistor
> >on a mosfet gate because I'm going to use a comparator
> >to turn it off, and I plan to limit the current the comparator sinks
> >to about 5 mA.
> >The N-mosfet source will be sitting at 15 volts and the gate drive
> >comes from a voltage doubler.
> >That's a bit less than 30 volts so the pull-up resistor needs to be in
> >the 6k ballpark.
> >I'm going to use the NTP90N02:
> >http://www.onsemi.com/pub/Collateral/NTB90N02-D.PDF
> >Speed doesn't matter -- this is basically an on-off switch, so I have
> >only to consider the dc behavior of the mosfet.
> >The mosfet has to conduct ten amps.
> >My question is, how much resistance can you put in front of the gate
> >before it gets too weak to turn the mosfet on hard?
>
> ---
> 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.
> ---
>
> >Mosfet models don't address this, so far as I can tell. I haven't
> >ordered the NTP90N02 yet so I can't simply experiment
> >on it, but I remember other mosfets having problems turning on when
> >the gate resistance gets into the tens of kilohms.
>
> ---
> 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.
>
> Question...
>
> 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?
>
> --
> JF- Hide quoted text -
>
> - Show quoted text -


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.

On Jan 18, 10:26=A0pm, "Bob Monsen" <rcmon...@gmail.com> wrote:
> 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.=

>
** There is no an external diode, that's a diagrammatic representation
of the mosfet's internal body diode. Yes, the mosfet conducts anytime
the generator comes on. That's what I want. It's not there to block
charging current. It's there to block the battery from _dis_charging
into the generator.

(snip)

> 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?

**Yes!

>If so, you probably want the Source to be next to the battery.

**No!

>However,
> that makes the substrate diode conduct when the generator is off, which yo=
u
> say you don't want.

**Right, that's why the mosfet is oriented the way it is. So that it
won't conduct when the generator is off.

> I wonder if you can go with a simpler circuit? Here is one, assuming I
> understand your requirements:
>
> =A0 generator--->|----(battery +)---->|---load
>
> When the generator is on, it'll charge the battery and power the load. Whe=
n
> the generator is off, the battery will drain through the load. You may wan=
t
> 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:
>
> =A0Gen =A0o------>|----o----------------------------------.
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0___ =A0 =A0 =A0 =A0 =A0 =
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 |
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0'---|___|----o-------->|-----------=
o
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A01 Ohm 5W =A0 | =A0 =A0 =A0 =A0 =
=A0 =A0 =A0 =A0 =A0 =A0 |
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0--- =A0 =A0=
=A0 =A0 =A0 =A0 =A0 =A0 =A0 .-.
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 - =A0 Batt=
ery =A0 =A0 =A0 =A0 =A0| |
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0--- =A0 =A0=
=A0 =A0 =A0 =A0 =A0 =A0 =A0 | | LOAD
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 - =A0 =A0 =
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0'-'
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 |
> =A0GND 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/DkSea ... 3D497-...=

>
> Here is a suitable resistor:
>
> http://search.digikey.com/scripts/DkSea ... 3D1.0W...=

>
> 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,
> =A0Bob Monsen

**I appreciate the effort you went to, Bob. The diagram I posted is
only a detail of a larger circuit, which includes voltage regulation.
But what I posted ought to be enough to understand what the mosfet is
doing. It's a blocking recitifier. At first blush, you might think
the mosfet is backwards, but it's not. The body diode acts as a
blocking diode (when the generator is turned off). Then, when the
generator turns on and the voltage at the source rises above the drain
voltage, the comparator detects it and relaxes its hold on the mosfet
gate. The voltage doubler then turns on the mosfet through the pullup
resistor, allowing it to conduct with much less loss than a mere diode
would. If you take a closer look at the circuit, you'll see how it
works.


>
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 Vboost
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|
> =A0 =A0,-----------------, =A0 =A0 ,-+-,
> =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 | =A0 |
> =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 Rp =A0|
> =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0,--+-----|---|---,
> =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0| =A0 =A0 | =A0 |/| |
> =A0 =A0 ) =A0 =A0 =A0 =A0 =A0 =A0 V S|_ =A0 =A0| =A0 /+|-'
> =A0 =A0 ) =A0 =A0 =A0 =A0 =A0 =A0 - =A0 _||--+--< =A0|
> =A0 =A0 ) =A0 =A0 =A0 =A0 =A0 =A0 | D| =A0 =A0 =A0 =A0 \-|-,
> =A0 =A0 ) =A0 =A0 =A0 =A0 =A0 =A0 | =A0| =A0 =A0 =A0 =A0 |\| |
> =A0 =A0 ) =A0 =A0 =A0 =A0 =A0 =A0 '--+---------|---'
> =A0 =A0 ) generator =A0 =A0 =A0| =A0 =A0 =A0 =A0 |
> =A0 =A0 ) windings =A0 =A0 =A0 | =A0 =A0 =A0 =A0 |
> =A0 =A0 ) =A0 =A0 =A0 =A0 =A0 =A0 =A0 ___+ =A0 =A0 =A0 |
> =A0 =A0 ) =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0_ =A0 =A0 =A0 =A0 |
> =A0 =A0 ) =A0 =A0 =A0 =A0 =A0 =A0 =A0 ___ =A0 =A0 =A0 =A0|
> =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 _ =A0 =A0 =A0 =A0 |
> =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0___ =A0 =A0 =A0 =A0|
> =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 _ - =A0 =A0 =A0 |
> =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 |
> =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 |
> =A0 =A0'-----------------+---------'
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0-----
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 ---
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0-
>

On Jan 19, 1:26 am, "Bob Monsen" <rcmon...@gmail.com> wrote:
> 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/DkSea ... ame=497-...
>
> Here is a suitable resistor:
>
> http://search.digikey.com/scripts/DkSea ... ame=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.
>
> "gearhead" <nos...@billburg.com> wrote in message
>
> news:43f4428d-486d-4c05-bfac-c1ebf080f1e1@j20g2000hsi.googlegroups.com...
> On Jan 14, 3:20 pm, John Fields <jfie...@austininstruments.com> wrote:
>
>
>
> > On Mon, 14 Jan 2008 12:15:57 -0800 (PST), gearhead
> > <nos...@billburg.com> wrote:
> > >I have a situation where I need to use a pretty stiff pull-up resistor
> > >on a mosfet gate because I'm going to use a comparator
> > >to turn it off, and I plan to limit the current the comparator sinks
> > >to about 5 mA.
> > >The N-mosfet source will be sitting at 15 volts and the gate drive
> > >comes from a voltage doubler.
> > >That's a bit less than 30 volts so the pull-up resistor needs to be in
> > >the 6k ballpark.
> > >I'm going to use the NTP90N02:
> > >http://www.onsemi.com/pub/Collateral/NTB90N02-D.PDF
> > >Speed doesn't matter -- this is basically an on-off switch, so I have
> > >only to consider the dc behavior of the mosfet.
> > >The mosfet has to conduct ten amps.
> > >My question is, how much resistance can you put in front of the gate
> > >before it gets too weak to turn the mosfet on hard?
>
> > ---
> > 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.
> > ---
>
> > >Mosfet models don't address this, so far as I can tell. I haven't
> > >ordered the NTP90N02 yet so I can't simply experiment
> > >on it, but I remember other mosfets having problems turning on when
> > >the gate resistance gets into the tens of kilohms.
>
> > ---
> > 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.
>
> > Question...
>
> > 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?
>
> > --
> > JF- Hide quoted text -
>
> > - Show quoted text -
>
> 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.

There are ORing-controller ICs that drive FETs
of your choice for the power-supply ORing job.
For example, the Intersil isl6144, TI tps2410
and 11, and LTC LT4351 for "+" supply lines,
and IRF IR5001s for "-" supply lines. All
of these feature built-in charge pumps, and
have comparators, MOSFET drivers, and other good
stuff. The LTC4412 uses a p-channel MOSFET.

Whether any of these ICs interest you or not,
studying their datasheets and app notes could
turn up aspects you hadn't considered.

Winfield Hill wrote:
>
> There are ORing-controller ICs that drive FETs
> of your choice for the power-supply ORing job.
> For example, the Intersil isl6144, TI tps2410
> and 11, and LTC LT4351 for "+" supply lines,
> and IRF IR5001s for "-" supply lines. All
> of these feature built-in charge pumps, and
> have comparators, MOSFET drivers, and other good
> stuff. The LTC4412 uses a p-channel MOSFET.
>
> Whether any of these ICs interest you or not,
> studying their datasheets and app notes could
> turn up aspects you hadn't considered.

Oh, and Maxim makes a half-dozen parts too.

On Jan 19, 10:33=A0am, Winfield Hill <h...@rowland.org> wrote:
>
> =A0There are ORing-controller ICs that drive FETs
> =A0of your choice for the power-supply ORing job.
> =A0For example, the Intersil isl6144, TI tps2410
> =A0and 11, and LTC LT4351 for "+" supply lines,
> =A0and IRF IR5001s for "-" supply lines. =A0All
> =A0of these feature built-in charge pumps, and
> =A0have comparators, MOSFET drivers, and other good
> =A0stuff. =A0The LTC4412 uses a p-channel MOSFET.
>
> =A0Whether any of these ICs interest you or not,
> =A0studying their datasheets and app notes could
> =A0turn up aspects you hadn't considered.

Just what the doctor ordered.
Thanks.