FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups 
 ProfileProfile   PreferencesPreferences   Log in to check your private messagesLog in to check your private messages   Log inLog in 
Forum index » Electronix » Basics
op amp controlling a mosfet
Post new topic   Reply to topic Page 1 of 2 [16 Posts] View previous topic :: View next topic
Goto page:  1, 2 Next
Author Message
jasen
electronics forum Guru Wannabe


Joined: 18 Jun 2006
Posts: 204

PostPosted: Fri Jul 21, 2006 9:01 am    Post subject: Re: op amp controlling a mosfet Reply with quote

On 2006-07-19, Ban <bansuri@masterweb.it> wrote:
Quote:
John Popelish wrote:

Why the opamp, wouldn't this be a *lot* simpler cheaper and have zero power
drain when off?

o-+----+^+-----+-o
| ||| |
| === |
| | ___ |
.-. +-|___|-'
| | |
| | |
'-' |
| |/
+--| NPN
|
|
o------+---------o

will that turn off?


--

Bye.
Jasen
Back to top
Ban
electronics forum Guru


Joined: 04 May 2005
Posts: 466

PostPosted: Thu Jul 20, 2006 3:55 pm    Post subject: Re: op amp controlling a mosfet Reply with quote

kell wrote:
Quote:
Ban wrote:
Why the opamp, wouldn't this be a *lot* simpler cheaper and have
zero power drain when off?

o-+----+^+-----+-o
| ||| |
| === |
| | ___ |
.-. +-|___|-'
| | |
| | |
'-' |
| |/
+--| NPN
|
|
o------+---------o

There could be a glitch when the generator's turning off, and you
would end up with battery current flowing through the windings of the
generator. That would surely keep the voltage at the base of the npn
high enough to keep it and the mosfet turned on.
Maybe this would work, though:


o-+------+^+-----+-o
| ||| |
| === |
|/ | |
/_ 12v | |
| zener| |
| | ___ |
.-. +-|___|-'
| | |
| | |
'-' |
| |/
+----| NPN
|
|
o--------+---------o
For the transistor to turn on, the voltage at the generator output has
to be at least 12.7 volts. If there's a glitch while turning off and
for some reason the voltage at the transistor's base doesn't fall far
enough to turn it off, the 12 volt zener gives you a little insurance.
The heavy drain on the battery as it discharges through the low
impedance armature windings will probably drag the voltage below 12.7
volts, turning off the npn transistor and the mosfet.
Adding a common diode in series with the 12 volt zener will bump the
insurance to over 13 volts.
I like it.
That will work better, in fact I had already drawn a similar circuit, but

you were faster. I have used another divider 1k/10k in the gate drive to
increase the voltage drop with a weak input.
But I think a Schottky diode will do the job as well with only 0.4V drop.
Put a transil into the input line in case the switch shuts off very fast to
avoid the spikes. A DC-generator will not be as reliable as a normal
alternator and there will be the problem with overcharging/regulating the
battery voltage as well.
--
ciao Ban
Apricale, Italy
Back to top
John Popelish
electronics forum Guru


Joined: 29 Apr 2005
Posts: 1601

PostPosted: Wed Jul 19, 2006 10:24 pm    Post subject: Re: op amp controlling a mosfet Reply with quote

Ban wrote:
Quote:
John Popelish wrote:

Imagine you have a vehicle that receives up to ten amps from a
generator

I think the opamp is needed to detect the small drop across the
channel resistance when the generator falls slightly below the battery
voltage, so that the generator does not "motor".


I have seen only alternators with diodes to the output, after all you do
want a DC-current to flow. So the whole idea is a bit say "strange"

I assumed he was talking about a shunt wound DC generator, not an
alternator.
Back to top
John Popelish
electronics forum Guru


Joined: 29 Apr 2005
Posts: 1601

PostPosted: Wed Jul 19, 2006 10:24 pm    Post subject: Re: op amp controlling a mosfet Reply with quote

kell wrote:

Quote:
I thought about it and the circuit I posted could have a problem when
transitioning from the charging condition when the generator turns off.
As the generator voltage drops, the op-amp output drops, dragging the
mosfet gate down, turning it on and allowing the battery to discharge
through the generator windings. I'd solve this pretty much as you
suggested, by reversing the op-amp inputs and using a transistor on the
op-amp output. Then the op-amp can turn the mosfet on ONLY when the
generator voltage is higher than the battery voltage; under any other
conditions, the mosfet's gate-source resistor will keep it turned off.


generator
|
,------------------------+
| |
| _ _|D
'--|+\ | STP80PF55
LF412| >--, ,--+-|| p-channel
,--|-/ | c/ | |_ mosfet
| '--R--| | |S
| e\ 10K |
| | | |
| gnd | |
'-------------------+----+
|
|
battery


If the LF412 really can stand at least 14 volts positive (and I think
it can) on its inputs when it is unpowered and you can protect both
the supply and input pins from larger damaging transients, it just
might work.
Back to top
kell
electronics forum Guru


Joined: 25 Mar 2005
Posts: 383

PostPosted: Wed Jul 19, 2006 4:27 pm    Post subject: Re: op amp controlling a mosfet Reply with quote

Ban wrote:
Quote:
Why the opamp, wouldn't this be a *lot* simpler cheaper and have zero power
drain when off?

o-+----+^+-----+-o
| ||| |
| === |
| | ___ |
.-. +-|___|-'
| | |
| | |
'-' |
| |/
+--| NPN
|
|
o------+---------o

There could be a glitch when the generator's turning off, and you would

end up with battery current flowing through the windings of the
generator. That would surely keep the voltage at the base of the npn
high enough to keep it and the mosfet turned on.
Maybe this would work, though:


o-+------+^+-----+-o
| ||| |
| === |
|/ | |
/_ 12v | |
| zener| |
| | ___ |
.-. +-|___|-'
| | |
| | |
'-' |
| |/
+----| NPN
|>
|
o--------+---------o
For the transistor to turn on, the voltage at the generator output has
to be at least 12.7 volts. If there's a glitch while turning off and
for some reason the voltage at the transistor's base doesn't fall far
enough to turn it off, the 12 volt zener gives you a little insurance.
The heavy drain on the battery as it discharges through the low
impedance armature windings will probably drag the voltage below 12.7
volts, turning off the npn transistor and the mosfet.
Adding a common diode in series with the 12 volt zener will bump the
insurance to over 13 volts.
I like it.
Back to top
kell
electronics forum Guru


Joined: 25 Mar 2005
Posts: 383

PostPosted: Wed Jul 19, 2006 3:53 pm    Post subject: Re: op amp controlling a mosfet Reply with quote

kell wrote:
Quote:
Imagine you have a vehicle that receives up to ten amps from a
generator, and with the generator off you need a blocking rectifier to
keep the 12 volt battery from discharging into the generator windings,
so you decide to use the body diode of mosfet as the blocking
rectifier; during charging you can have the mosfet turned on to reduce
dissipation. The blocking has to occur in the positive rail because
ground consists of the vehicle chassis.
What is the best way? I was thinking about something along these
lines:

generator
|
,------------+
| |
| _ _|D
'--|-\ | STP80PF55
LF412| >-+-|| p-channel
,--|+/ | |_ mosfet
| | |S
| 100K |
| | |
| | |
'-------+----+
|
|
battery

Somebody told me the LF412 works with common mode inputs at the
positive rail, but if I should be looking at some other op-amp let me
know.
http://www.national.com/ds/LF/LF412.pdf


Quote:
I expect to power the op-amp from the generator.

Then you have a problem to solve. How do you keep the gate voltage at
the battery positive (to keep the gate to source at zero) when the
generator is off and the only voltage the opamp can output is zero?


That may be practical if you add an open collector or open drain
output to the opamp so that it can pull down or let go, only. That
way, when the opamp is unpowered, the gate to source resistor can pull
the gate voltage up to the source voltage. you may want to lower the
gate to source pull up resistor.


I thought about it and the circuit I posted could have a problem when
transitioning from the charging condition when the generator turns off.
As the generator voltage drops, the op-amp output drops, dragging the
mosfet gate down, turning it on and allowing the battery to discharge
through the generator windings. I'd solve this pretty much as you
suggested, by reversing the op-amp inputs and using a transistor on the
op-amp output. Then the op-amp can turn the mosfet on ONLY when the
generator voltage is higher than the battery voltage; under any other
conditions, the mosfet's gate-source resistor will keep it turned off.


generator
|
,------------------------+
| |
| _ _|D
'--|+\ | STP80PF55
LF412| >--, ,--+-|| p-channel
,--|-/ | c/ | |_ mosfet
| '--R--| | |S
| e\ 10K |
| | | |
| gnd | |
'-------------------+----+
|
|
battery


I like the simplicity of your circuit, Ban.

o-+----+^+-----+-o
| ||| |
| === |
| | ___ |
.-. +-|___|-'
| | |
| | |
'-' |
| |/
+--| NPN
|>
|
o------+---------o

But What happens when you turn the motor off and the generator voltage
starts to fall? The npn transistor will keep the mosfet turned on,
the battery will discharge through the mosfet into the generator
windings, which will keep the voltage up at the transistor base. From
the looks of it, the transistor will never turn off.
Back to top
Ban
electronics forum Guru


Joined: 04 May 2005
Posts: 466

PostPosted: Wed Jul 19, 2006 1:22 pm    Post subject: Re: op amp controlling a mosfet Reply with quote

John Popelish wrote:
Quote:
Imagine you have a vehicle that receives up to ten amps from a
generator

I think the opamp is needed to detect the small drop across the
channel resistance when the generator falls slightly below the battery
voltage, so that the generator does not "motor".

I have seen only alternators with diodes to the output, after all you do
want a DC-current to flow. So the whole idea is a bit say "strange"
--
ciao Ban
Apricale, Italy
Back to top
John Popelish
electronics forum Guru


Joined: 29 Apr 2005
Posts: 1601

PostPosted: Wed Jul 19, 2006 12:17 pm    Post subject: Re: op amp controlling a mosfet Reply with quote

Ban wrote:

Quote:
Why the opamp, wouldn't this be a *lot* simpler cheaper and have zero power
drain when off?

o-+----+^+-----+-o
| ||| |
| === |
| | ___ |
.-. +-|___|-'
| | |
| | |
'-' |
| |/
+--| NPN
|
|
o------+---------o
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)


I think the opamp is needed to detect the small drop across the
channel resistance when the generator falls slightly below the battery
voltage, so that the generator does not "motor".
Back to top
Ban
electronics forum Guru


Joined: 04 May 2005
Posts: 466

PostPosted: Wed Jul 19, 2006 11:44 am    Post subject: Re: op amp controlling a mosfet Reply with quote

John Popelish wrote:
Quote:

. + -----+--- Rsense ----+----> To Batt (+)
. | |
. Rc |
. +------------, |
. | __ | |
. c\| / +|--' |
. NPN |---< | |
. e/| \_-|-----'
. |
. Rload
. |
. - -----+--------------------> To Batt (-)

with the Rsense being the drain-source resistance of the mosfet.


I think the + input has to connect directly to Rsense. Rc is just the
source to gate resistor, so it is effectively on the battery side of
Rsense (the channel resistance). You're getting there.

Why the opamp, wouldn't this be a *lot* simpler cheaper and have zero power
drain when off?

o-+----+^+-----+-o
| ||| |
| === |
| | ___ |
.-. +-|___|-'
| | |
| | |
'-' |
| |/
+--| NPN
|>
|
o------+---------o
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

--
ciao Ban
Apricale, Italy
Back to top
jasen
electronics forum Guru Wannabe


Joined: 18 Jun 2006
Posts: 204

PostPosted: Wed Jul 19, 2006 8:52 am    Post subject: Re: op amp controlling a mosfet Reply with quote

On 2006-07-18, kell <kellrobinson@billburg.com> wrote:
Quote:

John Popelish wrote:
kell wrote:
Imagine you have a vehicle that receives up to ten amps from a
generator, and with the generator off you need a blocking rectifier to
keep the 12 volt battery from discharging into the generator windings,
so you decide to use the body diode of mosfet as the blocking
rectifier; during charging you can have the mosfet turned on to reduce
dissipation. The blocking has to occur in the positive rail because
ground consists of the vehicle chassis.
What is the best way? I was thinking about something along these
lines:

generator
|
,------------+
| |
| _ _|D
'--|-\ | STP80PF55
LF412| >-+-|| p-channel
,--|+/ | |_ mosfet
| | |S
| 100K |
| | |
| | |
'-------+----+
|
|
battery


So are you saying that conceptually it's ok, but that actual
implementation is impractical? I get the impresson you don't like the
idea of powering the op-amp from the generator.

If you power the op-amp from the generator when the generator is stopped the
non-inverting input is 12V above the positive rail.

and how will you get the gate high enough to turn the mosfet off ?

It may be easiest to just use a Shottky power diode.

Bye.
Jasen
Back to top
John Popelish
electronics forum Guru


Joined: 29 Apr 2005
Posts: 1601

PostPosted: Wed Jul 19, 2006 3:25 am    Post subject: Re: op amp controlling a mosfet Reply with quote

kell wrote:
Quote:
John Popelish wrote:

Then you have a problem to solve. How do you keep the gate voltage at
the battery positive (to keep the gate to source at zero) when the
generator is off and the only voltage the opamp can output is zero?


When the generator voltage falls below the battery voltage the op-amp
should exhibit an active high output equal or near the generator
voltage. As the generator voltage continues to fall, the op-amp's
ouput should continue in an active high, tracking the generator
voltage, until the generator voltage falls too low to drive the op-amp.
Without the generator to power it, the op-amp's totem pole output will
most likely be left open. Whereupon the gate-source resistor will take
over in keeping the mosfet turned off. So I don't see a problem. And
there's a relatively simple workaround if the op-amp output does go
wack.

(Snipping some stuff)

As a side note, this blocking rectifier will be part of a voltage
regulator. I built a previous version that used a regular diode, but I
would like to make an improved version that doesn't require as much
heat sinking.
That's another reason I want to use the mosfet.
The mosfet can do double duty -- not only as a blocking recitifier, but
I can exploit the Rds as a sense resistor for the current limiting
feature I'll be implementing. So in addition to eliminating much of
the heat of a rectifier, I get to eliminate a separate sense resistor
and its associated heat. My idea is to minimize the size of this
voltage regulator and its heat sink. Then instead of a big chunky
thing that needs its own separate mount, it will be a slender thing I
can hide under the battery cover.
I can use the second op-amp in the LF412 for the current limiting, with
this circuit that I received in response to a question I posted to this
newsgroup:

. + -----+--- Rsense ----+----> To Batt (+)
. | |
. Rc |
. +------------, |
. | __ | |
. c\| / +|--' |
. NPN |---< | |
. e/| \_-|-----'
. |
. Rload
. |
. - -----+--------------------> To Batt (-)

with the Rsense being the drain-source resistance of the mosfet.


I think the + input has to connect directly to Rsense. Rc is just the
source to gate resistor, so it is effectively on the battery side of
Rsense (the channel resistance). You're getting there.
Back to top
kell
electronics forum Guru


Joined: 25 Mar 2005
Posts: 383

PostPosted: Wed Jul 19, 2006 3:07 am    Post subject: Re: op amp controlling a mosfet Reply with quote

Quote:
John Popelish wrote:

kell wrote:

Imagine you have a vehicle that receives up to ten amps from a
generator, and with the generator off you need a blocking rectifier to
keep the 12 volt battery from discharging into the generator windings,
so you decide to use the body diode of mosfet as the blocking
rectifier; during charging you can have the mosfet turned on to reduce
dissipation. The blocking has to occur in the positive rail because
ground consists of the vehicle chassis.
What is the best way? I was thinking about something along these
lines:

generator
|
,------------+
| |
| _ _|D
'--|-\ | STP80PF55
LF412| >-+-|| p-channel
,--|+/ | |_ mosfet
| | |S
| 100K |
| | |
| | |
'-------+----+
|
|
battery

Somebody told me the LF412 works with common mode inputs at the
positive rail, but if I should be looking at some other op-amp let me
know.
http://www.national.com/ds/LF/LF412.pdf


I doubt this simple circuit will do. The concept is to drive the gate
negative (down to ground) with respect to the source any time the
battery is more negative than the generator. So it gets real
important what powers the opamp, and that it (both the supply pins and
inputs) is protected from voltage spikes. Are you going to power the
opamp at all times from the battery?

I expect to power the op-amp from the generator.

Then you have a problem to solve. How do you keep the gate voltage at
the battery positive (to keep the gate to source at zero) when the
generator is off and the only voltage the opamp can output is zero?

When the generator voltage falls below the battery voltage the op-amp
should exhibit an active high output equal or near the generator
voltage. As the generator voltage continues to fall, the op-amp's
ouput should continue in an active high, tracking the generator
voltage, until the generator voltage falls too low to drive the op-amp.
Without the generator to power it, the op-amp's totem pole output will
most likely be left open. Whereupon the gate-source resistor will take
over in keeping the mosfet turned off. So I don't see a problem. And
there's a relatively simple workaround if the op-amp output does go
wack.
Quote:

(Snipping some stuff)


As a side note, this blocking rectifier will be part of a voltage
regulator. I built a previous version that used a regular diode, but I
would like to make an improved version that doesn't require as much
heat sinking.
That's another reason I want to use the mosfet.
The mosfet can do double duty -- not only as a blocking recitifier, but
I can exploit the Rds as a sense resistor for the current limiting
feature I'll be implementing. So in addition to eliminating much of
the heat of a rectifier, I get to eliminate a separate sense resistor
and its associated heat. My idea is to minimize the size of this
voltage regulator and its heat sink. Then instead of a big chunky
thing that needs its own separate mount, it will be a slender thing I
can hide under the battery cover.
I can use the second op-amp in the LF412 for the current limiting, with
this circuit that I received in response to a question I posted to this
newsgroup:

.. + -----+--- Rsense ----+----> To Batt (+)
.. | |
.. Rc |
.. +------------, |
.. | __ | |
.. c\| / +|--' |
.. NPN |---< | |
.. e/| \_-|-----'
.. |
.. Rload
.. |
.. - -----+--------------------> To Batt (-)

with the Rsense being the drain-source resistance of the mosfet.
Back to top
John Popelish
electronics forum Guru


Joined: 29 Apr 2005
Posts: 1601

PostPosted: Tue Jul 18, 2006 11:04 pm    Post subject: Re: op amp controlling a mosfet Reply with quote

kell wrote:
Quote:
John Popelish wrote:

kell wrote:

Imagine you have a vehicle that receives up to ten amps from a
generator, and with the generator off you need a blocking rectifier to
keep the 12 volt battery from discharging into the generator windings,
so you decide to use the body diode of mosfet as the blocking
rectifier; during charging you can have the mosfet turned on to reduce
dissipation. The blocking has to occur in the positive rail because
ground consists of the vehicle chassis.
What is the best way? I was thinking about something along these
lines:

generator
|
,------------+
| |
| _ _|D
'--|-\ | STP80PF55
LF412| >-+-|| p-channel
,--|+/ | |_ mosfet
| | |S
| 100K |
| | |
| | |
'-------+----+
|
|
battery

Somebody told me the LF412 works with common mode inputs at the
positive rail, but if I should be looking at some other op-amp let me
know.
http://www.national.com/ds/LF/LF412.pdf


I doubt this simple circuit will do. The concept is to drive the gate
negative (down to ground) with respect to the source any time the
battery is more negative than the generator. So it gets real
important what powers the opamp, and that it (both the supply pins and
inputs) is protected from voltage spikes. Are you going to power the
opamp at all times from the battery?

I expect to power the op-amp from the generator.

Then you have a problem to solve. How do you keep the gate voltage at
the battery positive (to keep the gate to source at zero) when the
generator is off and the only voltage the opamp can output is zero?

Quote:
I didn't indicate power pins, bypass caps, input resistors and such on
the circuit diagram. I was more interested in the concept, the utility
of the LF412 and and the like so I didn't flesh out the circuit like I
guess I should have.
So are you saying that conceptually it's ok, but that actual
implementation is impractical? I get the impresson you don't like the
idea of powering the op-amp from the generator.

That may be practical if you add an open collector or open drain
output to the opamp so that it can pull down or let go, only. That
way, when the opamp is unpowered, the gate to source resistor can pull
the gate voltage up to the source voltage. you may want to lower the
gate to source pull up resistor.
Back to top
kell
electronics forum Guru


Joined: 25 Mar 2005
Posts: 383

PostPosted: Tue Jul 18, 2006 10:30 pm    Post subject: Re: op amp controlling a mosfet Reply with quote

John Popelish wrote:
Quote:
kell wrote:
Imagine you have a vehicle that receives up to ten amps from a
generator, and with the generator off you need a blocking rectifier to
keep the 12 volt battery from discharging into the generator windings,
so you decide to use the body diode of mosfet as the blocking
rectifier; during charging you can have the mosfet turned on to reduce
dissipation. The blocking has to occur in the positive rail because
ground consists of the vehicle chassis.
What is the best way? I was thinking about something along these
lines:

generator
|
,------------+
| |
| _ _|D
'--|-\ | STP80PF55
LF412| >-+-|| p-channel
,--|+/ | |_ mosfet
| | |S
| 100K |
| | |
| | |
'-------+----+
|
|
battery

Somebody told me the LF412 works with common mode inputs at the
positive rail, but if I should be looking at some other op-amp let me
know.
http://www.national.com/ds/LF/LF412.pdf

I doubt this simple circuit will do. The concept is to drive the gate
negative (down to ground) with respect to the source any time the
battery is more negative than the generator. So it gets real
important what powers the opamp, and that it (both the supply pins and
inputs) is protected from voltage spikes. Are you going to power the
opamp at all times from the battery?
I expect to power the op-amp from the generator.

I didn't indicate power pins, bypass caps, input resistors and such on
the circuit diagram. I was more interested in the concept, the utility
of the LF412 and and the like so I didn't flesh out the circuit like I
guess I should have.
So are you saying that conceptually it's ok, but that actual
implementation is impractical? I get the impresson you don't like the
idea of powering the op-amp from the generator.
Back to top
John Popelish
electronics forum Guru


Joined: 29 Apr 2005
Posts: 1601

PostPosted: Tue Jul 18, 2006 10:04 pm    Post subject: Re: op amp controlling a mosfet Reply with quote

kell wrote:
Quote:
Imagine you have a vehicle that receives up to ten amps from a
generator, and with the generator off you need a blocking rectifier to
keep the 12 volt battery from discharging into the generator windings,
so you decide to use the body diode of mosfet as the blocking
rectifier; during charging you can have the mosfet turned on to reduce
dissipation. The blocking has to occur in the positive rail because
ground consists of the vehicle chassis.
What is the best way? I was thinking about something along these
lines:

generator
|
,------------+
| |
| _ _|D
'--|-\ | STP80PF55
LF412| >-+-|| p-channel
,--|+/ | |_ mosfet
| | |S
| 100K |
| | |
| | |
'-------+----+
|
|
battery

Somebody told me the LF412 works with common mode inputs at the
positive rail, but if I should be looking at some other op-amp let me
know.
http://www.national.com/ds/LF/LF412.pdf

I doubt this simple circuit will do. The concept is to drive the gate

negative (down to ground) with respect to the source any time the
battery is more negative than the generator. So it gets real
important what powers the opamp, and that it (both the supply pins and
inputs) is protected from voltage spikes. Are you going to power the
opamp at all times from the battery?
Back to top
Google

Back to top
Display posts from previous:   
Post new topic   Reply to topic Page 1 of 2 [16 Posts] Goto page:  1, 2 Next
View previous topic :: View next topic
The time now is Sat Nov 18, 2017 11:55 am | All times are GMT
Forum index » Electronix » Basics
Jump to:  

Similar Topics
Topic Author Forum Replies Last Post
No new posts Controlling a relay via cellular phone..... EsDee Basics 49 Wed Jul 12, 2006 9:26 pm
No new posts mosfet vnp49n04 meow components 2 Tue Jul 11, 2006 6:23 am
No new posts Effective model for MOSFET channel resistance, etc. ER Yost cad 5 Mon Jul 10, 2006 2:19 pm
No new posts Need low power 2-gate MOSFET for RF amp CryptoManceR design 2 Thu Jul 06, 2006 5:56 am
No new posts FA: @$10>11 NOS HITACHI 2SK176 K176 POWER MOSFET TRANSIST... rich components 1 Tue Jul 04, 2006 2:14 pm

Copyright © 2004-2005 DeniX Solutions SRL
Other DeniX Solutions sites: Unix/Linux blog |  Unix/Linux documentation |  Unix/Linux forums |  Medicine forum |  Science forum  |  Send and track newsletters


Powered by phpBB © 2001, 2005 phpBB Group