op amp controlling a mosfet
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Posted by kell on July 18, 2006, 1:20 pm
Please log in for more thread options 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 | |||||||||||||||||||||||||
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Posted by John Popelish on July 18, 2006, 6:06 pm
Please log in for more thread options kell wrote: > Imagine you have a vehicle that receives up to ten amps from a
I doubt this simple circuit will do. The concept is to drive the gate
> 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 > 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? | |||||||||||||||||||||||||
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Posted by kell on July 18, 2006, 6:30 pm
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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
I expect to power the op-amp from the generator.
> 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 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. | |||||||||||||||||||||||||
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Posted by John Popelish on July 18, 2006, 7:04 pm
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kell wrote: > 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? > 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. | |||||||||||||||||||||||||
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Posted by kell on July 18, 2006, 11:07 pm
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> > 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. >
(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. | |||||||||||||||||||||||||
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