Driving common source MOSFets with OptoMosfet

[OBones]

Hi

How would I go about driving a common source MOSFet configuration (Two N
type) with the output of an Opto Mosfet (N type)

Basically, the schematic would be this:


| D
||-+
.------------. ||<-
| --- -||-+
----- | | G | S
| | ||-+ | |
| V -> ||<- | |
| - -||-+ | G | S
| | | | -||-+
----- ---- ||<-
'------------' ||-+
| D


But I'm not sure as to how to connect the optically driven mosfet to the
two others.
Here are a bit more details:
What is connected to both D's is an AC signal, and that's the only
"power" source on that side.
When the LED is on in the OptoMosfet, the two Gates must be On so as to
turn the two mosfets On.
When the LED is off in the OptoMosfet, the two Gates must be driven to
zero so as to stop the mosfets.

And finally, if I replace the Two N type mosfets with two P type
mosfets, would that still work? Would the common source configuration
still be valid or would I need to go for a common drain configuration?

Thanks a lot for the help.
Olivier

 

[Andrew Holme]

Hi

How would I go about driving a common source MOSFet configuration (Two N
type) with the output of an Opto Mosfet (N type)

Basically, the schematic would be this:


| D
||-+
.------------. ||<-
| --- -||-+
----- | | G | S
| | ||-+ | |
| V -> ||<- | |
| - -||-+ | G | S
| | | | -||-+
----- ---- ||<-
'------------' ||-+
| D


But I'm not sure as to how to connect the optically driven mosfet to the
two others.
Here are a bit more details:
What is connected to both D's is an AC signal, and that's the only
"power" source on that side.
When the LED is on in the OptoMosfet, the two Gates must be On so as to
turn the two mosfets On.
When the LED is off in the OptoMosfet, the two Gates must be driven to
zero so as to stop the mosfets.

Didn't you previously post about "normally-closed" opto-MOSFETs?

For normally-open, how about the TLP598G? This is GBP 1.63 for 100+
from rswww.com

 

[OBones]

Didn't you previously post about "normally-closed" opto-MOSFETs?

I did, but I actually two NC and two NO, hence I'm looking at different
ways to achieve the same result. And yes, I know it would be easier to
use a regular electromechanical relay, but there is no challenge here.

For normally-open, how about the TLP598G? This is GBP 1.63 for 100+
from rswww.com

I only need two of each, and that's the main problem. My design is just
for hobby, not for production.

 

[Winfield Hill]

OBones wrote...

How would I go about driving a common source MOSFet configuration
(Two N type) with the output of an Opto Mosfet (N type)

| D
||-+
.------------. ||<-
| --- -||-+
----- | | G | S
| | ||-+ | |
| V -> ||<- | |
| - -||-+ | G | S
| | | | -||-+
----- ---- ||<-
'------------' ||-+
| D

But I'm not sure as to how to connect the optically driven mosfet
to the two others.

You're missing an important element above, the stack of photodiodes
required to receive the light and create an isolated gate voltage.

Here are a bit more details:
What is connected to both D's is an AC signal, and that's the only
"power" source on that side. When the LED is on in the OptoMosfet,
the two Gates must be On so as to turn the two mosfets On.

Yes, so tie both gates together, and connect them to the output
of the PD stack, through a diode. The bottom end of the PD stack
goes to the sources.

When the LED is off in the OptoMosfet, the two Gates must be driven
to zero so as to stop the mosfets.

Yes, so add a high-value resistor across the PD stack, to pull it
down to zero, and add a PNP emitter follower in parallel with the
diode to pull down the FET gates following the high-value resistor.
You can edit the drawing for us (or you can find one I posted here).

And finally, if I replace the Two N type mosfets with two P type
mosfets, would that still work? Would the common source configuration
still be valid or would I need to go for a common drain configuration?

Yes of course, by reversing the polarities and using an NPN trannie.

 

[Fred Bloggs]

Hi

How would I go about driving a common source MOSFet configuration (Two N
type) with the output of an Opto Mosfet (N type)

Basically, the schematic would be this:


| D
||-+
.------------. ||<-
| --- -||-+
----- | | G | S
| | ||-+ | |
| V -> ||<- | |
| - -||-+ | G | S
| | | | -||-+
----- ---- ||<-
'------------' ||-+
| D


But I'm not sure as to how to connect the optically driven mosfet to the
two others.
Here are a bit more details:
What is connected to both D's is an AC signal, and that's the only
"power" source on that side.
When the LED is on in the OptoMosfet, the two Gates must be On so as to
turn the two mosfets On.
When the LED is off in the OptoMosfet, the two Gates must be driven to
zero so as to stop the mosfets.

And finally, if I replace the Two N type mosfets with two P type
mosfets, would that still work? Would the common source configuration
still be valid or would I need to go for a common drain configuration?

Thanks a lot for the help.
Olivier

Won't work- you need a floating VGS source referenced to the common
source connection.

 

[OBones]

You're missing an important element above, the stack of photodiodes
required to receive the light and create an isolated gate voltage.

Yes, but aren't these included into the package and not available on any
of the pins of the package?

Yes, so tie both gates together, and connect them to the output
of the PD stack, through a diode. The bottom end of the PD stack
goes to the sources.
Fine.


Yes, so add a high-value resistor across the PD stack, to pull it
down to zero, and add a PNP emitter follower in parallel with the
diode to pull down the FET gates following the high-value resistor.
You can edit the drawing for us (or you can find one I posted here).

Do you have a reference or a keyword for that message you posted here?

Yes of course, by reversing the polarities and using an NPN trannie.

Indeed.

Thanks

 

[Pooh Bear]

Hi

How would I go about driving a common source MOSFet configuration (Two N
type)

You can't wire 2 N types like that.

What are you trying to do ?


Graham

 

[OBones]

You can't wire 2 N types like that.

Yes you can, see Solid State Relay schematics. It works because of the
parasitic anti parallell diode in the Mosfet.

What are you trying to do ?

Turn On and Off AC signals (phone, actually)

 

[Wolfgang Mahringer]

Hi,

Turn On and Off AC signals (phone, actually)

Why don't you use a simple bridge rectifier?
Connect your AC source to the AC terminals of the rectifier,
and let the opto-mosfet directly "short" the + and - terminals
of the rectifier?
If you can't afford the 2 diode drops, make a rectifier with
4 schottky diodes.

HTH
Wolfgang

 

[OBones]

Hi,



Why don't you use a simple bridge rectifier?
Connect your AC source to the AC terminals of the rectifier,
and let the opto-mosfet directly "short" the + and - terminals
of the rectifier?
If you can't afford the 2 diode drops, make a rectifier with
4 schottky diodes.

Would you have a schematic for this?

 

[Genome]

"Winfield Hill"

OBones wrote... [snip]

And finally, if I replace the Two N type mosfets with two P type
mosfets, would that still work? Would the common source configuration
still be valid or would I need to go for a

common drain configuration?

Yes of course,

Just to make the butter clear.....

Keep the common source connection. The drive
polarity is reversed.

Shoot me.

DNA

 

[Genome]

"Pooh Bear"

You can't wire 2 N types like that.

What are you trying to do ?


Graham

Irrespective of what Obones is trying to achieve
he has the right to wire up his mosfets any way he
ooooooooooooooooooooooo, **** it, fucking likes.

DNA

 

[Wolfgang Mahringer]

Hi,

Would you have a schematic for this?

+-+---------+
A A |
your AC load terminals o-----+ | D +-||
o-----(-+ ->|| Opto-MOSFET
A A S +-||
| | |
+-+---------+

so simple....
Just be sure to use a rectifier that can withstand the
"open" AC voltage, and the "closed" current. Your opto-mosfet
should also be capable of that....

HTH
Wolfgang

 

[OBones]

D'oh, so simple, should have thought of that before...
Brain must have gone away, it's the end of the week after all...

 

[Fred Bloggs]

D'oh, so simple, should have thought of that before...
Brain must have gone away, it's the end of the week after all...

It runs deeper than that- your opto FET is bidirectional, sorry-assed
p.o.s....

 

[Winfield Hill]

Pooh Bear (Graham) wrote...

You can't wire 2 N types like that.
What are you trying to do ?

Wassamatter with you, Graham, Of course you can! I've completed
O'Bones drawing below. The result is a bog-standard ac SSR.

_____________________________|____
| gste | |
| PD stack diode |--D |
| ,----+-|>|--+--+--||<-, |
| | | | | |--S |
| V | |/e | | |
----- LED - +----| | | |
| | -> | | |\ | | |
| V -> V \ pnp | | | |
| - -> - / R +--|------+ |
| | | \pull- | | | |
----- V | down | | |--S |
| - | | '--||<-' |
| |____|______| |--D |
|_____________________________|____|
|

If you want to use your own powerful high-voltage MOSFETs, the
inner LED / PD-stack combo can be provded by an IR PVI5050, etc.

 

[OBones]

Do you have a reference or a keyword for that message you posted here?

Doing a mix of your proposition and Wolfgang's one, I came up with this:


In1
|
N |
.-----+-+ |
||-+ A A |
||<- | +---'
.-||-+ +-)---.
.---------. | | A A |
----. .-------+--------+ '--+--+-+ |
| | | | .-. | | |
| V -> A | |R| | Vss |
| - A | | | | +--- Out
| | | | '-' | |
----' '-------+ | |
'---------' | | .-----+-+ |
| | ||-+ A A |
Vss | ||-> | +---'
'-||-+ +-)---.
| A A |
P '--+--+-+ |
| |
Vss |
In2


I drive the Mosfet driver with whatever logic I want, and this will act
as a SPDT "relay", selecting In1 if the LED is on, and In2 if the LED if
off. The R resistor discharges the gates and cell when the LED is off,
which effectively turns off the N mosfet and on the P one. Is there a
specific value to use?
Finally, to get a DPDT relay I simply duplicate the Mosfet + Brige part
so as to switch the second line of the phone signal.
Of course, the bridges and mosfets must whistand the voltage and
amperage on the phone line.
In my mind, this looks good, but please correct me if I'm wrong.

Thanks a lot for the help.
Cheers
Olivier

PS: Schematic created by AACircuit v1.28.5 beta 02/06/05
www.tech-chat.de, this soft is awesome

 

[OBones]

If you want to use your own powerful high-voltage MOSFETs, the
inner LED / PD-stack combo can be provded by an IR PVI5050, etc.

Shouldn't the PDiode stack be in the other direction (upside down)?

I have a hard time understanding how the gates get +V when the LED is on.

 

[John Larkin]

Hi

How would I go about driving a common source MOSFet configuration (Two N
type) with the output of an Opto Mosfet (N type)

Basically, the schematic would be this:


| D
||-+
.------------. ||<-
| --- -||-+
----- | | G | S
| | ||-+ | |
| V -> ||<- | |
| - -||-+ | G | S
| | | | -||-+
----- ---- ||<-
'------------' ||-+
| D


But I'm not sure as to how to connect the optically driven mosfet to the
two others.
Here are a bit more details:
What is connected to both D's is an AC signal, and that's the only
"power" source on that side.
When the LED is on in the OptoMosfet, the two Gates must be On so as to
turn the two mosfets On.
When the LED is off in the OptoMosfet, the two Gates must be driven to
zero so as to stop the mosfets.

And finally, if I replace the Two N type mosfets with two P type
mosfets, would that still work? Would the common source configuration
still be valid or would I need to go for a common drain configuration?

Thanks a lot for the help.
Olivier

Just buy a small solid-state relay or two. See cel.com for some
examples.

John

 

[Larry Brasfield]

Shouldn't the PDiode stack be in the other direction (upside down)?

Mr. Hill's drawing looked fine to me.

I have a hard time understanding how the gates get +V when the LED is on.

An open photodiode becomes forward biased when
illuminated with the right wavelengths. This mode is
called the photovoltaic mode.