Driving common source MOSFets with OptoMosfet

O

OBones

Guest
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 wrote:

Didn't you previously post about "normally-closed" opto-MOSFETs?
Click to expand...
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
Click to expand...
I only need two of each, and that's the main problem. My design is just
for hobby, not for production.
 
OBones wrote:
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
Click to expand...
Won't work- you need a floating VGS source referenced to the common
source connection.
 
Winfield Hill wrote:

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

Thanks
 
OBones wrote:

Hi

How would I go about driving a common source MOSFet configuration (Two N
type)
Click to expand...
You can't wire 2 N types like that.

What are you trying to do ?


Graham
 
Pooh Bear wrote:

You can't wire 2 N types like that.
Click to expand...
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 ?
Click to expand...
Turn On and Off AC signals (phone, actually)
 
Hi,

OBones wrote:
What are you trying to do ?

Turn On and Off AC signals (phone, actually)
Click to expand...
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

--
From-address is Spam trap
Use: wolfgang (dot) mahringer (at) sbg (dot) at
 
Wolfgang Mahringer wrote:

Hi,

OBones wrote:


What are you trying to do ?


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.
Click to expand...
Would you have a schematic for this?
 
"Winfield Hill"
<hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote
in message news:d35o9k0pbe@drn.newsguy.com...
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,

--
Thanks,
- Win
Click to expand...
Just to make the butter clear.....

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

Shoot me.

DNA
 
"Pooh Bear"
<rabbitsfriendsandrelations@hotmail.com> wrote in
message news:42566F9B.7233B194@hotmail.com...
OBones wrote:

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

DNA
 
Hi,

OBones wrote:
Would you have a schematic for this?
Click to expand...

+-+---------+
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



--
From-address is Spam trap
Use: wolfgang (dot) mahringer (at) sbg (dot) at
 
D'oh, so simple, should have thought of that before...
Brain must have gone away, it's the end of the week after all...
 
OBones wrote:
D'oh, so simple, should have thought of that before...
Brain must have gone away, it's the end of the week after all...
Click to expand...
It runs deeper than that- your opto FET is bidirectional, sorry-assed
p.o.s....
 
Pooh Bear (Graham) wrote...
OBones wrote:

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 ?
Click to expand...
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 |
|_____________________________|____|
|
Click to expand...
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.


--
Thanks,
- Win
 
OBones wrote:

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?
Click to expand...
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
 
Winfield Hill wrote:

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.
Click to expand...
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.
 
On Fri, 08 Apr 2005 12:00:52 +0200, OBones
<obones_gfd_@_gfd_altern.org> wrote:

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
Click to expand...

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

John
 
"OBones" <obones_gfd_@_gds_altern.org> wrote in
message news:4257034f$0$1313$636a15ce@news.free.fr...
Winfield Hill wrote:

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)?
Click to expand...
Mr. Hill's drawing looked fine to me.

I have a hard time understanding how the gates get +V when the LED is on.
Click to expand...
An open photodiode becomes forward biased when
illuminated with the right wavelengths. This mode is
called the photovoltaic mode.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
 
John Larkin wrote:

Just buy a small solid-state relay or two. See cel.com for some
examples.
Click to expand...
I wish I could simply buy 2 1-Form-C SSRs, but in France, they are
really hard to come by. And ordering from the US is just too costly (20$
S&H + Customs for two 4$ parts). Hence the desire to replace those with
what I get my hands on.
 
OBones wrote...
John Larkin wrote:

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

I wish I could simply buy 2 1-Form-C SSRs, but in France, they are
really hard to come by. And ordering from the US is just too costly
(20$ S&H + Customs for two 4$ parts). Hence the desire to replace
those with what I get my hands on.
Click to expand...
Keep trying - I'm sure all SSR types are widely used in France,
so find out who's selling them industrially. Post the question
in sci.electronics.components

Hey, maybe you can get some free samples. :>)


--
Thanks,
- Win
 
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