How to quickly turn on/off N-FET switching high side of 55V?

[Fred Bloggs]

Don't need all that:
View in a fixed-width font such as Courier.

.
. HV
. |
. 12V>--+-|>|-+--------+-----+-|<|------. |-
. | | | | | .--||
. | | C | | | |>
. | | .--B npn | | | |
. | | | E | | | |
. | [4.7K] | +-[51]-----------|-----+ |
. | | | E | | | |
. | '----+--B pnp |0.1u | | |
. [1K] | C === | | |
. | | | |Cbst | | | 1/2W
. | | '-----+----------------|-----+--[10k]-.
. | | | | | | |
. | |- +--+-|>|---+ | |> ---
. +-------||SSN1N45B | | | '--|| sgnd
. | |> | | [100] |-
. \| | | | | |
. 2N7000 ||-------+ -/ | === +---> out
. <| | 12VZ ^ ----- |0.01u |
. | | | |C555 |--' -
. --- | | |ASTBL| ^
. sgnd | | |10KHz| |
. | | ----- |
. | | | ---
. CMOS ON >----' +----' pgnd
. |
. /|\ 10mA
. CMOS EN >------------ \V/
. |
. ---
. sgnd
.
.
.



And here's a slightly simplified version. Add the PV isolator for DC
drive.

ftp://66.117.156.8/Gate_Driver.JPG

One could also use a dip dc/dc converter and the relay.

John

What makes that simpler?

Um, having 1/5 as many parts? Being easy to analyze?

And actually, as long as we know the MOSFETs
can take fairly hefty transient peak power dissipation , I would prefer
to redo the input interface and stick an integrating capacitor in there
somewhere to slow the turn on/off to 15V/us range, especially now with
several of these circuits driving a common power bus, preventing
unrestrained slew rates from inducing transient currents into high
impedance nodes.

The gate resistor controls slew rate.

Also, I would want to at least attempt a backup
interlock by connecting OUT to the gate of the 2N7000 through a resistor
divider, or something, keeping the switching off while a second power
source was on the power bus, but not turning itself off Then there's
fusing on the HV lead to consider as well as protection of the LV stuff
in the event of HV component failure, possibly UVLO and maybe thermal
shutdown and other things...

More parts!

The dc/dc converter + relay is ideal for a one-off by someone who
doesn't want to do a lot of design and pcb fab. You could build one in
20 minutes, no simulation or debugging needed.

You could even use latching relays if that would help.

John

The OP may have significant investment in the system he's driving and
should be made aware of real world problems and damage possible with
oversimplified quick-make circuits...

 

[John Larkin]

John Larkin wrote:

Don't need all that:
View in a fixed-width font such as Courier.

.
. HV
. |
. 12V>--+-|>|-+--------+-----+-|<|------. |-
. | | | | | .--||
. | | C | | | |>
. | | .--B npn | | | |
. | | | E | | | |
. | [4.7K] | +-[51]-----------|-----+ |
. | | | E | | | |
. | '----+--B pnp |0.1u | | |
. [1K] | C === | | |
. | | | |Cbst | | | 1/2W
. | | '-----+----------------|-----+--[10k]-.
. | | | | | | |
. | |- +--+-|>|---+ | |> ---
. +-------||SSN1N45B | | | '--|| sgnd
. | |> | | [100] |-
. \| | | | | |
. 2N7000 ||-------+ -/ | === +---> out
. <| | 12VZ ^ ----- |0.01u |
. | | | |C555 |--' -
. --- | | |ASTBL| ^
. sgnd | | |10KHz| |
. | | ----- |
. | | | ---
. CMOS ON >----' +----' pgnd
. |
. /|\ 10mA
. CMOS EN >------------ \V/
. |
. ---
. sgnd
.
.
.



And here's a slightly simplified version. Add the PV isolator for DC
drive.

ftp://66.117.156.8/Gate_Driver.JPG

One could also use a dip dc/dc converter and the relay.

John


What makes that simpler?

Um, having 1/5 as many parts? Being easy to analyze?

And actually, as long as we know the MOSFETs
can take fairly hefty transient peak power dissipation , I would prefer
to redo the input interface and stick an integrating capacitor in there
somewhere to slow the turn on/off to 15V/us range, especially now with
several of these circuits driving a common power bus, preventing
unrestrained slew rates from inducing transient currents into high
impedance nodes.

The gate resistor controls slew rate.

Also, I would want to at least attempt a backup
interlock by connecting OUT to the gate of the 2N7000 through a resistor
divider, or something, keeping the switching off while a second power
source was on the power bus, but not turning itself off Then there's
fusing on the HV lead to consider as well as protection of the LV stuff
in the event of HV component failure, possibly UVLO and maybe thermal
shutdown and other things...

More parts!

The dc/dc converter + relay is ideal for a one-off by someone who
doesn't want to do a lot of design and pcb fab. You could build one in
20 minutes, no simulation or debugging needed.

You could even use latching relays if that would help.

John

The OP may have significant investment in the system he's driving and
should be made aware of real world problems and damage possible with
oversimplified quick-make circuits...

Or overcomplicated ones.

John

 

[Fred Bloggs]

John Larkin wrote:


Don't need all that:
View in a fixed-width font such as Courier.

.
. HV
. |
. 12V>--+-|>|-+--------+-----+-|<|------. |-
. | | | | | .--||
. | | C | | | |>
. | | .--B npn | | | |
. | | | E | | | |
. | [4.7K] | +-[51]-----------|-----+ |
. | | | E | | | |
. | '----+--B pnp |0.1u | | |
. [1K] | C === | | |
. | | | |Cbst | | | 1/2W
. | | '-----+----------------|-----+--[10k]-.
. | | | | | | |
. | |- +--+-|>|---+ | |> ---
. +-------||SSN1N45B | | | '--|| sgnd
. | |> | | [100] |-
. \| | | | | |
. 2N7000 ||-------+ -/ | === +---> out
. <| | 12VZ ^ ----- |0.01u |
. | | | |C555 |--' -
. --- | | |ASTBL| ^
. sgnd | | |10KHz| |
. | | ----- |
. | | | ---
. CMOS ON >----' +----' pgnd
. |
. /|\ 10mA
. CMOS EN >------------ \V/
. |
. ---
. sgnd
.
.
.



And here's a slightly simplified version. Add the PV isolator for DC
drive.

ftp://66.117.156.8/Gate_Driver.JPG

One could also use a dip dc/dc converter and the relay.

John


What makes that simpler?


Um, having 1/5 as many parts? Being easy to analyze?



And actually, as long as we know the MOSFETs
can take fairly hefty transient peak power dissipation , I would prefer
to redo the input interface and stick an integrating capacitor in there
somewhere to slow the turn on/off to 15V/us range, especially now with
several of these circuits driving a common power bus, preventing
unrestrained slew rates from inducing transient currents into high
impedance nodes.


The gate resistor controls slew rate.




Also, I would want to at least attempt a backup
interlock by connecting OUT to the gate of the 2N7000 through a resistor
divider, or something, keeping the switching off while a second power
source was on the power bus, but not turning itself off Then there's
fusing on the HV lead to consider as well as protection of the LV stuff
in the event of HV component failure, possibly UVLO and maybe thermal
shutdown and other things...


More parts!

The dc/dc converter + relay is ideal for a one-off by someone who
doesn't want to do a lot of design and pcb fab. You could build one in
20 minutes, no simulation or debugging needed.

You could even use latching relays if that would help.

John

The OP may have significant investment in the system he's driving and
should be made aware of real world problems and damage possible with
oversimplified quick-make circuits...

Or overcomplicated ones.

John

Just because it's not an NIF type of end use does not mean we trivialize
and get sloppy with the electronics.

 

[Winfield]

Fred Bloggs wrote:
Don't need all that:
View in a fixed-width font such as Courier.
.
. HV
. |
. 12V>--+-|>|-+--------+-----+-|<|------. |-
. | | | | | .--||
. | | C | | | |>
. | | .--B npn | | | |
. | | | E | | | |
. | [4.7K] | +-[51]-----------|-----+ |
. | | | E | | | |
. | '----+--B pnp |0.1u | | |
. [1K] | C === | | |
. | | | |Cbst | | | 1/2W
. | | '-----+----------------|-----+--[10k]-.
. | | | | | | |
. | |- +--+-|>|---+ | |> ---
. +-------||SSN1N45B | | | '--|| sgnd
. | |> | | [100] |-
. \| | | | | |
. 2N7000 ||-------+ -/ | === +---> out
. <| | 12VZ ^ ----- |0.01u |
. | | | |C555 |--' -
. --- | | |ASTBL| ^
. sgnd | | |10KHz| |
. | | ----- |
. | | | ---
. CMOS ON >----' +----' pgnd
. |
. /|\ 10mA
. CMOS EN >------------ \V/
. |
. ---
. sgnd
And here's a slightly simplified version. Add the PV isolator for DC
drive. ftp://66.117.156.8/Gate_Driver.JPG
One could also use a dip dc/dc converter and the relay.

Here's John's Gate_Driver.jpg sketch as an ASCII drawing.

Silly Gate Driver JL Dec '07 V+
|
D1 R1 K1 |
+12 --|>|--/\/\/--+---------o R2 |--'
,--->| ,o---/\/\---||
/ _|_ C1 ,---o' |-->
opt + ---' --- | : |
PV - ---, | | : |
\ | | : |
'--->'-----+---------------------+
: |
---uuuuu--- |
LOAD

Well, John's idea is certainly more simple than Fred's.

About the size of a sugar cube. An electronic driver circuit
could well be bigger.

The idea of using a relay to control a 30A MOSFET means
the relay can be small; a 30A relay would be a beast.

My favorite small relay type is Panasonic's TN series
(formerly under Mitsubishi's NAiS Aromat brand name),
http://pewa.panasonic.com/pcsd/product/sign/pdf/mech_eng_tn.pdf
These measure a tiny 0.22 x 0.55" and are 0.39" high.
I often use the TN2-4.5V part, rated for use at 4.5V,
but turns on at 3.38V max. Its 145-ohm coil takes
31mA at 4.5V, so it's easy to drive.

Bounce won't matter; the initial contact hit charges the gate
all the way.

True.

 

[Fred Bloggs]

John Larkin wrote:

Fred Bloggs wrote:

Don't need all that:
View in a fixed-width font such as Courier.

.
. HV
. |
. 12V>--+-|>|-+--------+-----+-|<|------. |-
. | | | | | .--||
. | | C | | | |>
. | | .--B npn | | | |
. | | | E | | | |
. | [4.7K] | +-[51]-----------|-----+ |
. | | | E | | | |
. | '----+--B pnp |0.1u | | |
. [1K] | C === | | |
. | | | |Cbst | | | 1/2W
. | | '-----+----------------|-----+--[10k]-.
. | | | | | | |
. | |- +--+-|>|---+ | |> ---
. +-------||SSN1N45B | | | '--|| sgnd
. | |> | | [100] |-
. \| | | | | |
. 2N7000 ||-------+ -/ | === +---> out
. <| | 12VZ ^ ----- |0.01u |
. | | | |C555 |--' -
. --- | | |ASTBL| ^
. sgnd | | |10KHz| |
. | | ----- |
. | | | ---
. CMOS ON >----' +----' pgnd
. |
. /|\ 10mA
. CMOS EN >------------ \V/
. |
. ---
. sgnd

And here's a slightly simplified version. Add the PV isolator for DC
drive. ftp://66.117.156.8/Gate_Driver.JPG
One could also use a dip dc/dc converter and the relay.

Here's John's Gate_Driver.jpg sketch as an ASCII drawing.

Silly Gate Driver JL Dec '07 V+
|
D1 R1 K1 |
+12 --|>|--/\/\/--+---------o R2 |--'
,--->| ,o---/\/\---||
/ _|_ C1 ,---o' |-->
opt + ---' --- | : |
PV - ---, | | : |
\ | | : |
'--->'-----+---------------------+
: |
---uuuuu--- |
LOAD

Well, John's idea is certainly more simple than Fred's.

About the size of a sugar cube. An electronic driver circuit
could well be bigger.

The idea of using a relay to control a 30A MOSFET means
the relay can be small; a 30A relay would be a beast.

My favorite small relay type is Panasonic's TN series
(formerly under Mitsubishi's NAiS Aromat brand name),
http://pewa.panasonic.com/pcsd/product/sign/pdf/mech_eng_tn.pdf
These measure a tiny 0.22 x 0.55" and are 0.39" high.
I often use the TN2-4.5V part, rated for use at 4.5V,
but turns on at 3.38V max. Its 145-ohm coil takes
31mA at 4.5V, so it's easy to drive.

Bounce won't matter; the initial contact hit charges the gate
all the way.

True.

Well- okay, this does simplify things for the OP and it does seem
relatively bullet proof as far as protecting his low voltage stuff.
But I still think he should fuse each battery. The automotive
aftermarket is supplying 42V blade type fuses these days. The Littlefuse
MAXI 42V line, ( and maybe Bussman has similar), is a 58VDC fuse with
1000A interrupt capability and available in high current. The OP could
use a 30 or 40A job. These things are rugged.

 

[Winfield]

I still think he should fuse each battery. The automotive
aftermarket is supplying 42V blade type fuses these days.
The Littlefuse MAXI 42V line, ( and maybe Bussman has
similar), is a 58VDC fuse with 1000A interrupt capability
and available in high current. The OP could use a 30 or
40A job. These things are rugged.

Or 50A. I hadn't seen your fuse suggestion before,
but I certainly agree, with these serious batteries,
that's a mandatory addition.

 

[John Larkin]

John Larkin wrote:
Fred Bloggs wrote:

Don't need all that:
View in a fixed-width font such as Courier.

.
. HV
. |
. 12V>--+-|>|-+--------+-----+-|<|------. |-
. | | | | | .--||
. | | C | | | |>
. | | .--B npn | | | |
. | | | E | | | |
. | [4.7K] | +-[51]-----------|-----+ |
. | | | E | | | |
. | '----+--B pnp |0.1u | | |
. [1K] | C === | | |
. | | | |Cbst | | | 1/2W
. | | '-----+----------------|-----+--[10k]-.
. | | | | | | |
. | |- +--+-|>|---+ | |> ---
. +-------||SSN1N45B | | | '--|| sgnd
. | |> | | [100] |-
. \| | | | | |
. 2N7000 ||-------+ -/ | === +---> out
. <| | 12VZ ^ ----- |0.01u |
. | | | |C555 |--' -
. --- | | |ASTBL| ^
. sgnd | | |10KHz| |
. | | ----- |
. | | | ---
. CMOS ON >----' +----' pgnd
. |
. /|\ 10mA
. CMOS EN >------------ \V/
. |
. ---
. sgnd

And here's a slightly simplified version. Add the PV isolator for DC
drive. ftp://66.117.156.8/Gate_Driver.JPG
One could also use a dip dc/dc converter and the relay.

Here's John's Gate_Driver.jpg sketch as an ASCII drawing.

Silly Gate Driver JL Dec '07 V+
|
D1 R1 K1 |
+12 --|>|--/\/\/--+---------o R2 |--'
,--->| ,o---/\/\---||
/ _|_ C1 ,---o' |-->
opt + ---' --- | : |
PV - ---, | | : |
\ | | : |
'--->'-----+---------------------+
: |
---uuuuu--- |
LOAD

Well, John's idea is certainly more simple than Fred's.

About the size of a sugar cube. An electronic driver circuit
could well be bigger.

The idea of using a relay to control a 30A MOSFET means
the relay can be small; a 30A relay would be a beast.

My favorite small relay type is Panasonic's TN series
(formerly under Mitsubishi's NAiS Aromat brand name),
http://pewa.panasonic.com/pcsd/product/sign/pdf/mech_eng_tn.pdf
These measure a tiny 0.22 x 0.55" and are 0.39" high.
I often use the TN2-4.5V part, rated for use at 4.5V,
but turns on at 3.38V max. Its 145-ohm coil takes
31mA at 4.5V, so it's easy to drive.

We use these a lot,

http://www.fujitsu.com/downloads/MICRO/fcai/relays/ftr-b3.pdf

fairly similar. The latching versions are handy, and have zero thermal
emf's. They switch in under a millisecond, and we've had zero failures
so far.

I still like relays.

John

 

[John Larkin]

John Larkin wrote:

John Larkin wrote:


Don't need all that:
View in a fixed-width font such as Courier.

.
. HV
. |
. 12V>--+-|>|-+--------+-----+-|<|------. |-
. | | | | | .--||
. | | C | | | |>
. | | .--B npn | | | |
. | | | E | | | |
. | [4.7K] | +-[51]-----------|-----+ |
. | | | E | | | |
. | '----+--B pnp |0.1u | | |
. [1K] | C === | | |
. | | | |Cbst | | | 1/2W
. | | '-----+----------------|-----+--[10k]-.
. | | | | | | |
. | |- +--+-|>|---+ | |> ---
. +-------||SSN1N45B | | | '--|| sgnd
. | |> | | [100] |-
. \| | | | | |
. 2N7000 ||-------+ -/ | === +---> out
. <| | 12VZ ^ ----- |0.01u |
. | | | |C555 |--' -
. --- | | |ASTBL| ^
. sgnd | | |10KHz| |
. | | ----- |
. | | | ---
. CMOS ON >----' +----' pgnd
. |
. /|\ 10mA
. CMOS EN >------------ \V/
. |
. ---
. sgnd
.
.
.



And here's a slightly simplified version. Add the PV isolator for DC
drive.

ftp://66.117.156.8/Gate_Driver.JPG

One could also use a dip dc/dc converter and the relay.

John


What makes that simpler?


Um, having 1/5 as many parts? Being easy to analyze?



And actually, as long as we know the MOSFETs
can take fairly hefty transient peak power dissipation , I would prefer
to redo the input interface and stick an integrating capacitor in there
somewhere to slow the turn on/off to 15V/us range, especially now with
several of these circuits driving a common power bus, preventing
unrestrained slew rates from inducing transient currents into high
impedance nodes.


The gate resistor controls slew rate.




Also, I would want to at least attempt a backup
interlock by connecting OUT to the gate of the 2N7000 through a resistor
divider, or something, keeping the switching off while a second power
source was on the power bus, but not turning itself off Then there's
fusing on the HV lead to consider as well as protection of the LV stuff
in the event of HV component failure, possibly UVLO and maybe thermal
shutdown and other things...


More parts!

The dc/dc converter + relay is ideal for a one-off by someone who
doesn't want to do a lot of design and pcb fab. You could build one in
20 minutes, no simulation or debugging needed.

You could even use latching relays if that would help.

John

The OP may have significant investment in the system he's driving and
should be made aware of real world problems and damage possible with
oversimplified quick-make circuits...

Or overcomplicated ones.

John

Just because it's not an NIF type of end use does not mean we trivialize
and get sloppy with the electronics.

The dc/dc converter and relay isn't a bit sloppy. It's ideal for
someone who doesn't want to do a lot of design and analysis and
electronic assembly. If I had this switching problem myself, that's
the way I'd do it.

Playing with various driver circuit ideas is fun, but the OP is
probably better off with a simple, bulletproof circuit he can wire up
in a few minutes from a few common parts.

John

 

[ehsjr]

On Thu, 20 Dec 2007 04:40:13 -0800 (PST), Winfield




We use these a lot,

http://www.fujitsu.com/downloads/MICRO/fcai/relays/ftr-b3.pdf

fairly similar. The latching versions are handy, and have zero thermal
emf's. They switch in under a millisecond, and we've had zero failures
so far.

I still like relays.

John

Know of anything small like that capable of switching
~750 mA inductive at ~18V 60 Hz? Using a G2R-14
at present.

Ed

 

[John Larkin]

Know of anything small like that capable of switching
~750 mA inductive at ~18V 60 Hz? Using a G2R-14
at present.

Ed

The Fujitsu should be OK for infrequent switching. But if it's going
to switch a lot, and you need small, an SSR would probably be more
reliable.


John

 

[John Fields]

Bounce won't matter; the initial contact hit charges the gate all the
way.

 

[John Fields]

The dc/dc converter and relay isn't a bit sloppy.

 

[John Larkin]

"touting its use as a panacea"?

What, are you studying journalism in night school?

John

 

[John Larkin]

How so? A mosfet gate will stay charged for hours or days. If the
contact bounces on break, nothing happens to the gate voltage until
the wiper hits the opposite contact.

The contact bounce will *not* be visible on the gate voltage.

John

 

[John Fields]

How so? A mosfet gate will stay charged for hours or days. If the
contact bounces on break, nothing happens to the gate voltage until
the wiper hits the opposite contact.

The contact bounce will *not* be visible on the gate voltage.

---
Don't know much about contact bounce, do you?

It could easily look something like this on ON make and OFF make
(when I said "break" I meant "OFF make") View in Courier
_____ . . .___
___/ \
/ \_
__/ \
___/ \____


Looking at your silly gate driver, since there's no way to determine
what the hash on the contact will look like when the common contact
bounces against the +12V contact, there's no way to tell how long
the contacts will stay mated until the first, (and subsequent)
bounces. Ergo, since the gate capacitance has to charge through the
series resistor the RC to Vth may be longer than the first make
time.

The same will be true when the relay common contact bounces against
the ground contact.

Also, depedending on how long it takes the MOSFET to transition from
full OFF to full ON, or full ON to full OFF, the power dissipated in
the channel may be more than trivial.

 

[John Fields]

"touting its use as a panacea"?

What, are you studying journalism in night school?

 

[John Larkin]

I know that machanical contacts can generate clean sub-nanosecond
edges, at tens to thousands of volts. And I've recently measured the
bounce behavior of the cute little Fujitsu relays.

It could easily look something like this on ON make and OFF make
(when I said "break" I meant "OFF make") View in Courier
_____ . . .___
___/ \
/ \_
__/ \
___/ \____

Consider that first transition:

When the wiper lifts off the "ground" side, the gate voltage is frozen
low. If the wiper bounces off the low side a few times before it
finally lets go, the gate voltage stays put.

While the wiper is "in the air" between contacts, the gate stays low.

When the wiper finally hits the +12 (or whatever) contact, the initial
connection, before the first bounce, will last many microseconds,
plenty of time to charge the gate to +12. Subsequent bounces, which
might last a few hundred microseconds in a small relay, don't matter,
because the gate is already charged.

Try it.

Looking at your silly gate driver, since there's no way to determine
what the hash on the contact will look like when the common contact
bounces against the +12V contact, there's no way to tell how long
the contacts will stay mated until the first, (and subsequent)
bounces. Ergo, since the gate capacitance has to charge through the
series resistor the RC to Vth may be longer than the first make
time.

Well, make the gate resistor the right value. That's what engineering
is about.

The same will be true when the relay common contact bounces against
the ground contact.

Also, depedending on how long it takes the MOSFET to transition from
full OFF to full ON, or full ON to full OFF, the power dissipated in
the channel may be more than trivial.

That's true for any gate drive. Fact is, in the 10's of volts and up
range, the rising edge of a mechanical contact closure rivals the
speed of the fastest semiconductors; subnanosecond edges are easy.

Try it.


John

 

[Fred Bloggs]

Well, two batteries can deliver current at one time if they're at the
exact same voltage, but yes uninterrupted current flow is necesarry.
And yes, it'd be problematic if one battery started charging another,
especially at 500A (the batteries I'm using are rated for 20A
continuous, 30A for up to 10 seconds, though I'm not sure about how
they handle really large transients). It's unfortunate that I have to
throw some power away in a Schottky, but as far as I can see my
options are very limited, unless I'm missing something. Doing source
to source FETs will require incredibly accurate timing along with a
large capacitor to make uninterrupted, again unless I'm missing
something. Thus it seems like the Schottky + single FET is the best
solution?

View in a fixed-width font such as Courier.

..
..
.. F1 SD1
.. --BATT1>--~---|>|---.
.. | |
.. | |
.. | | ------
.. | F2 SD2 | | |
.. +--BATT2>--~---|>|---+-------|MOSFET|------o
.. | | | SW | LOAD
.. | | ------
.. | | | o
.. | F3 SD3 | | |
.. +--BATT3>--~---|>|---' | ---
.. | | ///
.. --- |
.. /// |
.. |
.. |
.. |
.. ON/OFF >-----'
..
..
..
..

 

[Fred Bloggs]

View in a fixed-width font such as Courier.

.
.
. F1 SD1
. --BATT1>--~---|>|---.
. | |
. | |
. | | ------
. | F2 SD2 | | |
. +--BATT2>--~---|>|---+-------|MOSFET|------o
. | | | SW | LOAD
. | | ------
. | | | o
. | F3 SD3 | | |
. +--BATT3>--~---|>|---' | ---
. | | ///
. --- |
. /// |
. |
. |
. |
. ON/OFF >-----'
.
.
.
.

If you want some extra control over individual battery discharge then
there's this:
View in a fixed-width font such as Courier.

..
..
.. F1 SCR1
.. --BATT1>--~---|>|---.
.. | / |
.. | |
.. | | ------
.. | F2 SCR2 | | |
.. +--BATT2>--~---|>|---+-------|MOSFET|------o
.. | / | | SW | LOAD
.. | | ------
.. | | | o
.. | F3 SCR3 | | |
.. +--BATT3>--~---|>|---' | ---
.. | / | ///
.. --- |
.. /// |
.. |
.. |
.. |
.. ON/OFF >-----'
..
..
..
..

 

[Fred Bloggs]

Doing source
to source FETs will require incredibly accurate timing along with a
large capacitor to make uninterrupted, again unless I'm missing
something. ...

You are missing a great deal, a little something called "working
knowledge" of the practice of the art and its components. All you have
to do is explain the application with some amount of quantitative
detail, which you're not doing very well at all, and we'll decide what
is or is not "incredible."