P-channel MOSFET gate driver...

[James]

I am building a transfer switch that has two 12v DC inputs and one 12v
DC output, current is 10A peak and 4A normally. The switch could
comprise 2 pairs of back-to-back P-channel MOSFETs, (the back-to-back
prevents reverse flows through the MOSFET\'s body diode). Should one
input fail I need to switch between inputs fast enough such that the
output remains on but it is not switching at high frequency.

The inherent gate capacitance of MOSFETs causes a current surge on
switch on/off. I see the advantages of a gate driver [1] but need help
with selection. The surge current is calculated multiplying the total
charge by the switch time. The suggestion by diyodemag for high-side
P-channel [1] is a TPS2812P [2] which has a 2A peak current.

Q: If the MOSFET Qg x dt says more than the peak supplied by the gate
driver what happens?
A. The gate driver blows up.
B. The current is limited and the switching time is extended.

If the switch time is extended I assume there is a little more internal
heating because it is part-on for longer. Does one add a series
resister to the gate drive output to limit current?

Microchip Application Note 799 [3] helps and its Table 3 matches devices
to gate capacitance. There are devices with higher peak currents, eg,
the TC4420/TC4421 [4] [5] deliver 6A/9A. These appear to pull the
output between Vdd and 0V but is this source/sink, compatible with N- or
P-channel, high/low side switching?

The spec sheets talk about peak output current but presumably it is an
input current when the output voltage is falling. Can one assume the
input current is the same as the output current?

Any other suggestions? eg, fancy chips like the LTC4416-1 [6] but I
don\'t need its internal comparators and just the ability to flip on/off
is enough.


Thank you.


1. https://diyodemag.com/education/mosfet_drivers_why_you_need_them
2. https://www.ti.com/lit/gpn/tps2812
3. https://ww1.microchip.com/downloads/en/Appnotes/00799b.pdf
4. https://ww1.microchip.com/downloads/en/DeviceDoc/21419D.pdf
5. https://ww1.microchip.com/downloads/en/DeviceDoc/20001420F.pdf
6.
https://www.analog.com/media/en/technical-documentation/data-sheets/4416fa.pdf

 

[John Larkin]

On Sat, 21 Jan 2023 15:16:17 +0000, James <news@oxdrove.co.uk> wrote:

I am building a transfer switch that has two 12v DC inputs and one 12v
DC output, current is 10A peak and 4A normally. The switch could
comprise 2 pairs of back-to-back P-channel MOSFETs, (the back-to-back
prevents reverse flows through the MOSFET\'s body diode). Should one
input fail I need to switch between inputs fast enough such that the
output remains on but it is not switching at high frequency.

The inherent gate capacitance of MOSFETs causes a current surge on
switch on/off. I see the advantages of a gate driver [1] but need help
with selection. The surge current is calculated multiplying the total
charge by the switch time. The suggestion by diyodemag for high-side
P-channel [1] is a TPS2812P [2] which has a 2A peak current.

Q: If the MOSFET Qg x dt says more than the peak supplied by the gate
driver what happens?
A. The gate driver blows up.
B. The current is limited and the switching time is extended.

If the switch time is extended I assume there is a little more internal
heating because it is part-on for longer. Does one add a series
resister to the gate drive output to limit current?

Microchip Application Note 799 [3] helps and its Table 3 matches devices
to gate capacitance. There are devices with higher peak currents, eg,
the TC4420/TC4421 [4] [5] deliver 6A/9A. These appear to pull the
output between Vdd and 0V but is this source/sink, compatible with N- or
P-channel, high/low side switching?

The spec sheets talk about peak output current but presumably it is an
input current when the output voltage is falling. Can one assume the
input current is the same as the output current?

Any other suggestions? eg, fancy chips like the LTC4416-1 [6] but I
don\'t need its internal comparators and just the ability to flip on/off
is enough.


Thank you.


1. https://diyodemag.com/education/mosfet_drivers_why_you_need_them
2. https://www.ti.com/lit/gpn/tps2812
3. https://ww1.microchip.com/downloads/en/Appnotes/00799b.pdf
4. https://ww1.microchip.com/downloads/en/DeviceDoc/21419D.pdf
5. https://ww1.microchip.com/downloads/en/DeviceDoc/20001420F.pdf
6.
https://www.analog.com/media/en/technical-documentation/data-sheets/4416fa.pdf

If you post a schematic of your proposed circuit, we could understand
the situation better.

Why not two schottky diodes? I\'m designing some boxes that can run off
either USB 5 volts or a wall wart up to 48v, and that turns out to be
the most sensible way: diode OR at the 5v level.

 

[James]

On 21/01/2023 15:56, John Larkin wrote:

https://www.analog.com/media/en/technical-documentation/data-sheets/4416fa.pdf

If you post a schematic of your proposed circuit, we could understand
the situation better.

Figure 4 in the LTC4416 documentation is it. V1 and V2 in, Vs out. G1
to one pair of back-to-back N-channel MOSFETS controlling V1, G2 to
another pair for V2. E1 and E2 are logic control inputs. I assumed
this was a classic arrangement.

The Block Diagram of the LTC4416 shows pins are connected to comparators
which allows auto control with external resisters. I just need logic
inputs and no load sharing.

The LTC4416 has 0.5A output. The example MOSFET in figure 4 is an
Si7483ADP, 120 / 33 * 2 (charge / dt by 2) exceeds the drive gate
current. What gives?

Why not two schottky diodes? I\'m designing some boxes that can run off
either USB 5 volts or a wall wart up to 48v, and that turns out to be
the most sensible way: diode OR at the 5v level.

Power / voltage drop and lack of control. I want to turn on/off. It is
a transfer switch not [just] an ideal diode OR.

Thank you.

 

[Fred Bloggs]

On Saturday, January 21, 2023 at 10:16:30 AM UTC-5, James wrote:

I am building a transfer switch that has two 12v DC inputs and one 12v
DC output, current is 10A peak and 4A normally. The switch could
comprise 2 pairs of back-to-back P-channel MOSFETs, (the back-to-back
prevents reverse flows through the MOSFET\'s body diode).

You don\'t need \"back-to-back\" P-FETs. If you think about it for even a minute, back-to-back is only necessary to block current flow from the battery to the load. If your load voltage is being maintained by backup, there will be no such current flow because the body diode is reverse biased. In your situation you need only 2x P-FETs, drains to individual power source batt or whatever, and sources joined to the load. One clever trick technique is to pull-up the gate drive of the alternate source P-FET with primary battery voltage. The primary source fet is controlled by a window comparator, you will need hysteresis and immunity to transient voltage dips due to load surge.

 

[John Larkin]

On Sat, 21 Jan 2023 19:37:58 +0000, James <news@oxdrove.co.uk> wrote:

On 21/01/2023 15:56, John Larkin wrote:

https://www.analog.com/media/en/technical-documentation/data-sheets/4416fa.pdf

If you post a schematic of your proposed circuit, we could understand
the situation better.

Figure 4 in the LTC4416 documentation is it. V1 and V2 in, Vs out. G1
to one pair of back-to-back N-channel MOSFETS controlling V1, G2 to
another pair for V2. E1 and E2 are logic control inputs. I assumed
this was a classic arrangement.

The Block Diagram of the LTC4416 shows pins are connected to comparators
which allows auto control with external resisters. I just need logic
inputs and no load sharing.

The LTC4416 has 0.5A output. The example MOSFET in figure 4 is an
Si7483ADP, 120 / 33 * 2 (charge / dt by 2) exceeds the drive gate
current. What gives?


Why not two schottky diodes? I\'m designing some boxes that can run off
either USB 5 volts or a wall wart up to 48v, and that turns out to be
the most sensible way: diode OR at the 5v level.

Power / voltage drop and lack of control. I want to turn on/off. It is
a transfer switch not [just] an ideal diode OR.

Thank you.

Two diodes looks easier.

 

[James]

On 21/01/2023 23:07, John Larkin wrote:

> Two diodes looks easier.

....but I can not control with a micro processor or any logic input.
Please, as per my original question if anyone knows about MOSFETs and
gate drivers.

 

[whit3rd]

On Saturday, January 21, 2023 at 7:16:30 AM UTC-8, James wrote:

I am building a transfer switch that has two 12v DC inputs and one 12v
DC output, current is 10A peak and 4A normally. The switch could
comprise 2 pairs of back-to-back P-channel MOSFETs, (the back-to-back
prevents reverse flows through the MOSFET\'s body diode). Should one
input fail I need to switch between inputs fast enough such that the
output remains on but it is not switching at high frequency.

The inherent gate capacitance of MOSFETs causes a current surge on
switch on/off.

So? That current will flow to/from the voltage sources (harmless) or to the
load, which is presumably low-impedance (draws lots of current anyhow).
If you want the load to stay ON during a switch, it has to have an input
capacitor anyhow, which will totally dominate the gate capacitance.

 

[James]

On 22/01/2023 17:27, whit3rd wrote:

On Saturday, January 21, 2023 at 7:16:30 AM UTC-8, James wrote:
I am building a transfer switch that has two 12v DC inputs and one 12v
DC output, current is 10A peak and 4A normally. The switch could
comprise 2 pairs of back-to-back P-channel MOSFETs, (the back-to-back
prevents reverse flows through the MOSFET\'s body diode). Should one
input fail I need to switch between inputs fast enough such that the
output remains on but it is not switching at high frequency.

The inherent gate capacitance of MOSFETs causes a current surge on
switch on/off.

So? That current will flow to/from the voltage sources (harmless) or to the
load, which is presumably low-impedance (draws lots of current anyhow).

I\'m asking about the gate current and its driver, not the source-drain.

 

[Jan Panteltje]

On a sunny day (Mon, 23 Jan 2023 09:00:40 +0000) it happened James
<news@oxdrove.co.uk> wrote in <tqlibo$3i6lm$1@dont-email.me>:

On 22/01/2023 17:27, whit3rd wrote:
On Saturday, January 21, 2023 at 7:16:30 AM UTC-8, James wrote:
I am building a transfer switch that has two 12v DC inputs and one 12v
DC output, current is 10A peak and 4A normally. The switch could
comprise 2 pairs of back-to-back P-channel MOSFETs, (the back-to-back
prevents reverse flows through the MOSFET\'s body diode). Should one
input fail I need to switch between inputs fast enough such that the
output remains on but it is not switching at high frequency.

The inherent gate capacitance of MOSFETs causes a current surge on
switch on/off.

So? That current will flow to/from the voltage sources (harmless) or to the
load, which is presumably low-impedance (draws lots of current anyhow).

I\'m asking about the gate current and its driver, not the source-drain.

Look up how MOSFETs work
The gate electrode is separated from the main current channel by a thin insulating layer
So normally no \'gate current\' is needed, much like a grid in a electron tube.
There is however, due to that construction, a CAPACITANCE, the gate and its
insulating layer and main channel form a capacitor.

This maybe be a very small value capacitor for for example small signal RF MOSFETs (a few pF)
or a rather big one for power MOSFETs (several nF).
The drive circuit impedance then determines how fast his capacitor is loaded, so
how fast the rise-time of the gate voltage and the controlled current as result of that.
There are some other capacitive effects too (drain gate for example).
Look up the datasheet of the MOSFET for the value of the Cgs (gate source capacitance).

The other important parameter is where the conduction happens versus gate voltage.
So I drain versus Vgs (drain current versus gate source voltage).
And beware of exceeding maximum allowed gate voltage, exceeding that will break down the gate insulating layer,

Also some MOSFETS have a reverse diode in parallel, with its own reverse breakdown characteristics.

Not sure if that the question was, but those are things that count when designing.

 

[James]

On 23/01/2023 09:51, Jan Panteltje wrote:

The gate electrode is separated from the main current channel by a thin insulating layer
So normally no \'gate current\' is needed, much like a grid in a electron tube.
There is however, due to that construction, a CAPACITANCE, the gate and its
insulating layer and main channel form a capacitor.
This maybe be a very small value capacitor for for example small signal RF MOSFETs (a few pF)
or a rather big one for power MOSFETs (several nF).

[As I understand] There is no gate current when just on/off. The
capacitance causes a current flow when going between on and off. The
charge is small but the period is short which leads to the values as
high as 10A being suggested. Gate drives are deigned to supply this
brief transient current. [There is a power requirement if cycling
rapidly at high frequency - I am not.]

The drive circuit impedance then determines how fast his capacitor is loaded, so
how fast the rise-time of the gate voltage and the controlled current as result of that.

So a 1 Amp gate driver will not fuse if the capacitance asks for more
than 1A? It will just fill/dump more slowly and switch on/off more
slowly? ie, gate drivers are current limiting devices.

The other important parameter is where the conduction happens versus gate voltage.
So I drain versus Vgs (drain current versus gate source voltage).
And beware of exceeding maximum allowed gate voltage, exceeding that will break down the gate insulating layer,

OK, thanks.

> Also some MOSFETS have a reverse diode in parallel, with its own reverse breakdown characteristics.

Yes, it is why I need the back-to-back arrangement to stop the reverse
current flow.


Thank you.

 

[Jan Panteltje]

On a sunny day (Mon, 23 Jan 2023 11:38:09 +0000) it happened James
<news@oxdrove.co.uk> wrote in <tqlrj2$3jo73$1@dont-email.me>:


>[As I understand] There is no gate current when just on/off.

Right.

The capacitance causes a current flow when going between on and off. The
charge is small but the period is short which leads to the values as
high as 10A being suggested. Gate drives are deigned to supply this
brief transient current. [There is a power requirement if cycling
rapidly at high frequency - I am not.]


The drive circuit impedance then determines how fast his capacitor is loaded, so
how fast the rise-time of the gate voltage and the controlled current as result of that.

So a 1 Amp gate driver will not fuse if the capacitance asks for more
than 1A? It will just fill/dump more slowly and switch on/off more
slowly? ie, gate drivers are current limiting devices.

There are no infinities in practice, so all drivers have a current limit....


The famous equation Q = C.U = i.t goes.
So the charge time t to voltage U by a constant current i is t = (C.U) / i
If the drive circuit is resistive and not a constant current than the normal exponential RC charge curve time goes.

Note that a very fast drive pulse rise - or fall time may still exceed gate capabilities as then
the gate current peaks to very high values.
That is more a theoretical case, normally that would not happen.

If the rise time of the gate voltage is slow then the MOSFET will dissipate more during the linear
part of the drain current Ids versus gate voltage Vgs curve.
Not what you want in a switching application.

My idea, get some MOSFET and play with it,,,,,

 

[whit3rd]

On Monday, January 23, 2023 at 3:38:17 AM UTC-8, James wrote:

On 23/01/2023 09:51, Jan Panteltje wrote:
The gate electrode is separated from the main current channel by a thin insulating layer
So normally no \'gate current\' is needed, much like a grid in a electron tube.
There is however, due to that construction, a CAPACITANCE, the gate and its
insulating layer and main channel form a capacitor.
This maybe be a very small value capacitor for for example small signal RF MOSFETs (a few pF)
or a rather big one for power MOSFETs (several nF).
[As I understand] There is no gate current when just on/off. The
capacitance causes a current flow when going between on and off. The
charge is small but the period is short which leads to the values as
high as 10A being suggested. Gate drives are deigned to supply this
brief transient current. [There is a power requirement if cycling
rapidly at high frequency - I am not.]
The drive circuit impedance then determines how fast his capacitor is loaded, so...

As a practical matter, the driver either has intrinsic current limiting, like a
transistor with emitter resistor, or one can add a simple series resistor to the gate.

 

[Fred Bloggs]

On Monday, January 23, 2023 at 4:07:42 PM UTC-5, whit3rd wrote:

On Monday, January 23, 2023 at 3:38:17 AM UTC-8, James wrote:
On 23/01/2023 09:51, Jan Panteltje wrote:
The gate electrode is separated from the main current channel by a thin insulating layer
So normally no \'gate current\' is needed, much like a grid in a electron tube.
There is however, due to that construction, a CAPACITANCE, the gate and its
insulating layer and main channel form a capacitor.
This maybe be a very small value capacitor for for example small signal RF MOSFETs (a few pF)
or a rather big one for power MOSFETs (several nF).
[As I understand] There is no gate current when just on/off. The
capacitance causes a current flow when going between on and off. The
charge is small but the period is short which leads to the values as
high as 10A being suggested. Gate drives are deigned to supply this
brief transient current. [There is a power requirement if cycling
rapidly at high frequency - I am not.]
The drive circuit impedance then determines how fast his capacitor is loaded, so...

As a practical matter, the driver either has intrinsic current limiting, like a
transistor with emitter resistor, or one can add a simple series resistor to the gate.

He doesn\'t have to \"switch\" anything. The FETs have a VDS on the order of a diode drop across them. All he has to do is enhance the channel with a 10mA grade gate drive, no need to shake the house down with a multi-ampere job.

 

[James]

On 23/01/2023 12:26, Jan Panteltje wrote:

The drive circuit impedance then determines how fast his capacitor is loaded, so
how fast the rise-time of the gate voltage and the controlled current as result of that.

So a 1 Amp gate driver will not fuse if the capacitance asks for more
than 1A? It will just fill/dump more slowly and switch on/off more
slowly? ie, gate drivers are current limiting devices.

There are no infinities in practice, so all drivers have a current limit....

How does the current limit manifest itself? If I plug a cooker into a
1A supply it is current limited, the fuse blows / trips and if it didn\'t
fail it wouldn\'t bake a cake at 50 degC in 10 hours.

The workings of the driver are opaque to me; I assume there is some
cleverness to justify the their existence else we would just use a
single transistor.
Do I need a gate driver with enough \"zap\" to match the MOSFET gate
charge? [Does the cleverness mean at high frequency the charge coming
out is stored and fed back in on the next cycle?]
Will the driver fuse?
Will it run out of power halfway though switch on?
Do I need to add a current limiting resister between the driver and the
gate? [As shown on some application schematics but no actual values are
given, how to calculate?]

If the rise time of the gate voltage is slow then the MOSFET will dissipate more during the linear
part of the drain current Ids versus gate voltage Vgs curve.
Not what you want in a switching application.

This is probably only an issue at a high frequency. I think mine will
be acceptable to switch in 60ns rather than 30ns.


> My idea, get some MOSFET and play with it,,,,,

Buy 10 at a time. Prepare for whiffs of smoke...

I\'m still not clear if the eg Microchip TC44xx series work with N or P
channel.

Thank you.

 

[Jan Panteltje]

On a sunny day (Wed, 25 Jan 2023 09:52:20 +0000) it happened James
<news@oxdrove.co.uk> wrote in <tqqu4m$k564$1@dont-email.me>:

On 23/01/2023 12:26, Jan Panteltje wrote:

The drive circuit impedance then determines how fast his capacitor is loaded, so
how fast the rise-time of the gate voltage and the controlled current as result of that.

So a 1 Amp gate driver will not fuse if the capacitance asks for more
than 1A? It will just fill/dump more slowly and switch on/off more
slowly? ie, gate drivers are current limiting devices.

There are no infinities in practice, so all drivers have a current limit....

How does the current limit manifest itself? If I plug a cooker into a
1A supply it is current limited, the fuse blows / trips and if it didn\'t
fail it wouldn\'t bake a cake at 50 degC in 10 hours.

Yep, always something will give way, no infinities.
mamaticians came up with infinities doing their divide by zero etc ..

The workings of the driver are opaque to me; I assume there is some
cleverness to justify the their existence else we would just use a
single transistor.

No, you likely need a totempole:
delivering power for voltage going high and voltage going low,
to be able to charge / discharge the MOSFET gate capacitance fast enough.

Do I need a gate driver with enough \"zap\" to match the MOSFET gate
charge?

Yes

[Does the cleverness mean at high frequency the charge coming
out is stored and fed back in on the next cycle?]
Will the driver fuse?

Well even in a totempole its drive will limit current.

Will it run out of power halfway though switch on?

?

Do I need to add a current limiting resister between the driver and the
gate? [As shown on some application schematics but no actual values are
given, how to calculate?]

Likely not, more often gate resistors are added to prevent oscillation in a linear MOSFET circuit
at some very high frequency, but in a switcher these are likely not needed.

If the rise time of the gate voltage is slow then the MOSFET will dissipate more during the linear
part of the drain current Ids versus gate voltage Vgs curve.
Not what you want in a switching application.

This is probably only an issue at a high frequency. I think mine will
be acceptable to switch in 60ns rather than 30ns.

That seems pretty fast to me...
You can start with the switching time required, find the gate voltage swing needed for full conductance to no conductance
look up the gate capacitance, then you can calculate how much current the driver needs to supply.

My idea, get some MOSFET and play with it,,,,,

Buy 10 at a time. Prepare for whiffs of smoke...

Na, just measure one on the bench, Vgs versus Id at some Vdd voltage... etc

I\'m still not clear if the eg Microchip TC44xx series work with N or P
channel.

Never used that particular chip....
seems every semiconductor company makes driver chips...

Maybe these days using just transistors is safer
as no teling if a particular chip is still available in a few years.
Same for transistors, but there is more choice there..

Strange no chips no cars, old diesel would just work and get you everywhere.
Can people still find their way without GPS these days???

I only got lost once at night, a $1 compass helped me out...

Too much tronix is no good ...

 

[whit3rd]

On Wednesday, January 25, 2023 at 1:52:29 AM UTC-8, James wrote:

On 23/01/2023 12:26, Jan Panteltje wrote:

The drive circuit impedance then determines how fast his capacitor is loaded, so
how fast the rise-time of the gate voltage and the controlled current as result of that.

So a 1 Amp gate driver will not fuse if the capacitance asks for more
than 1A? It will just fill/dump more slowly and switch on/off more
slowly? ie, gate drivers are current limiting devices.

There are no infinities in practice, so all drivers have a current limit...

How does the current limit manifest itself?

Maybe by adding a resistor in series with the gate, maybe with an inductor to
tame the surge (might be just a ferrite bead).

The workings of the driver are opaque to me; I assume there is some
cleverness to justify the their existence else we would just use a
single transistor.

Don\'t assume that! Make the \"driver\" manufacturer show you a
justification.

I\'m still not clear if the eg Microchip TC44xx series work with N or P
channel.

The marketers of that series reuse the same prefix for a variety of different
units, some for single NMOS drive, some for PMOS, some for bridge drive... so
the best answer is \'yes, no, and maybe\'.

 

[piglet]

On 25/01/2023 9:52 am, James wrote:

On 23/01/2023 12:26, Jan Panteltje wrote:

The drive circuit impedance then determines how fast his capacitor
is loaded, so
how fast the rise-time of the gate voltage and the controlled
current as result of that.

So a 1 Amp gate driver will not fuse if the capacitance asks for more
than 1A?  It will just fill/dump more slowly and switch on/off more
slowly?  ie, gate drivers are current limiting devices.

There are no infinities in practice, so all drivers have a current
limit....

How does the current limit manifest itself?  If I plug a cooker into a
1A supply it is current limited, the fuse blows / trips and if it didn\'t
fail it wouldn\'t bake a cake at 50 degC in 10 hours.

The workings of the driver are opaque to me; I assume there is some
cleverness to justify the their existence else we would just use a
single transistor.
Do I need a gate driver with enough \"zap\" to match the MOSFET gate
charge?  [Does the cleverness mean at high frequency the charge coming
out is stored and fed back in on the next cycle?]
Will the driver fuse?
Will it run out of power halfway though switch on?
Do I need to add a current limiting resister between the driver and the
gate? [As shown on some application schematics but no actual values are
given, how to calculate?]

If the rise time of the gate voltage is slow then the MOSFET will
dissipate more during the linear
part of the drain current Ids versus gate voltage Vgs curve.
Not what you want in a switching application.

This is probably only an issue at a high frequency.  I think mine will
be acceptable to switch in 60ns rather than 30ns.


My idea, get some MOSFET and play with it,,,,,

Buy 10 at a time.  Prepare for whiffs of smoke...

I\'m still not clear if the eg Microchip TC44xx series work with N or P
channel.

Thank you.

The TC44xx series will work fine with N channel devices if the mosfet
source is at or close to GND potential and will work fine with P channel
devices if the mosfet source is at or close to VDD potential.

But you said you were not switching at many kHZ - this is just a DC
transfer switch? So assuming your load has some bypass capacitance to
ride out a microsecond glitch then you really shouldn\'t need any kind of
rapid gate driver - a simple DIY descrete circuit will be fine.

You might be able to reduce your 4 mosfets to just 3 and still retain
the isolation and swicthing you want.

piglet

 

[James]

On 25/01/2023 22:07, piglet wrote:

The TC44xx series will work fine with N channel devices if the mosfet
source is at or close to GND potential and will work fine with P channel
devices if the mosfet source is at or close to VDD potential.

Perfect answer, thank you.

Sorry for not understanding this but the specifications does not make it
explicit although I could see no reason from the functional diagram why
not (but I am not an electrical engineer, hence my need to ask the
experts). Also eg www.mouser.co.uk filters them as \"low side\".

But you said you were not switching at many kHZ - this is just a DC
transfer switch? So assuming your load has some bypass capacitance to
ride out a microsecond glitch then you really shouldn\'t need any kind of
rapid gate driver - a simple DIY descrete circuit will be fine.

Correct and understood. I need some isolation between the 3.3v logic to
the power side and some current amplification device - connecting a
ESP32 through a resistor (40mA max) or using 4N25 works for low powers
but maybe not at when switching 10 Amps with high capacitance gates. I
thought a gate driver was as easy as anything and at under £2 for dual
device affordable.

You might be able to reduce your 4 mosfets to just 3 and still retain
the isolation and swicthing you want.

I want to run either input on with the other off. The feeds in are
off-the-shelf parts so I can\'t know if back EMF is a problem, caution
says reverse protect both.


Thank you.


James.

 

[piglet]

On 26/01/2023 11:33 am, James wrote:

On 25/01/2023 22:07, piglet wrote:

The TC44xx series will work fine with N channel devices if the mosfet
source is at or close to GND potential and will work fine with P channel
devices if the mosfet source is at or close to VDD potential.

Perfect answer, thank you.

Sorry for not understanding this but the specifications does not make it
explicit although I could see no reason from the functional diagram why
not (but I am not an electrical engineer, hence my need to ask the
experts).  Also eg www.mouser.co.uk filters them as \"low side\".


But you said you were not switching at many kHZ - this is just a DC
transfer switch? So assuming your load has some bypass capacitance to
ride out a microsecond glitch then you really shouldn\'t need any kind of
rapid gate driver - a simple DIY descrete circuit will be fine.

Correct and understood.  I need some isolation between the 3.3v logic to
the power side and some current amplification device - connecting a
ESP32 through a resistor (40mA max) or using 4N25 works for low powers
but maybe not at when switching 10 Amps with high capacitance gates.  I
thought a gate driver was as easy as anything and at under £2 for dual
device affordable.


You might be able to reduce your 4 mosfets to just 3 and still retain
the isolation and swicthing you want.

I want to run either input on with the other off.  The feeds in are
off-the-shelf parts so I can\'t know if back EMF is a problem, caution
says reverse protect both.


Thank you.


James.

Is your 3.3V logic that controls the switching itself powered by the
switch. Just curious if a startup issue could arise?

piglet

 

[James]

On 26/01/2023 14:14, piglet wrote:

Is your 3.3V logic that controls the switching itself powered by the
switch. Just curious if a startup issue could arise?

One side is battery backed (and solar powered) and can be assumed always
available. Even depleted it will be enough to power the controller. I
do need to consider the default power path and maybe add some \"passive\"
components to achieve this (not wait for a micro controller to boot/react).

Now you have made me wonder if each MOSFET pair and driver needs to be
powered from its own input side. The voltages on each side are within
0.1v of each other so gate should be close enough to source to turn off
from either.