load-switch

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1

Just done a little load-switch MOSFET board based on an N-P complementary
FET

Should work down to around 1.8V and up to about 20V.

I will probably make a version of this without the extra gate pin as well.

Will run off a few boards tomorrow to have a play.

Ben Dooks, http://www.fluff.org/ben/
bjdooks@googlemail.cbjdooks@googlemail.com
om

loadswitch1.pdf (10.9 KB)

2

Errm… what’s the benefit of this over a single MOSFET?On 21 December 2012 19:27, Ben Dooks bjdooks@googlemail.com wrote:

Just done a little load-switch MOSFET board based on an N-P complementary
FET

Should work down to around 1.8V and up to about 20V.

I will probably make a version of this without the extra gate pin as well.

Will run off a few boards tomorrow to have a play.

–
Ben Dooks, http://www.fluff.org/ben/ bjdooks@googlemail.com

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Alan Burlison

3

You don’t have to have both on the same supply. IN can be much higher than
the SW.On 21 December 2012 20:02, Alan Burlison alan.burlison@gmail.com wrote:

Errm… what’s the benefit of this over a single MOSFET?

On 21 December 2012 19:27, Ben Dooks bjdooks@googlemail.com wrote:

Just done a little load-switch MOSFET board based on an N-P complementary
FET

Should work down to around 1.8V and up to about 20V.

I will probably make a version of this without the extra gate pin as
well.

Will run off a few boards tomorrow to have a play.

–
Ben Dooks, http://www.fluff.org/ben/ bjdooks@googlemail.com

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4

I switched a couple of amps at 12V with a FQP30N06L MOSFET and a 5V
AVR, they are rated up to 60V and 30A.On 21 December 2012 21:24, Ben Dooks bjdooks@googlemail.com wrote:

You don’t have to have both on the same supply. IN can be much higher than
the SW.

On 21 December 2012 20:02, Alan Burlison alan.burlison@gmail.com wrote:

Errm… what’s the benefit of this over a single MOSFET?

On 21 December 2012 19:27, Ben Dooks bjdooks@googlemail.com wrote:

Just done a little load-switch MOSFET board based on an N-P
complementary
FET

Should work down to around 1.8V and up to about 20V.

I will probably make a version of this without the extra gate pin as
well.

Will run off a few boards tomorrow to have a play.

–
Ben Dooks, http://www.fluff.org/ben/ bjdooks@googlemail.com

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Alan Burlison

5

The gate drive of a FET won’t fully switch off unless it gets close to the
VIN… and the protection diodes on the IO pins will stop the line getting
all the way to VINOn 22 December 2012 00:12, Alan Burlison alan.burlison@gmail.com wrote:

I switched a couple of amps at 12V with a FQP30N06L MOSFET and a 5V
AVR, they are rated up to 60V and 30A.

On 21 December 2012 21:24, Ben Dooks bjdooks@googlemail.com wrote:

You don’t have to have both on the same supply. IN can be much higher
than
the SW.

On 21 December 2012 20:02, Alan Burlison alan.burlison@gmail.com wrote:

Errm… what’s the benefit of this over a single MOSFET?

On 21 December 2012 19:27, Ben Dooks bjdooks@googlemail.com wrote:

Just done a little load-switch MOSFET board based on an N-P
complementary
FET

Should work down to around 1.8V and up to about 20V.

I will probably make a version of this without the extra gate pin as
well.

Will run off a few boards tomorrow to have a play.

–
Ben Dooks, http://www.fluff.org/ben/ bjdooks@googlemail.com

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6

This board should be good for about 3AOn 22 December 2012 00:47, Ben Dooks bjdooks@googlemail.com wrote:

The gate drive of a FET won’t fully switch off unless it gets close to the
VIN… and the protection diodes on the IO pins will stop the line getting
all the way to VIN

On 22 December 2012 00:12, Alan Burlison alan.burlison@gmail.com wrote:

I switched a couple of amps at 12V with a FQP30N06L MOSFET and a 5V
AVR, they are rated up to 60V and 30A.

On 21 December 2012 21:24, Ben Dooks bjdooks@googlemail.com wrote:

You don’t have to have both on the same supply. IN can be much higher
than
the SW.

On 21 December 2012 20:02, Alan Burlison alan.burlison@gmail.com wrote:

Errm… what’s the benefit of this over a single MOSFET?

On 21 December 2012 19:27, Ben Dooks bjdooks@googlemail.com wrote:

Just done a little load-switch MOSFET board based on an N-P
complementary
FET

Should work down to around 1.8V and up to about 20V.

I will probably make a version of this without the extra gate pin as
well.

Will run off a few boards tomorrow to have a play.

–
Ben Dooks, http://www.fluff.org/ben/ bjdooks@googlemail.com

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Ben Dooks, http://www.fluff.org/ben/ bjdooks@googlemail.com

7

The gate drive of a FET won’t fully switch off unless it gets close to the
VIN… and the protection diodes on the IO pins will stop the line getting
all the way to VIN

So what’s the practical upshot? That there will be a voltage drop
across the MOSFET when it is turned on? I’m not sure what you mean by
it not switching off fully, because it certainly did.

8

Depending on the FET it may not turn off fully, leakage current would be
dependant on the voltage from gate-source and the FET involved.

You may have been lucky and your FET was mostly off at that point.

Also, it puts un-necessary strain on IO pins and could lead to back-drive
in the MCU is turned off.On 22 December 2012 01:03, Alan Burlison alan.burlison@gmail.com wrote:

The gate drive of a FET won’t fully switch off unless it gets close to
the
VIN… and the protection diodes on the IO pins will stop the line
getting
all the way to VIN

So what’s the practical upshot? That there will be a voltage drop
across the MOSFET when it is turned on? I’m not sure what you mean by
it not switching off fully, because it certainly did.

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Ben Dooks, http://www.fluff.org/ben/ bjdooks@googlemail.com

9

So what would be a better way to drive a FQP30N06L? There’s a long
discussion about these logic-level MOSFETs on
https://www.sparkfun.com/products/10213 and after reading it I’m more
confused than illuminated - I assumed that these particular MOSFETs
were intended to be driven directly by a 5V logic signal - they turn
on completely at around 3.5V.On 22 December 2012 02:24, Ben Dooks bjdooks@googlemail.com wrote:

Depending on the FET it may not turn off fully, leakage current would be
dependant on the voltage from gate-source and the FET involved.

You may have been lucky and your FET was mostly off at that point.

Also, it puts un-necessary strain on IO pins and could lead to back-drive in
the MCU is turned off.

On 22 December 2012 01:03, Alan Burlison alan.burlison@gmail.com wrote:

The gate drive of a FET won’t fully switch off unless it gets close to
the
VIN… and the protection diodes on the IO pins will stop the line
getting
all the way to VIN

So what’s the practical upshot? That there will be a voltage drop
across the MOSFET when it is turned on? I’m not sure what you mean by
it not switching off fully, because it certainly did.

–
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Ben Dooks, http://www.fluff.org/ben/ bjdooks@googlemail.com

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Alan Burlison

10

Ah. If you are using N channel this is fine. However I need to switch the
positive supply in this case so a P and N combination provides the best way
of doing it.On Dec 22, 2012 12:10 PM, “Alan Burlison” alan.burlison@gmail.com wrote:

So what would be a better way to drive a FQP30N06L? There’s a long
discussion about these logic-level MOSFETs on
https://www.sparkfun.com/products/10213 and after reading it I’m more
confused than illuminated - I assumed that these particular MOSFETs
were intended to be driven directly by a 5V logic signal - they turn
on completely at around 3.5V.

On 22 December 2012 02:24, Ben Dooks bjdooks@googlemail.com wrote:

Depending on the FET it may not turn off fully, leakage current would be
dependant on the voltage from gate-source and the FET involved.

You may have been lucky and your FET was mostly off at that point.

Also, it puts un-necessary strain on IO pins and could lead to
back-drive in
the MCU is turned off.

On 22 December 2012 01:03, Alan Burlison alan.burlison@gmail.com wrote:

The gate drive of a FET won’t fully switch off unless it gets close to
the
VIN… and the protection diodes on the IO pins will stop the line
getting
all the way to VIN

So what’s the practical upshot? That there will be a voltage drop
across the MOSFET when it is turned on? I’m not sure what you mean by
it not switching off fully, because it certainly did.

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11

Also if using a N channel on the high side you need to get thr gate above
Vin to make sure it turns on. Many of the switch ICs integrate a mini
charge pump to get Vgs to the necessary full on voltage.On Dec 22, 2012 12:10 PM, “Alan Burlison” alan.burlison@gmail.com wrote:

So what would be a better way to drive a FQP30N06L? There’s a long
discussion about these logic-level MOSFETs on
https://www.sparkfun.com/products/10213 and after reading it I’m more
confused than illuminated - I assumed that these particular MOSFETs
were intended to be driven directly by a 5V logic signal - they turn
on completely at around 3.5V.

On 22 December 2012 02:24, Ben Dooks bjdooks@googlemail.com wrote:

Depending on the FET it may not turn off fully, leakage current would be
dependant on the voltage from gate-source and the FET involved.

You may have been lucky and your FET was mostly off at that point.

Also, it puts un-necessary strain on IO pins and could lead to
back-drive in
the MCU is turned off.

On 22 December 2012 01:03, Alan Burlison alan.burlison@gmail.com wrote:

The gate drive of a FET won’t fully switch off unless it gets close to
the
VIN… and the protection diodes on the IO pins will stop the line
getting
all the way to VIN

So what’s the practical upshot? That there will be a voltage drop
across the MOSFET when it is turned on? I’m not sure what you mean by
it not switching off fully, because it certainly did.

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12

Ah, that all makes sense now, thanks for the explanation :slight_smile: I used it on
the low side which explains why I got it to work OK.On Dec 22, 2012 12:16 PM, “Ben Dooks” bjdooks@googlemail.com wrote:

Also if using a N channel on the high side you need to get thr gate above
Vin to make sure it turns on. Many of the switch ICs integrate a mini
charge pump to get Vgs to the necessary full on voltage.
On Dec 22, 2012 12:10 PM, “Alan Burlison” alan.burlison@gmail.com wrote:

So what would be a better way to drive a FQP30N06L? There’s a long
discussion about these logic-level MOSFETs on
https://www.sparkfun.com/products/10213 and after reading it I’m more
confused than illuminated - I assumed that these particular MOSFETs
were intended to be driven directly by a 5V logic signal - they turn
on completely at around 3.5V.

On 22 December 2012 02:24, Ben Dooks bjdooks@googlemail.com wrote:

Depending on the FET it may not turn off fully, leakage current would be
dependant on the voltage from gate-source and the FET involved.

You may have been lucky and your FET was mostly off at that point.

Also, it puts un-necessary strain on IO pins and could lead to
back-drive in
the MCU is turned off.

On 22 December 2012 01:03, Alan Burlison alan.burlison@gmail.com wrote:

The gate drive of a FET won’t fully switch off unless it gets close
to

the
VIN… and the protection diodes on the IO pins will stop the line
getting
all the way to VIN

So what’s the practical upshot? That there will be a voltage drop
across the MOSFET when it is turned on? I’m not sure what you mean by
it not switching off fully, because it certainly did.

–
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13

Yep.

I got a couple of them to etch ok. Will need to clean off the fine areas
and redo the etch as the bits under the TSOT26 did not etch very well. I
suslect they still have some paper on them.On Dec 22, 2012 3:06 PM, “Alan Burlison” alan.burlison@gmail.com wrote:

Ah, that all makes sense now, thanks for the explanation :slight_smile: I used it on
the low side which explains why I got it to work OK.
On Dec 22, 2012 12:16 PM, “Ben Dooks” bjdooks@googlemail.com wrote:

Also if using a N channel on the high side you need to get thr gate above
Vin to make sure it turns on. Many of the switch ICs integrate a mini
charge pump to get Vgs to the necessary full on voltage.
On Dec 22, 2012 12:10 PM, “Alan Burlison” alan.burlison@gmail.com wrote:

So what would be a better way to drive a FQP30N06L? There’s a long
discussion about these logic-level MOSFETs on
https://www.sparkfun.com/products/10213 and after reading it I’m more
confused than illuminated - I assumed that these particular MOSFETs
were intended to be driven directly by a 5V logic signal - they turn
on completely at around 3.5V.

On 22 December 2012 02:24, Ben Dooks bjdooks@googlemail.com wrote:

Depending on the FET it may not turn off fully, leakage current would
be
dependant on the voltage from gate-source and the FET involved.

You may have been lucky and your FET was mostly off at that point.

Also, it puts un-necessary strain on IO pins and could lead to
back-drive in
the MCU is turned off.

On 22 December 2012 01:03, Alan Burlison alan.burlison@gmail.com wrote:

The gate drive of a FET won’t fully switch off unless it gets close
to

the
VIN… and the protection diodes on the IO pins will stop the line
getting
all the way to VIN

So what’s the practical upshot? That there will be a voltage drop
across the MOSFET when it is turned on? I’m not sure what you mean by
it not switching off fully, because it certainly did.

–
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Ben Dooks, http://www.fluff.org/ben/ bjdooks@googlemail.com

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