# Is there a way to use a P channel fet like a switch and hold it closed even when there is 0V across it?

#### mike _Jacobs

Joined Jun 9, 2021
122
Is there a way to use a P channel fet like a switch and hold it closed even when there is 0V across it?

Was trying to think of how to do that without a bipolar power supply.

I only have a single available.

Anyone have any ideas?

#### DickCappels

Joined Aug 21, 2008
10,104
0 volts across which two terminals?

#### mike _Jacobs

Joined Jun 9, 2021
122
0 volts across which two terminals?
sorry good point

0V across source and drain

#### crutschow

Joined Mar 14, 2008
33,339
0V across source and drain
The MOSFET will stay ON (closed) as long as the gate-source voltage is at or above the value given in its data sheet for the minimum Rds on-resistance.
This is typically -10V for a standard P-MOSFET.

#### mike _Jacobs

Joined Jun 9, 2021
122
The MOSFET will stay ON (closed) as long as the gate-source voltage is at or above the value given in its data sheet for the minimum Rds on-resistance.
This is typically -10V for a standard P-MOSFET.
As i said, i dont have a negative supply.

So if the source = 0V and the lowest voltage i have is ground then 0-0 = 0
Hence my question.

#### crutschow

Joined Mar 14, 2008
33,339
Perhaps if you explain exactly what you are trying to do with the MOSFET, we could better come up with a solution to that.

#### WBahn

Joined Mar 31, 2012
29,493
sorry good point

0V across source and drain
If there is 0 V across source and drain, then it doesn't matter if the transistor is conducting or not, since no current will flow in either case.

#### WBahn

Joined Mar 31, 2012
29,493
As i said, i dont have a negative supply.

So if the source = 0V and the lowest voltage i have is ground then 0-0 = 0
Hence my question.
It's the gate-source voltage, Vgs, that matters.

That means that the gate voltage would need to be 10 V below the source voltage. If the source voltage is 20 V, then the gate voltage only needs to be 10 V or less.

There are many PFETs that are fully turned on with less the ten volts. You just need to check the specs.

You REALLY need to post a schematic (or sketch) of how you want to use this transistor.

When used as a switch, the source of the PFET is normally connected directly to the positive supply voltage.

#### mike _Jacobs

Joined Jun 9, 2021
122
There is no circuit to post. I am wanting to treat a PFET like a solid state relay.
I want it to stay closed for a very particular reason. It has to do with something much more complex from a transient standpoint.
Just trying to explore if there is a way to make a PFET stay closed with 0v from S->D without a negative supply.

#### WBahn

Joined Mar 31, 2012
29,493
There is no circuit to post. I am wanting to treat a PFET like a solid state relay.
I want it to stay closed for a very particular reason. It has to do with something much more complex from a transient standpoint.
Just trying to explore if there is a way to make a PFET stay closed with 0v from S->D without a negative supply.
Again, if there is 0 V from source-to-drain, what does it matter whether it is closed or not? No current will flow in either case.

If you want to keep it in the linear region, the you need to keep the gate voltage lower than the source voltage by at least the threshold voltage. That does NOT mean that you need a negative supply.

Again, if the source is at 20 V and the there is 0 V between source and drain, that just means that the drain is also at 20 V. If the threshold voltage is -8 V, then applying 5 V to the gate will give you a gate-source voltage of -15 V and the transistor will be on (i.e., closed) very hard.

#### sghioto

Joined Dec 31, 2017
5,088
If there is 20 volts on the source and drain then the FET is already ON.

#### crutschow

Joined Mar 14, 2008
33,339
I am wanting to treat a PFET like a solid state relay.
Again, what is the purpose of using a PFET for this purpose?
What do you want to switch?
Is it some secret application?
You must have some usage in mind.

If you only have a positive supply to bias the gate, the use an N-MOSFET.

#### AnalogKid

Joined Aug 1, 2013
10,778
Try this . . .

1. What voltage is powering your circuit?

2. What voltage do you want the FET to switch on and off?

3. How much current will be going through the FET?

To be "turned on", the gate of a p-channel FET must be negative *with respect to its source*. That is *not* to say it needs a voltage that is negative with respect to the circuit GND. If you have a circuit with a +12 V supply, and that is what you want to switch with the FET, then when the gate is tied to GND, it is at -12 V with respect to the source, which is up at +12 V.

Positive and negative always are relative to something. In this case, the gate is negative relative to the source.

ak

#### WBahn

Joined Mar 31, 2012
29,493
If there is 20 volts on the source and drain then the FET is already ON.
Not if the gate is also at 20 V.

The TS seems to be struggling with the very critical distinction between the voltage at a node (as measured relative to some arbitrarily defined "ground") and voltage between two nodes.

#### cmartinez

Joined Jan 17, 2007
8,122
Is there a way to use a P channel fet like a switch and hold it closed even when there is 0V across it?

Was trying to think of how to do that without a bipolar power supply.

I only have a single available.

Anyone have any ideas?
Hello, Mike. Try googling "high side pFet".

If I'm understanding you correctly, you'd like a circuit that would "latch" its state even if voltage to the pFet is removed?

#### LowQCab

Joined Nov 6, 2012
3,575
Please provide at least a Block-Diagram-Drawing and
a full and detailed description of the problem that You are trying to solve.
.
.
.

#### mike _Jacobs

Joined Jun 9, 2021
122
Hello, Mike. Try googling "high side pFet".

If I'm understanding you correctly, you'd like a circuit that would "latch" its state even if voltage to the pFet is removed?
Correct

that is a good way to think of it.

#### cmartinez

Joined Jan 17, 2007
8,122
Correct

that is a good way to think of it.
Maybe you could describe to us, step by step, exactly what you'd like your circuit to do.

#### DickCappels

Joined Aug 21, 2008
10,104
And in addition to @cmartinez 's question, how it the P-channel switch to be connected between the + power supply and the load. Is the load grounded? This should not be had to get right once we understand what you want to do.

#### AnalogKid

Joined Aug 1, 2013
10,778
If I'm understanding you correctly, you'd like a circuit that would "latch" its state even if voltage to the pFet is removed?
Correct that is a good way to think of it.
OK . . . how is that supposed to work? I probably missed something here, but . . . The most basic solid state latch + power switch requires two transistors, and transistors need electrical energy to function. Other than a flash memory cell, how is the latch state to be remembered without power?

ak