Is this correct way to use a P-Channel mosfet, with the load placed between mosfet and ground?


Or, is this second schema the way it should almost always be done?

 

The first is how it's usually done.

The second can be used, but the voltage required to turn the MOSFET on depends on the voltage drop on the load. The preferred way doesn't have that problem.

 

Why do you have an additional negative supply?
Do it like this:

 

The first is a common-source circuit, the second is a source follower.
The first applies full supply voltage to the load, the second applies the supply voltage minus the gate-source turn-on voltage.

 

It is more common to put the load on the high side of an N-channel MOSFET between the drain and the positive supply rail.
Use a logic level MOSFET such as IRL540 if switching from 5V logic.

 

Why do you have an additional negative supply?
Do it like this:

Okay, is this better? (I'm unfamiliar with schematic software, so I have to cut and past from the internet, then monkey around in paint to get something legible.)

 

The first is a common-source circuit, the second is a source follower.
The first applies full supply voltage to the load, the second applies the supply voltage minus the gate-source turn-on voltage.

So a "source-follower" might have a problem turning on at low supply voltages?

I realized the following isn't a schematic, but it's what I am working with today...
If I understand, in my following pictogram, my 2n7000 is a "source-follower" and the
bs250 is a "common-source"?

 

Probably going to be told I'm wrong, but... The easiest way for me to remember is to look at the diode given in most drawings of the mosfet, P or N. If you look at how the intrinsic diode gets biased it will tell you the correct way to use the mosfet.

 

So a "source-follower" might have a problem turning on at low supply voltages?

Yes.
But the common-source also needs to have Vgs well above its threshold voltage (typically a Vgs of 10V) to fully turn on as a switch.

If you look at how the intrinsic diode gets biased it will tell you the correct way to use the mosfet.

To make sure that the intrinsic diode is always reversed biased (except in certain applications, such blocking reverse voltage to a circuit or when used as an active fly-back diode in a switch-mode power supply).

 

The part number of many Mosfets shows the Vgs for them to fully turn on. IRFxxx need 10V but IRLxxx need 4.5V.

 

Horowitz and Hill describe both transistors and mosfets from the standpoint of sourcing or sinking current in their usual configuration. PNP transistors and P-channel mosfets source current; NPN transistors and N-channel mosfets sink. That doesn't mean N-channel mosfets and NPN transistors can't be used to source.



Of course, as shortbus said, the diode is a sure sign how to connect mosfets. For BJT's, the arrow always points toward ground.

 

So a "source-follower" might have a problem turning on at low supply voltages?

With low supply voltages, you can use logic level MOSFETs that have lower threshold voltages.

I used an A03401 in a current limiting circuit and I limited the gate voltage swing to a couple volts. With that voltage range, typical on resistance at 1A is ~60 milliohms.

I limited the voltage swing because I also wanted to be able to use an NTA4151P that had a maximum Vgs of 6V. The difference in on resistance for the A03401 at Vgs=10V wasn't significant for my circuit.