If I want to use an NMOS or PMOS as a voltage controlled switch, when would I know to use one over the other? I know a PMOS activates with a LOW at the gate and for an NMOS when HIGH at the gate, but would there be a different choice to be made depending on where in the circuit they are oriented? What is the difference in operation of a CMOS inverter with an inverter->NMOS replacing the PMOS?

  • \$\begingroup\$ in general: high side = pmos, low side = nmos. low side is more common, probably so that switching can be done by MCUs. \$\endgroup\$ – dandavis Feb 25 '19 at 18:28

NMOS is more easily available, switches faster, and is more efficient than PMOS.

There is only one time you would choose PMOS over NMOS:

  1. When your particular circuit has a connection for the source terminal that provides a convenient reference for your gate drive voltage
  2. and when this simplicity outweighs the efficiency of an NMOS.

For both NMOS and PMOS, the DIFFERENCE in the voltage between gate and source terminal determines whether it is open or closed. This is where I must make a critical correction to your opening post:

"I know a PMOS activates with a LOW at the gate and for an NMOS when HIGH at the gate"

This is wrong. A PMOS closes when the voltage on the gate is more negative than the source terminal. An NMOS closes when the voltage on the gate is more positive than the source terminal. Remebering that the the gate voltage is referenced to the the source voltage (not just a generic HI or LO, or assuming reference to ground) is very important because your source terminal is not always connected to a fixed voltage in some circuits, and these are the circuits that are representative of when to use a PMOS and when to use an NMOS.

It is most convenient to have the source terminal connected to a fixed rail so that you can easily supply a gate drive voltage relative to the source terminal to produce the required Vgs. Obviously, having ground as this fixed rail is most convenient of all.

In some circuits like an H-bridge, the high side MOSFETs provide some challenges. If you use a PMOS the source pins are connected directly to the positive supply rail which makes it relatively easy to provide a gate-source voltage difference to switch the MOSFET.

But if you need an NMOS for efficiency, speed, or power handling, the source pin is not connected to a fixed rail. It floats with the load voltage so you need a gate voltage referenced to this floating voltage in order to provide the appropriate Vgs to switch the MOSFET.

90%+ of the time, there is a way to re-arrange the circuit use an NMOS while maintaining simplicity. The most common times you run into times when a PMOS might be simpler is the high-side MOSFETs for half-bridges, push-pull circuits, H-bridges, reverse polarity protection circuits, and load-switches where the ground connection cannot be interrupted. And even in a lot of these cases, an NMOS and the complication of driving its floating source pin is chosen so over a PMOS due to the efficiency.

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