NMOS is an abbreviation of N-channel MOSFET while PMOS is P-channel MOSFET. When to use NMOS and PMOS depends on the application. It depends mainly on where you want to place the switching device in the circuit and how the MOSFET will be driven.
As a general rule:
- Use an NMOS when you want to switch the connection to the low side, usually toward ground or a lower potential.
- Use a PMOS when you want to switch the connection to the high side, usually toward the positive supply.
This is why NMOS devices are commonly associated with low-side switches, while PMOS devices are often used for high-side switches.
Put in mind, this is a general guideline rather than an absolute rule. NMOS devices can also be used for high-side switching such as in power switching converter, but they normally require a more complex high-side gate-drive circuits.
Why Is NMOS Commonly Preferred for Low-Side Switching?
An NMOS is relatively easy to turn on when its gate is driven sufficiently positive with respect to its source. In a low-side switching circuit, the source terminal of an NMOS is typically connected to ground or a low potential.
This makes the gate-drive requirement simple. To site an example, if the source is at 0 V level, applying a positive voltage to the gate produces a positive VGS, which turns the MOSFET on once the VGS(th) is reached.
This is one reason NMOS devices are very popular for low-side switching applications.
Common NMOS Applications
1. Low-Side Switch
NMOS devices are commonly used for low-power, low-side switch such as:
- Low-side relay drivers
- Solenoid drivers
- Signal inverter circuits
- General-purpose load switching
For a low-side relay driver, for example, the relay coil can be connected between the positive supply and the NMOS drain, while the source is connected to ground.
When the NMOS turns on, current flows through the relay coil and the MOSFET to ground.
Take note, when switching an inductive load such as a relay or contactor coil, an appropriate flyback or clamp circuit should be added to the circuit to protect the MOSFET from the voltage spike generated when the coil is switched off.
2. NMOS Used to Drive a PMOS
An NMOS can also be used to control a PMOS high-side switch.
This can be useful when the control signal cannot directly pull the PMOS gate sufficiently low to turn it on.
The NMOS acts as a level-shifting or gate-pulling device, allowing a lower-voltage control signal to control the PMOS connected to the positive supply.
This arrangement is particularly useful when the switched supply voltage is higher than the available control-signal voltage.
3. Reverse Battery Protection
An NMOS can also be used for reverse-battery protection.
In this arrangement, the NMOS is placed in series with the low or negative potential path. The MOSFET can be controlled so that it allows normal current flow while blocking current when the battery is connected with the wrong polarity.
The main advantage of using a MOSFET for reverse battery protection instead of a conventional diode is that the MOSFET has very low RDS(on) that will result to a very low voltage drop when it is fully turned on.
However, need to pay attention that the exact MOSFET orientation and gate-drive arrangement are important things to consider when designing reverse-polarity protection.
4. Half-Bridge and Full-Bridge Circuits
NMOS devices are also widely used in half-bridge and full-bridge switching circuits. These circuit are often used in power supply stage like in DC-DC stage.
Examples include:
- LLC resonant converters
- Phase-shifted full-bridge (PSFB) converters
- Motor drives
- Other power-conversion circuits
For these applications, NMOS devices are preferred because they generally offer lower RDS(on) and better switching performance than comparable PMOS devices. However, an NMOS used on the high side of a bridge requires a little complex high-side gate driver circuit because the source voltage moves with the switching node.
When to Use PMOS
A PMOS is commonly used when the MOSFET needs to switch the connection to the positive supply.
This is known as high-side switching.
A PMOS can be convenient for simple high-side switching because its source is connected to the positive supply, and pulling the gate sufficiently below the source turns the device on.
For example, if the PMOS source is connected to a 12 V supply, the gate needs to be driven sufficiently below 12 V to create a negative VGS and turn the MOSFET on. When the gate is brought close to the source voltage, VGS approaches zero and the PMOS turns off.
Common PMOS Applications
1. High-Side Switch
PMOS devices are commonly used for:
- Simple low-power high-side switches
- High-side relay drivers
- Circuit power switches
- Battery-powered circuits
- Load-disconnect circuits
A PMOS is particularly convenient when a simple control circuit is needed to connect or disconnect a load from the positive supply.
2. Electronically Controlling a Power Supply
A PMOS is useful when you want to electronically turn a voltage source on and off to power another circuit.
For example, a PMOS can be placed in series with the positive supply rail. When the PMOS is turned on, power is delivered to the downstream circuit. When it is turned off, the circuit is disconnected from the supply.
This technique is often used useful for:
- Power sequencing
- Battery-powered equipment
- Power management
- Turning circuit sections on and off
3. Reverse Battery Protection
A PMOS can also be used for reverse-battery protection as the NMOS. However, the way it is connected in the circuit is different.
In this configuration, the PMOS is placed in series with the positive supply rail. It is in the high side section.
When the battery is connected correctly, the initial current path is to the body diode. When the current reaches the source pin of the MOSFET, the gate to source voltage is become negative (as the source is higher than the gate). This will turn on the PMOS. Then, the
current path afterwards will be on the MOSFET channel since it has less voltage drop than the body diode.
When the battery polarity is reversed, the current cannot flow to the body diode. As a result, there is no voltage delivered to the source pin. The MOSFET will not turn on.
NMOS vs. PMOS – A Quick Comparison
| Application | Typical Choice |
| Low-side switching | NMOS |
| Low-side relay driver | NMOS |
| Low-side solenoid driver | NMOS |
| High-side switching | PMOS for simple circuits |
| High-side relay driver | PMOS for simple circuits |
| Circuit power switch | PMOS |
| Half-bridge (for high power converter) | NMOS |
| Full-bridge (for high power converter) | NMOS |
| High-performance power converter | NMOS |
| Reverse-battery protection | NMOS or PMOS, depending on the circuit |
PMOS Is Easier to Drive for High-Side Switching – But it Is Not Always Preferred
A PMOS is easier to drive for simple high-side switch. But in high power application such in switching converter and power supply, NMOS is preferred despite of a need of a complex gate drive circuit. It is because PMOS devices generally have higher RDS(on) and lower current capability than comparable NMOS devices of similar size and cost. In terms of supply chain, NMOS has the advantage as well.
This is one reason NMOS devices dominate high-current and high-frequency power applications.
In using NMOS for high-side switch, the gate must be driven relative to the source. As the source voltage rises, the gate voltage must also be driven appropriately to maintain the required VGS.
This requires a high-side gate driver, bootstrap circuit, charge pump, or another suitable gate-drive technique.
Therefore, the choice can be summarized as follows:
For a simple low-side switch, NMOS is usually the natural choice.
For a simple high-side switch, PMOS is often the easier choice.
For high-current or high-frequency switching, NMOS is usually preferred, even for high-side applications, because of its lower RDS(on) and better power-switching performance.
Conclusion
Choosing between NMOS and PMOS becomes much easier once you identify the application and where the MOSFET will be placed in the circuit.
If the MOSFET is intended to connect or disconnect a load from ground or the low-potential side, generally an NMOS is usually the best choice.
If the MOSFET is intended to connect or disconnect a load from the positive supply, a PMOS can be considered a best choice for simple and low-power applications.
For high power applications such as LLC converters, PSFB converters, motor drives, and high-current switching, NMOS devices are generally preferred because of their lower conduction resistance and better switching characteristics. Only that the NMOS needs a complex gate drive circuit.
The key takeaway is:
NMOS is generally the first choice for low-side switching, while PMOS is convenient for simple high-side switching. For high-performance power switching, NMOS is usually preferred—even when it is used on the high side.
Understanding this basic distinction will make it much easier to choose the right MOSFET topology for your application.