In electronics, two commonly used semiconductor devices for switching applications are the BJT (Bipolar Junction Transistor) and the MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor). Although these devices have different characteristics and require different methods of control, they can perform the same basic function when configured as electronic switches: connecting or disconnecting a load from a circuit.

Both BJT and MOSFET switches can be configured as either low-side switches or high-side switches.

What Is a Low-Side Switch?

A low-side switch is a switching configuration in which the transistor connects the load to ground or the circuit’s low-potential node when it is turned on.

In electronics, ground is commonly associated with the terms low, low potential, or logic low. This is where the term low-side switch comes from.

A low-side switch is normally placed between the load and ground. When the transistor turns on, current can flow from the power supply, through the load, and then through the transistor to ground.

For a switch to operate properly, the BJT or MOSFET must be driven sufficiently into its low-resistance ON state. When commanded OFF, the device should stop conducting so that the circuit is effectively disconnected from ground.

For a detailed explanation of BJT saturation in switching applications, see BJT Saturation Guide.

For MOSFET switch design, see Designing MOSFET Switch – Comprehensive.

What Is a High-Side Switch?

A high-side switch is a switching configuration in which the transistor connects the load to the positive power supply when it is turned on.

The positive supply rail is often referred to as the high-potential side of the circuit. This is the reason the configuration is called a high-side switch.

A high-side switch is normally placed between the power supply and the load. When the transistor turns on, it supplies the load with the required voltage.

The main difference between the two configurations is therefore the location of the switching device:

• Low-side switch: transistor is between the load and ground.
• High-side switch: transistor is between the power supply and the load.

Low-Side Switch: Requirements and Characteristics

BJT Low-Side Switch

For a BJT low-side switch, an NPN transistor is commonly used.

The important requirements are:

• The NPN BJT must be driven sufficiently to turn on.
• The required base-emitter voltage (VBE) must be established for conduction.
• Sufficient base current must be provided to drive the transistor into saturation when hard switching is required.
• To turn the transistor off, the base-emitter junction must no longer be forward biased.
• When the transistor is turned on, it provides a conducting path between the load and ground.
• When the transistor is turned off, the conducting path to ground is removed.

The amount of base current is particularly important when the BJT is being used as a saturated switch. Insufficient base drive can prevent the transistor from reaching the desired ON state.

MOSFET Low-Side Switch

For a MOSFET low-side switch, an N-channel MOSFET (NMOS) is commonly used.

The important requirements are:

• The NMOS must be driven sufficiently to turn on.
• The required gate-to-source voltage (VGS) must be applied.
• The MOSFET’s threshold voltage (VGS(th)) indicates the beginning of conduction, but it should not be treated as the voltage required to fully turn on the MOSFET.
• Unlike a BJT, the MOSFET gate does not require continuous DC current to maintain its ON state.
• The gate-to-source voltage must be reduced sufficiently (below the VGS threshold) to turn the MOSFET off.
• When turned on, the MOSFET provides a low-resistance path between the load and ground.
• When turned off, the conducting path to ground is removed.

For switching applications, the MOSFET should be selected and driven according to its specified RDS(on) and the gate voltage at which that resistance is guaranteed

High-Side Switch: Requirements and Characteristics

BJT High-Side Switch

For a BJT high-side switch, a PNP transistor is commonly used.

The important requirements are:

• The PNP BJT must be driven sufficiently to turn on.
• The required base-emitter voltage (VBE) must be established with the correct polarity.
• Sufficient base current must be provided to drive the transistor into saturation when hard switching is required.
• To turn the transistor off, the base-emitter junction must no longer be sufficiently forward biased.
• When the transistor is turned on, it provides a conducting path from the positive supply to the load.
• When the transistor is turned off, the connection between the supply and the load is removed.

A major consideration with a PNP high-side switch is that the base voltage must be controlled relative to the emitter voltage, rather than simply relative to ground.

MOSFET High-Side Switch

For a MOSFET high-side switch, a P-channel MOSFET (PMOS) can be used.

The important requirements are:

• The PMOS must be driven sufficiently to turn on.
• The required gate-to-source voltage (VGS) must be applied with the correct polarity.
• A PMOS turns on when its gate is sufficiently lower than its source.
• To turn the PMOS off, the gate-to-source voltage must be brought sufficiently close to zero.
• When turned on, the PMOS provides a low-resistance path between the positive supply and the load.
• When turned off, the connection between the positive supply and the load is removed.

For higher-power or higher-efficiency applications, an N-channel MOSFET may also be used as a high-side switch, but it generally requires a suitable gate-drive arrangement because the source voltage rises toward the supply voltage when the MOSFET turns on.

Low-Side vs. High-Side Switching

The basic difference can be summarized as follows:

Conclusion

Both low-side and high-side switches are widely used in electronic circuits. The choice between them depends on how the load needs to be connected and controlled.

A low-side switch controls the connection between the load and ground. An NPN BJT or NMOS MOSFET is commonly used for this configuration.

A high-side switch controls the connection between the load and the positive supply. A PNP BJT or PMOS MOSFET can be used for this configuration.

Understanding the difference between these two arrangements is important when designing transistor switching circuits because the transistor type, control voltage, and drive requirements depend not only on whether the device is a BJT or MOSFET, but also on whether it is being used on the low side or high side.