What Is MOSFET Switching?

MOSFET switching means using a MOSFET as an electronic switch that rapidly alternates between a low-conduction state and a high-conduction state.

In the OFF state, the MOSFET blocks current between drain and source.

In the ON state, the MOSFET is strongly enhanced and presents a relatively low resistance between drain and source.

OFF:

Drain ─── X ─── Source


ON:

Drain ─── LOW RESISTANCE ─── Source

Unlike a mechanical switch, a MOSFET can change state thousands or millions of times per second depending on the device and circuit.

MOSFET as an Electronic Switch

For switching applications, the MOSFET is normally operated so that it spends most of its time either fully OFF or strongly ON.

The goal of a power switching design is generally to minimize the time the MOSFET spends in the transition between these two states.

N-Channel MOSFET Low-Side Switching

The simplest and most common MOSFET switching configuration uses an N-channel MOSFET as a low-side switch.

             +V
              │
             LOAD
              │
              │
              D
           N-MOSFET
              S
              │
             GND

Control ───── G

The load is connected between the positive supply and the MOSFET drain. The source is connected to ground.

When the MOSFET turns ON, current flows from the positive supply through the load and then through the MOSFET to ground.

Low-Side Switch OFF

When the gate voltage is low relative to the source:

VGS ≈ 0 V

MOSFET = OFF

The MOSFET channel is not sufficiently enhanced and the load current is interrupted.

             +V
              │
             LOAD
              │
              X
              │
           N-MOSFET
              │
             GND

The load therefore receives little or no current through the switching path.

Low-Side Switch ON

When the gate voltage rises sufficiently above the source:

VGS = positive

MOSFET = ON

The channel becomes conductive.

             +V
              │
             LOAD
              │
              │
              D
           N-MOSFET
              S
              │
             GND
             
 The MOSFET's drain-source resistance is now relatively low and current can flow through the load. 
 

Why N-Channel MOSFETs Are Preferred for Low-Side Switching

N-channel MOSFETs are particularly convenient in low-side circuits because the source can be connected directly to ground.

This means a controller can often drive the gate using a voltage referenced to the same ground.

For example:

 Source = 0 V Gate = 0 V VGS = 0 V → OFF Source = 0 V Gate = 5 V VGS = +5 V → ON 

This simple relationship makes N-channel MOSFET low-side switching very popular.

Gate Voltage Must Be Measured Relative to Source

One of the most important rules when working with MOSFETs is that the gate voltage is not an absolute value.

The relevant voltage is:

 VGS = VG - VS 

For example, if:

 VG = 12 V VS = 10 V ``` 
 then: 
 
 VGS = 12 - 10 VGS = +2 V 
 
 The MOSFET therefore sees only 2 V between gate and source. 
 
 This is particularly important in high-side switching circuits. 
 

High-Side Switching

In a high-side switch, the MOSFET is placed between the positive supply and the load.

 +V │ MOSFET │ LOAD │ GND 

High-side switching is useful because the load can remain referenced to ground while its positive supply is switched.

Both P-channel and N-channel MOSFETs can be used for high-side switching, but their gate-drive requirements are different.

P-Channel MOSFET High-Side Switching

A P-channel MOSFET is often used when a simple high-side switching circuit is required.

 +V │ S P-MOSFET D │ LOAD │ GND Gate HIGH ≈ Source → OFF Gate LOW → ON 

The source is connected to the positive supply.

To turn the MOSFET ON, the gate is pulled sufficiently below the source.

To turn it OFF, the gate is brought close to the source voltage.

N-Channel MOSFET High-Side Switching

N-channel MOSFETs can provide lower conduction resistance than many P-channel devices, but high-side switching requires the gate to be driven above the source voltage.

 +V │ D N-MOSFET S │ LOAD │ GND Gate must be: VS + required VGS ``` 
 As the source voltage rises, the gate must rise with it while maintaining the required VGS. 
 
 This normally requires a suitable high-side gate-driver circuit. 
 

High-Side Gate Driver

Common methods for driving a high-side N-channel MOSFET include:

• Bootstrap drivers
• Charge-pump drivers
• Isolated gate drivers
• Dedicated high-side driver ICs
• Transformer-isolated gate drive

The appropriate method depends on the supply voltage, switching frequency, duty cycle and circuit topology.

Gate Resistor

A gate resistor is commonly placed between the MOSFET driver and the gate.

 Driver ─── R ─── Gate ``` 
 The resistor controls the rate at which the gate capacitance is charged and discharged. 
 
 It can help reduce: 
 
 
Gate ringing
 
Oscillation
 
EMI
 
Switching overshoot
 
Excessive instantaneous driver current
 
 
 A smaller resistor generally allows faster switching, while a larger resistor generally slows the transition. 
 

Gate Pull-Down Resistor

An N-channel MOSFET gate should not normally be left floating.

A pull-down resistor can keep the MOSFET OFF when the controller output is high impedance or disconnected.

 Control ───── Gate │ R │ GND 

When the control signal is removed, the resistor pulls the gate toward ground.

Gate Pull-Up Resistor

A P-channel MOSFET used as a high-side switch commonly uses a pull-up resistor from gate to source.

 +V │ ├── Source │ P-MOSFET │ ├── Gate │ R │ +V 

The pull-up keeps the gate close to the source and therefore keeps the MOSFET OFF when no active pull-down signal is present.

Gate Capacitance

A MOSFET gate behaves approximately like a capacitor.

The driver must charge this capacitance when turning the MOSFET ON and discharge it when turning the MOSFET OFF.

This is why a MOSFET does not switch instantaneously even though its steady-state gate current is extremely small.

Gate Charge

The total gate charge is commonly specified as Qg.

It represents the amount of charge that must be supplied to the gate under the manufacturer's specified test conditions.

For repetitive switching, the approximate average gate-drive current is related to:

 I ≈ Qg × f 

where:

• Qg = gate charge
• f = switching frequency

The actual driver current during transitions is much higher than this average value.

Turn-On Process

When a gate driver applies voltage to the MOSFET gate, the gate voltage does not immediately reach its final value.

The general process is:

1. The gate capacitance begins charging.
2. VGS rises toward the threshold region.
3. The channel begins conducting.
4. The drain current increases.
5. The drain voltage changes.
6. The MOSFET becomes strongly enhanced.

The exact waveform depends on the gate driver, MOSFET and load circuit.

Turn-Off Process

When the driver removes the gate charge, the reverse process occurs.

1. The gate charge begins to discharge.
2. VGS falls.
3. The channel becomes less conductive.
4. The drain voltage changes.
5. The drain current falls.
6. The MOSFET eventually reaches the OFF state.

The driver must discharge the gate quickly enough for the required switching speed.

Switching Transition

During the transition between OFF and ON, the MOSFET simultaneously experiences significant voltage and current.

This produces switching power loss.

 Voltage │\ │ \ │ \ │ \ │ \____ │ └──────────── Time Current ____ / / / _____________/ ``` 
 During the overlap between significant voltage and current, the instantaneous power can be substantial. 
 

Switching Loss

A simplified approximation for switching loss is:

 Psw ≈ ½ × VDS × ID × (tr + tf) × f 

where:

• VDS = drain-source voltage
• ID = drain current
• tr = rise time
• tf = fall time
• f = switching frequency

This is a simplified model. Real switching losses are also affected by device capacitances, gate charge, diode recovery, circuit parasitics and switching conditions.

Conduction Loss

When the MOSFET is ON, it still has a finite resistance.

The conduction loss is approximately:

 Pcond = I² × RDS(on) 

For example, if:

 I = 20 A RDS(on) = 5 mΩ ``` 
 then: 
 
 Pcond = 20² × 0.005 Pcond = 2 W 
 
 The MOSFET must dissipate this heat. 
 

Conduction Loss vs Switching Loss

PWM Switching

Pulse-width modulation, or PWM, rapidly turns the MOSFET ON and OFF to control the average power delivered to a load.

 ON ON ON ┌───┐ ┌───┐ │ │ │ │ │ │ │ │ ┘ └───┘ └─── OFF periods between pulses 

The ratio of ON time to the total period is called the duty cycle.

 Duty Cycle = TON / T × 100% 

PWM Duty Cycle

For example, with a 50% duty cycle, the MOSFET is ON for approximately half of each switching period.

 50% duty cycle: ON ┌────┐ │ │ ────┘ └──── │ │ └────┘ OFF 

Increasing the duty cycle generally increases the average power delivered to a resistive load, although the actual relationship depends on the load and circuit topology.

MOSFET Switching a Relay

An N-channel MOSFET can be used to switch a relay coil.

 +V │ Relay │ ├───────┐ │ │ │ |<| Flyback diode │ │ D │ N-MOSFET │ S │ │ │ GND──────┘ Control ─────── G 

The relay coil is an inductive load. When the MOSFET turns OFF, the stored magnetic energy in the coil can produce a large voltage spike.

A suitable flyback-protection circuit is therefore normally required.

MOSFET Switching an Inductive Load

Inductive loads include:

• Relays
• Solenoids
• Motors
• Transformers
• Inductors
• Speaker coils

An inductor resists sudden changes in current. When the switching transistor opens the current path, the inductor can generate a high voltage as it attempts to maintain current flow.

Without suitable protection, this voltage can damage the MOSFET.

Flyback Diode

A flyback diode provides a path for current when an inductive load is switched OFF.

For a simple DC relay or solenoid:

 +V │ COIL │───────┐ │ │ │ |<| │ │ D │ N-MOSFET │ S │ │ │ GND──────┘ 

The diode orientation must be selected so that it does not normally conduct while the MOSFET is ON but provides a current path when the inductive voltage reverses during turn-off.

MOSFET Switching a DC Motor

N-channel MOSFETs are commonly used to control DC motors.

 +V │ MOTOR │ D N-MOSFET S │ GND PWM ───────── G 

PWM can control the average motor power.

Because the motor is inductive, the switching circuit must provide a suitable current path and protection against voltage transients.

MOSFET Switching an LED

A MOSFET can switch an LED load when the required current exceeds what the controller output should safely provide.

 +V │ LED │ Resistor │ D N-MOSFET S │ GND GPIO ──────── G 

The LED must have appropriate current limiting unless it is part of a module that already contains a suitable current regulator or resistor.

Microcontroller MOSFET Switching

A microcontroller can often directly control a small logic-level N-channel MOSFET.

Before connecting the MOSFET gate directly to a GPIO, check:

• GPIO output voltage
• Required MOSFET VGS
• RDS(on) at that VGS
• Gate charge
• Maximum GPIO current
• Switching frequency

For high-current or high-frequency applications, a dedicated gate driver is usually preferable.

MOSFET Gate Driver

A gate-driver circuit is designed specifically to charge and discharge the MOSFET gate quickly.

A dedicated driver can provide much higher peak current than a typical microcontroller GPIO.

Gate drivers are especially useful when:

• The MOSFET has high gate charge.
• Switching frequency is high.
• Very fast transitions are required.
• Several MOSFETs are being driven.
• High-side switching is required.

Why a Weak Gate Driver Is a Problem

If the gate is charged too slowly, the MOSFET spends more time in its transition region.

During this period, both drain current and drain-source voltage can be significant.

The result is increased switching loss and heating.

A weak driver can therefore cause a MOSFET to become hot even when its RDS(on) appears very low on the datasheet.

MOSFET Switching Frequency

The switching frequency is the number of switching cycles per second.

It is measured in hertz:

• 1 kHz = 1,000 switching cycles per second
• 10 kHz = 10,000 cycles per second
• 100 kHz = 100,000 cycles per second
• 1 MHz = 1,000,000 cycles per second

Higher frequency can reduce the size of magnetic components in power converters but generally increases switching losses.

Dead Time in Half-Bridge Switching

When two MOSFETs are used in a half-bridge, they must not normally be turned ON simultaneously.

 +V │ HIGH MOSFET │ ├──── Output │ LOW MOSFET │ GND 

If both devices turn ON at the same time, a large current can flow directly from the supply to ground.

This condition is called shoot-through.

A short delay called dead time is often inserted between turning one MOSFET OFF and turning the other MOSFET ON.

MOSFET Shoot-Through

Shoot-through can cause extremely high current because both MOSFETs provide a low-resistance path between the supply rails.

Possible consequences include:

• MOSFET failure
• Driver failure
• Excessive supply current
• PCB damage
• Fuse operation
• Severe voltage transients

Proper gate-driver timing is therefore critical in bridge circuits.

MOSFET Switching in a Half-Bridge

Half-bridge circuits are widely used in:

• Motor controllers
• Class D amplifiers
• DC-DC converters
• Inverters
• Power supplies

The high-side and low-side MOSFETs alternately conduct while the gate driver controls their timing.

MOSFET Switching in a Full Bridge

A full bridge, also called an H-bridge, uses four switching devices.

 +V │ ┌───┴───┐ MOSFET MOSFET │ │ ├─LOAD┤ │ │ MOSFET MOSFET └───┬───┘ │ GND 

By controlling the MOSFETs in appropriate pairs, the direction of current through the load can be reversed.

H-bridges are widely used for DC motor control and other bidirectional power applications.

MOSFET Switching and EMI

Fast MOSFET switching creates rapid voltage and current transitions.

These transitions can produce electromagnetic interference and ringing due to parasitic inductance and capacitance.

Methods used to control these effects include:

• Gate resistors
• Snubber networks
• Short PCB connections
• Proper grounding
• Decoupling capacitors
• Controlled switching speed
• Appropriate gate-driver layout

PCB Layout for MOSFET Switching

PCB layout can have a major effect on switching performance.

High-current switching loops should generally be kept as short as practical.

Important considerations include:

• Short gate-driver traces
• Low-inductance power loops
• Good local decoupling
• Appropriate ground connections
• Wide high-current copper paths
• Careful placement of snubbers and protection components

Practical MOSFET Switching Checklist

1. Determine the load voltage.
2. Determine the maximum load current.
3. Choose the MOSFET voltage rating with suitable margin.
4. Check RDS(on) at the actual gate voltage.
5. Check gate charge.
6. Determine the switching frequency.
7. Calculate conduction losses.
8. Estimate switching losses.
9. Check thermal performance.
10. Provide suitable gate protection.
11. Protect the MOSFET from inductive voltage spikes.
12. Check PCB layout and current paths.

Key Points

• MOSFETs can operate as very fast electronic switches.
• N-channel MOSFETs are especially convenient for low-side switching.
• P-channel MOSFETs can simplify many high-side switching circuits.
• N-channel high-side switching requires the gate to be driven above the source.
• The important control voltage is VGS, not simply the gate voltage relative to ground.
• RDS(on) determines an important part of conduction loss.
• Gate charge affects switching speed and driver requirements.
• Gate resistors can help control switching behavior and ringing.
• Inductive loads require suitable voltage-spike protection.
• PWM controls average load power by rapidly switching the MOSFET.
• Half-bridge circuits require proper dead time to prevent shoot-through.
• Fast switching requires careful PCB layout.
• Thermal design must account for both conduction and switching losses.