A MOSFET can be a very simple and effective device for switching electronic loads. But, even for a basic MOSFET switch, there are conditions and requirements to make it operate properly and safely. What are the minimum requirements for a MOSFET switch? Below details provide complete answer to this question.
For a simple, low-power MOSFET switch, there are three basic requirements:
- A supply voltage
- A suitable gate-to-source voltage
- A means of limiting the drain current
If the gate-drive voltage can float when the MOSFET is commanded to turn off, an additional gate pull-down resistor is necessary.
For high-frequency switching applications, such as switching converters (buck, boost, LLC converter, Flyback and so on), a small gate series resistor may also be necessary. It helps control the gate current and can smooth-en ringing and voltage overshoot.
For power MOSFET applications, another important consideration is that the gate-to-source voltage must be driven sufficiently high; typically beyond the Miller plateau—to achieve low conduction losses.
There is an additional requirement when the MOSFET is used to switch an inductive load – need to add kickback voltage protection.
What Are the Minimum Requirements of a MOSFET Switch?
The Minimum Requirement: A Simple, Low-Power Switch
Consider a basic MOSFET switching circuit with three important elements:
- Vdd – the supply voltage
- Vgg – the gate-to-source drive voltage
- R2 – the drain-current-limiting resistor
Each has a specific purpose.
1. Vdd – The Supply Voltage
The supply voltage, Vdd, provides the voltage for the load and the MOSFET’s drain-to-source path.
Without a supply voltage, there is no electrical power for the MOSFET to switch to the load.
The required Vdd depends on the application and the voltage rating of the MOSFET. The MOSFET’s drain-to-source voltage rating, VDS(max), must be high enough to safely withstand the voltage that can appear across the device.
2. Vgg – The Gate-to-Source Drive Voltage
The gate-drive voltage, Vgg, provides the gate-to-source voltage needed to turn on the MOSFET.
Unlike a BJT, the MOSFET gate does not require continuous DC current to maintain turn on state. Instead, the gate voltage controls the MOSFET’s conduction.
However, the gate-to-source voltage must be high enough to turn the MOSFET on effectively but must be low enough to exceed the maximum gate to source voltage limit.
This is an important distinction: VGS(th) is not the recommended operating voltage for fully turning on a MOSFET. The threshold voltage indicates when the MOSFET begins to conduct a current. But this is not yet the low RDS(on) state. For practical switching, the gate voltage should be selected according to the MOSFET datasheet and the desired RDS(on).
3. R2 – The Drain-Current Limiter
The drain-current-limiting resistor, R2, limits the current flowing through the MOSFET and the load.
This is particularly important in a simple demonstration or low-power switching circuit because excessive drain current can cause the MOSFET to overheat or become damaged. In a real application, the load itself may naturally limit the current (such as a relay or contactor coil which has internal resistance already), so a separate resistor may not always be necessary. If using an inductive load, the effective resistance must be known to check if the drain current is still in its safe level.
Additional Requirement When the Gate Drive Can Float
What happens if the gate-drive source, Vgg, can become disconnected or float?
When the MOSFET is commanded to turn off, the gate should be pulled to a defined voltage—normally close to the source potential for a low-side N-channel MOSFET.
A gate pull-down resistor, such as R3 in the circuit below, can be added for this purpose.
R3 provides a discharge path for the gate and prevents the MOSFET gate from being left floating.
Without a defined gate voltage, the MOSFET can potentially turn on unintentionally because the gate is capacitive and can retain charge or pick up electrical noise. Therefore, when the gate-drive signal can float, a pull-down resistor is a simple way to ensure that the MOSFET remains in the intended OFF state.
Additional Requirement as Protection for Inductive Loads
When a MOSFET is used to switch an inductive load, such as a relay coil, contactor coil, solenoid, or motor winding, an additional protection circuit is often required.
By operation, an inductor stores energy while current is flowing through it. When the MOSFET suddenly turns off, the inductor attempts to maintain the current. This can produce a high-voltage transient, commonly called kickback or inductive voltage spike or back-EMF.
If this voltage is not controlled, it can exceed the MOSFET’s drain-to-source voltage rating and will damage the device.
Therefore, when switching an inductive load, a suitable surge or kickback protection circuit should be added to the circuit.
Depending on the application, possible protection methods include:
- Flyback or clamp diode – commonly used with DC relay and contactor coils.
- Zener or TVS clamp – limits the voltage to a controlled level and can allow faster turn-off than a simple diode in some applications.
- RCD clamp – useful in applications where the stored inductive energy and switching characteristics require a more controlled clamp network.
- RC snubber – can help control voltage spikes, ringing, and high-frequency transients.
The protection method should be selected based on the load, supply voltage, switching frequency, required turn-off time, and the MOSFET’s voltage rating.
For example, a simple flyback diode across a DC relay coil provides a path for the coil current when the MOSFET turns off, significantly reducing the voltage spike seen by the MOSFET. Very important point:
When switching an inductive load, do not assume that turning off the MOSFET is simply done by removing the current. The stored energy in the inductor must have a safe path to dissipate.
Additional Requirement for High-Frequency Switching
The requirements become more demanding when the MOSFET is used for high-frequency switching.
A MOSFET gate (especially power MOSFET) behaves largely like a capacitive load. Every time the MOSFET switches, the gate capacitances must be charged and discharged.
If the gate is driven directly by a low-impedance source, the initial gate-current pulse can be relatively large. In a high-frequency circuit, this can produce unwanted effects such as:
- Excessive gate-drive current – may damage the drive circuit
- Ringing – may contribute to EMI issues
- Voltage overshoot – may exceed the gate-to-source voltage rating
- Electromagnetic interference
- Stress on the gate-driver circuit
For these reasons, a small gate series resistor, such as R4, can be placed between the gate driver and the MOSFET gate.
The resistor limits the peak gate current and works with the MOSFET’s gate capacitance to slow the gate transition slightly. This can help reduce ringing and voltage overshoot. However, the resistor also slows the MOSFET’s switching transition. Therefore, its value should be selected carefully. A resistance that is too large can increase switching losses, while a resistance that is too small may not provide enough control over ringing and peak gate current. Read Does a MOSFET Need a Series Resistor on Its Gate? for more information.
Power MOSFETs and the Miller Plateau
For power MOSFETs used in switching converters, simply applying a voltage higher than VGS(th) is not sufficient.
During turn-on, the gate voltage eventually reaches the Miller plateau. For a power MOSFET to be driven into hard saturation, the gate-drive voltage should be sufficiently above the Miller plateau. This helps the MOSFET reach a low RDS(on) and reduces conduction losses.
The exact gate-drive voltage should always be determined from the MOSFET’s datasheet and the requirements of the application.
So, What Are the Minimum Requirements of a MOSFET Switch? The Key Takeaways
A MOSFET switch may look simple, but reliable switching requires more than simply connecting a voltage source to the gate.
For a basic MOSFET switch, the essential requirements are a supply voltage, an appropriate gate-to-source voltage, and a method of limiting excessive drain current.
If the gate-drive signal can float, a pull-down resistor helps ensure that the MOSFET remains off when it is supposed to be off.
When used to switch an inductive load, such as a relay or contactor coil, a suitable flyback diode, zener/TVS clamp, RCD clamp, or snubber should be considered to control the voltage generated when the MOSFET turns off and to protect the MOSFET from excessive drain-to-source voltage.
When the MOSFET is used for high-frequency switching like in switching converters and power electronics application, a gate series resistor can help control peak gate current, ringing, and voltage overshoot. The gate-drive voltage must be high enough to drive the MOSFET strongly on and should be evaluated with respect to the Miller plateau and RDS(on).