How to Configure a MOSFET as a Switch

In our earlier discussions we observed that the N-channel Enhancement-mode MOSFET (e-MOSFET) operates effectively when supplied with a positive input voltage.

This particular type of MOSFET boasts an exceptionally high input resistance—approaching infinity.

This facilitates its use as a switch. This characteristic allows us to interface the MOSFET seamlessly with virtually any logic gate or driver capable of delivering a positive output signal.

Furthermore we noted that the remarkable input (Gate) resistance of these devices permits us to connect multiple MOSFETs in parallel without concern. By doing so we can achieve the requisite current handling capacity that our applications demand.

However while the practice of paralleling various MOSFETs enables us to manage high currents or drive high-voltage loads, it also introduces significant challenges.

Specifically, this approach can become prohibitively expensive and impractical due to the increased number of components required and the additional circuit board space needed for their implementation.

To address these concerns, Power Field Effect Transistors commonly referred to as Power FETs, were developed as a more efficient solution.

We have come to understand that there are two primary distinctions between field effect transistors: the depletion-mode which is exclusive to JFETs and the dual capability of enhancement-mode and depletion-mode operation found in MOSFETs.

In this tutorial we will focus our attention on utilizing the Enhancement-mode MOSFET as a switch. These transistors necessitate a positive gate voltage to transition into the “ON” state and require a zero voltage to revert to the “OFF” state.

This operating feature makes them especially easy to understand as switches and allows for seamless connection with logic gates.

The functionality of the enhancement-mode MOSFET, often referred to as the e-MOSFET, can be most effectively illustrated through its I-V characteristic curves which are depicted below.

When the input voltage VIN applied to the gate of the transistor is at zero, the MOSFET allows virtually no current to flow, resulting in an output voltage V_OUT that is equivalent to the supply voltage V_DD.

In this scenario we observe that the MOSFET is in its “OFF” state, operating within what is known as the “cut-off” region.

Characteristics Curves of a MOSFET

So, when we are talking about the minimum ON-state gate voltage that we need to keep our MOSFET in the “ON” position while it is handling a specific drain current, we can actually figure this out by looking at those V-I transfer curves shown above.

Basically, when our input voltage V_IN is increased to HIGH or matches V_DD, the operating point of the MOSFET, the Q-point, shifts over to point A along that load line we have drawn out.

At this point, the drain current I_D starts to increase to its maximum value. This increase happens because the channel resistance drops down quite a bit.

Once we hit this sweet spot, I_D becomes a constant value that does not really change with V_DD, instead it only depends on the gate-source voltage V_GS.

So in this scenario the transistor is acting just like a closed switch. However it is worth noting that the channel ON-resistance does not completely vanish, it gets really small but does not go all the way down to zero because of its R_DS(on) value.

Now when V_IN is LOW or gets to zero, our MOSFET’s Q-point shifts from point A over to point B along that same load line. In this case, the channel resistance shoots up really high, which means our transistor is behaving like an open circuit, meaning no current can flow through the channel at all.

So if we keep toggling the gate voltage of our MOSFET between these two states, HIGH and LOW, it essentially acts like a “single-pole single-throw” (SPST) solid state switch. This whole back-and-forth action is what we define as:

Cut-off Region

In this region, the conditions for operating the transistor are such that we have zero input gate voltage (V_IN), zero drain current (I_D), and the output voltage (V_DS) is equal to V_DD. This means that for an enhancement type MOSFET, the conductive channel is completely closed, and the device is essentially switched “OFF.” Here are a few key points to consider:

Characteristics of the Cut-off Region

• The input and gate are grounded at 0 volts.
• The gate-source voltage is less than the threshold voltage, which means V_GS is less than V_TH.
• The MOSFET is in the “OFF” state, which we refer to as the cut-off region.
• There is no drain current flowing through, so I_D equals 0 amps.
• The output voltage, V_OUT, is equal to V_DS, which equals V_DD, and we can say it is “1.”
• In this situation, the MOSFET operates like an “open switch.”

Therefore when we work with an enhancement-mode MOSFET as a switch, we may choose the cut-off zone or “OFF mode.” This has been characterized by the fact that I_D equals 0 since the gate voltage is lower than the threshold voltage (V_GS < V_TH). A P-channel enhancement MOSFET requires a gate voltage that is more positive with respect to the source.

Saturation Region

When we talk about the saturation or linear region of a transistor, we’ve got its biasing set up in such a way that we’re increasing the gate voltage to its maximum value.

This setup makes the channel resistance, which we call R_DS(on), very low, allowing the maximum drain current to flow through our MOSFET switch.

So for enhancement-type MOSFETs, this means that the conductive channel is wide open and we can say the device is switched “ON”.

Characteristics of the Saturation Region

• Essentially our MOSFET acts like a low-resistance “closed switch,” making it super efficient.
• We connect the input and gate to V_DD.
• The gate-source voltage is way higher than the threshold voltage, so V_GS is greater than V_TH.
• This means our MOSFET is fully “ON” and operating in that saturation region.
• We can expect the maximum drain current to flow, which we can calculate as I_D = V_DD / R_L.
• In an ideal scenario, V_DS would be 0V, indicating perfect saturation.
• The minimum channel resistance R_DS(on) is less than 0.1Ω, which is pretty impressive.
• As a result of R_DS(on), we see that V_OUT is approximately 0.2V.

Therefore let us discuss the definitions of “saturation region” and “ON mode” when utilizing an e-MOSFET as a switch.

Essentially this mode occurs when the gate-source voltage (V_GS) is higher than the threshold voltage (V_TH).

When this occurs, we reach the point at which the drain current, I_D achieves its highest value.

If we are working with a P-channel enhancement MOSFET, it is important to remember that the gate potential must be lower than the source.

When we supply the correct drive voltage to the FET gate, an interesting phenomenon occurs: we are able to control the resistance of the drain-source channel, referred to as R_DS(on).

This resistance can vary between a very high “OFF-resistance” of hundreds of kΩ, functioning as an open circuit, to a very low “ON-resistance” of less than 1Ω, similar to a short circuit.

Now when we utilize our MOSFET as a switch, we enjoy incredible versatility. We have the option to switch it on rapidly or opt for a slower approach.

Additionally, we have the ability to regulate if it allows high currents or low currents. The capability to efficiently turn on and off the power MOSFET makes it an excellent switch.

Actually, it has the ability to transition at a much quicker rate than standard bipolar junction transistors.

Analyzing a MOSFET as a Switch Example

Let’s discuss the gate input voltage applied to the MOSFET which we refer to as V_GS. This voltage is adjusted to a suitable positive level in order to activate the device, thereby turning the lamp load either “ON” when VGS is positive, or “OFF” when V_GS is at a zero voltage level (0V).

Now if we were to swap out the resistive load of the lamp for something like an inductive load such as coils, solenoids, or relays, we would need to incorporate a “flywheel diode” in parallel with that load. This diode serves a crucial purpose.

It protects the MOSFET from any back electromotive force (back-emf) that can be generated by the inductive load itself.

What we have here is a straightforward circuit designed for switching a resistive load such as a lamp or an LED. But when we start using power MOSFETs to switch inductive or capacitive loads, it becomes crucial to implement some form of protection.

This is necessary to prevent potential damage to the MOSFET device.

It is interesting to note that driving an inductive load behaves quite differently compared to driving a capacitive load.

For instance consider a capacitor that has no electrical charge. In this state it essentially acts like a short circuit which leads to a significant “inrush” of current.

On the other hand when we cut off the voltage from an inductive load, we see a substantial reverse voltage buildup as the magnetic field collapses.

This situation generates an induced back-emf in the windings of the inductor which can be quite dangerous for the MOSFET.

So we can summarize the switching characteristics of both N-channel and P-channel type MOSFETs in the following table.

MOSFET Type	VGS ≪ 0	VGS = 0	VGS ≫ 0
N-channel Enhancement	OFF	OFF	ON
N-channel Depletion	OFF	ON	ON
P-channel Enhancement	ON	OFF	OFF
P-channel Depletion	ON	ON	OFF

An important difference between the N-channel MOSFET and the P-channel MOSFET is that, in the case of the N-channel MOSFET, we need to ensure that the gate terminal is made more positive than the source. This positive voltage attracts electrons which allows current to flow through the channel.

But when we work with the P-channel MOSFET, the situation changes significantly. Here the conduction occurs due to the movement of holes instead of electrons.

This means that for a P-channel MOSFET to function properly we must make the gate terminal more negative than the source. It is worth noting that this device will only cease conducting—essentially entering a “cut-off” state—when the gate voltage surpasses the source voltage, becoming more positive.

Now, if we want to use an enhancement-type power MOSFET as an analog switching device, we need to manage its operation carefully.

Specifically, we have to toggle it between two distinct regions, which are the “Cut-off Region,” where V_GS equals 0V (or V_GS is negative), and the “Saturation Region,” where V_GS(on) is positive.

The power dissipation (PD) within the MOSFET is influenced by several factors. It primarily depends on the current flowing through the channel (I_D) when in saturation, as well as on what we refer to as the “ON-resistance” of that channel, which is designated as R_DS(on).

For example let us consider how these parameters interact in a practical scenario. When we have a certain current flowing through our P-channel MOSFET while it is in saturation region, we can calculate how much power is being dissipated based on both I_D and R_DS(on). This understanding is crucial for effectively utilizing the MOSFETs in our circuits.

Solving a MOSFET Switch Circuit Problem #1

Let’s imagine we have a lamp that is rated at 12 volts and 25 watts, and it is operating at full capacity. We also have a standard MOSFET with a channel on-resistance (RDS(on)) of 0.12 ohms. We want to calculate the power that is dissipated in the MOSFET switching device under these conditions.

We will first calculate the current passing through the lamp using the following fomula:

P = V * I_D

∴ I_D = P/V = 25/12 = 2.08 Amps

Next, we will calculate the power dissipated in the MOSFET, in the following manner:

P = I² * R

PD = I²_D * R_DS

∴ PD = 2.08 * 0.12 = 0.25 watts

Understanding R_DS(on) of a MOSFET

While thinking about the complexities of utilizing a MOSFET to control DC motors or high inrush current loads, you might be reminded of the importance of the “ON” state resistance (R_DS(on)) across the drain and source terminals.

Then you may be asking yourself, “What is so crucial about this?”

However this parameter is very important. When we examine MOSFETs used to control DC motors, we find that they experience a significant surge of current as soon as the motor begins to turn.

This sudden increase happens because the initial current of the motor is mainly restricted by the extremely low resistance of its windings.

To further explain this you may consider the basic power relationship shown by the formula P = I²R.

A high R_DS(on) channel resistance value would result in a large amount of power being wasted in the MOSFET. If not handled properly.

This situation could lead to a significant increase in temperature, which can potentially cause the MOSFET to overheat and become damaged.

Conversely if the MOSFET has a lower R_DS(on) value for the channel resistance, it become a very sought-after quality.

This helps decrease the effective saturation voltage (V_DS(sat) = I_D * R_DS(on)) across the MOSFET, enabling it to function at a lower temperature.

Actually power MOSFETs generally have an R_DS(on) value that is lower than 0.01 ohms.

Having minimal resistance helps the MOSFET to stay cool which in turn increases their lifespan and improves their reliability in many different situations.

When We consider the usage of a MOSFET as a switching device, one of the first limits that comes to mind is the maximum drain current that it can successfully manage.

This restriction is critical since the R_DS(on) metric is a key measure of the MOSFET’s switching efficiency. In simple terms, this value represents the ratio of V_DS to I_D during the period the MOSFET remains in the “ON” state.

When using a MOSFET or any other form of field-effect transistor as a solid-state switching device, it is best to choose components with a low R_DS(on) value.

But if we have no access to these components, we should try placing them on a big enough heatsink.

This strategy is critical in reducing the dangers associated with the thermal runaway and potential damage.

Although the power MOSFETs built for switching applications often have surge-current safety, it seems that for high-current applications, a bipolar junction transistor is frequently a better option.

As We investigate the features of MOSFETs, one thing that sticks out is their extremely high input or gate resistance.

This property, along with their exceptionally rapid switching rates and simplicity of driving, makes them perfect for interfacing with operational amplifiers (op-amps) or ordinary logic gates.

But we must take caution while determining the optimum gate-source input voltage. When using the MOSFET as a switch, it is critical that the device has a low R_DS(on) channel resistance that is proportional to the input gate voltage.

It is worth noting that low-threshold power MOSFETs may not turn “on” right until the minimum of 3V or 4V is given to its gate.

If the logic gate’s output only offers +5V logic, it may not be sufficient to properly push the MOSFET into saturation. thankfully we have the choice of lower-threshold MOSFETs that are particularly developed for interfacing with TTL and CMOS logic gates, having thresholds ranging from 1.5V to 2.0V.

Moreover power MOSFETs may be used to control the acceleration of DC or brushless stepper motors using computer logic or pulse-width modulation (PWM) controllers.

The fact that a DC motor has a strong beginning torque that is exactly proportional to the armature current, we may utilize MOSFET switches in conjunction with PWM approaches to achieve a highly efficient speed control solution.

This combination not only ensures smooth and quiet motor running, but it also improves overall performance and control in a variety of applications.

Understanding the Working of a Simple MOSFET Motor Controller Circuit

Referring to the diagram above, when we are dealing with a motor load which is inductive, we need to connect a simple flywheel diode right across the inductive load.

This diode helps to get rid of any back emf that the motor generates when we turn the MOSFET “OFF.”

To make things even better, we can set up a clamping network by adding a zener diode in series with the flywheel diode. This setup allows for faster switching and gives us better control over the peak reverse voltage and how long it takes to drop out.

For some extra safety, we can also add another silicon or zener diode, which we can see as D1, across the channel of the MOSFET switch whenever we are working with inductive loads like motors, relays, or solenoids.

This extra diode helps to suppress those sneaky over-voltage switching transients and noise, giving our MOSFET switch some added protection if we need it.

And let us not forget about resistor R_GS! This resistor acts as a pull-down resistor to help pull the TTL output voltage down to 0V when we switch the MOSFET “OFF.”

How to Configure a P-Channel MOSFET as a Switch

Up to this point, we have been examining the N-channel MOSFET employed as a switch, with the MOSFET situated between the load and the ground.

This configuration allows us to refer to the gate drive or switching signal as grounded, known as low-side switching. However in certain situations, we genuinely require a P-channel enhancement-mode MOSFET, with the load directly linked to ground.

In this scenario the MOSFET switch is connected between the load and the positive supply rail, a method referred to as high-side switching—similar to the approach used with PNP BJTs.

Currently, with a P-channel device, the drain current travels in the negative direction. So, to activate the transistor, we must apply a negative voltage between the gate and source.

We are able to achieve this since the P-channel MOSFET is kind of “inverted,” with its source terminal connected to the positive supply, which we call +V_DD.

Thus when the switch is set to LOW, the MOSFET activates or turns “ON,” and when the switch is set to HIGH, it deactivates or turns “OFF.”

This inverted configuration of a P-channel enhancement-mode MOSFET switch enables us to connect it in series with an N-channel enhancement-mode MOSFET.

This integration forms a complementary or CMOS switching component that functions effectively with a dual power supply.

Understanding the Working of a Complementary MOSFET as a Switch Motor Controller

Looking at the above image, we configured two MOSFETs to create a bi-directional switch utilizing a dual power supply. In this setup, the motor is connected between the common drain point and the ground reference.

When the input signal is LOW, the P-channel MOSFET turns on due to its gate-source junction being negatively charged. This leads the motor to turn in one direction, and we are exclusively using the positive +V_DD power supply line to power it.

At present, when we apply a HIGH signal, the P-channel device turns OFF, while at the same time, the N-channel device turns ON due to its gate-source junction being positively biased.

This inversion causes the motor to rotate in the opposite direction because the voltage at the motor’s terminals has now been reversed; it is drawing its power from the negative -V_DD power source.

In this setup we use the P-channel MOSFET to connect the positive supply to the motor when we want it to move forward (this is known as high-side switching).

On the other hand, we employ the N-channel MOSFET to connect the negative supply to the motor when we want it to rotate in reverse (this method is known as low-side switching).

You may find numerous ways to configure the two MOSFETs for different applications. Both the P-channel and the N-channel devices can indeed be managed by one gate drive IC, as observed in certain configurations.

However the key point is this, to avoid cross conduction, when both the MOSFETs conduct simultaneously across the two polarities of the dual supply, we require rapid switching devices.

This is crucial because we aim to ensure there is a slight time interval between one MOSFET switching “OFF” and the other MOSFET switching “ON.”

A possible resolution to this problem is to control the gates of each MOSFET independently. This approach allows us to establish a third option that efficiently places the motor in a “STOP” state when both MOSFETs are in the “OFF” position.

This slight delay between the conduction of the two MOSFETs is also known as the deadtime.

Table to Indicate the Switching Control of two Complementary MOSFETs

MOSFET 1	MOSFET 2	Motor Function
OFF	OFF	Motor Stopped (OFF)
ON	OFF	Motor Rotates Forward
OFF	ON	Motor Rotates Reverse
ON	ON	NOT ALLOWED

Just a quick heads-up: it is really important that we do not allow any other combinations of inputs to be active at the same time. If we do, then it could lead to a situation where the power supply gets shorted out. This happens because both MOSFETs, FET1 and FET2, could end up being switched “ON” at the same time. And trust me, that would result in a big problem, just think of it as a fuse blowing with a loud bang! So we need to be careful!

References:

How to use a MOSFET as a switch. (controlled by arduino)

Driving a High-Side MOSFET Input Switch