How to use MOSFET as a Switch ? MOSFET as a Switch Explained

Summary

This video from the ALL ABOUT ELECTRONICS YouTube channel explains the use of MOSFETs as switches, highlighting their advantages over BJTs, particularly in high-frequency and power-efficient applications. It covers the basic operation of MOSFETs, their types, characteristics, and how to select them based on datasheet parameters. The script also discusses gate driving considerations, thermal stability, and the importance of total gate charge for fast switching applications, providing a comprehensive guide for electronics enthusiasts.

Takeaways

• 😀 The MOSFET is a voltage-controlled device that can be turned on or off by controlling the gate-to-source voltage, making it suitable for use as a switch.
• 🔌 In contrast to the MOSFET, the BJT is a current-controlled device that requires base current to be driven into saturation or cutoff for use as a switch.
• 💡 MOSFETs are easier to drive compared to BJTs, especially beneficial for applications requiring high drive currents or control by microcontrollers.
• 🚀 MOSFETs are more power efficient than BJTs in continuous switching applications due to their lower on-state drain-to-source resistance.
• 🔥 The switching loss of a MOSFET is lower than that of a BJT, making it preferable for high-frequency continuous switching applications.
• 🌡 MOSFETs exhibit positive temperature coefficient, which contributes to their thermal stability, unlike BJTs that have a negative temperature coefficient and risk of thermal runaway.
• 🛠 Enhancement type MOSFETs are typically used as switches due to their normally off state, as opposed to depletion type MOSFETs which are normally on.
• 📉 The RDS(ON) parameter is crucial for MOSFETs when used as a switch, as it represents the on-state resistance and affects conduction loss.
• 🔄 When using a MOSFET as a switch, it's important to consider the load line and operating point to ensure the MOSFET operates in the linear region for efficient switching.
• 🔌 A series resistor is recommended between the gate and control input to limit transient current and protect the control circuit when switching a MOSFET.
• ⏱ The total gate charge of a MOSFET is an important parameter for fast switching applications, as it affects the speed at which the MOSFET can be turned on or off.

Q & A

What is the primary function of a MOSFET in this video?

The primary function of a MOSFET in this video is to be used as a switch, controlled by the gate to source voltage.

How does a MOSFET differ from a BJT in terms of control mechanism?

A MOSFET is a voltage-controlled device, whereas a BJT is a current-controlled device. This means a MOSFET can be turned on or off by controlling the gate to source voltage, while a BJT requires base current to be driven into saturation or cutoff.

What are the advantages of using a MOSFET over a BJT for high current applications?

For high current applications, a MOSFET is easier to drive and requires less base current compared to a BJT. Additionally, MOSFETs are more power efficient and thermally stable, making them preferable for high frequency switching.

Why is a MOSFET considered more thermally stable than a BJT?

MOSFETs have a positive temperature coefficient, which means as the temperature increases, the on resistance increases, reducing the drain current and thus the temperature. In contrast, BJTs have a negative temperature coefficient, which can lead to thermal runaway if not properly managed.

What is the significance of the RDS(ON) parameter in MOSFETs?

RDS(ON) represents the drain to source resistance when the MOSFET is in the on condition. A lower RDS(ON) value is desirable as it reduces the conduction loss in the MOSFET.

How does the threshold voltage of a MOSFET affect its operation as a switch?

The threshold voltage determines the minimum gate to source voltage required to turn on the MOSFET. If it is low, the MOSFET can be easily operated using a microcontroller, making it suitable for switch applications.

What is the purpose of a series resistor when connecting a control signal to the gate of a MOSFET?

A series resistor is used to limit the transient current that can be drawn by the gate during the charging of the gate-source capacitor, preventing damage to the control circuit.

Why is a pull-down resistor necessary at the gate terminal of a MOSFET?

A pull-down resistor ensures that the MOSFET remains in the off condition during power-up of the control circuit, maintaining the default state until the control signal is applied.

How does the total gate charge of a MOSFET affect its switching speed?

The total gate charge is the amount of charge needed to fully turn on the MOSFET. A lower total gate charge allows for faster switching, which is beneficial for high-frequency applications.

What is the relationship between the switching speed of a MOSFET and its switching loss?

Faster switching speeds in a MOSFET reduce the time during which both drain current and voltage are present, thus reducing the switching loss and improving efficiency.

How can the load line be determined when using a MOSFET as a switch?

The load line can be determined by considering the maximum current through the load when the drain-source voltage is zero and the drain-source voltage when the current is zero, connecting these two points to form the line.

Outlines

00:00

🔋 MOSFET vs. BJT as Switches

This paragraph discusses the use of MOSFETs as switches, comparing them with BJTs. MOSFETs are voltage-controlled devices that can be turned on or off by controlling the gate-to-source voltage, making them advantageous for low drive current applications. BJTs, on the other hand, are current-controlled devices, requiring a base current to drive them into saturation or cutoff. While BJTs are cost-effective for low current applications, such as LED driving, they require a more robust driver circuit for high current applications. MOSFETs are easier to drive, especially with logic level MOSFETs that can be controlled directly by a microcontroller. The paragraph also touches on power efficiency, thermal stability, and the importance of gate driving for high-frequency switching applications.

05:03

🛠 Characteristics of Enhancement and Depletion MOSFETs

The second paragraph delves into the characteristics of enhancement and depletion type MOSFETs. Enhancement type MOSFETs, which are typically used as switches, only conduct when the voltage VGS exceeds the threshold voltage, making them normally off devices. Depletion type MOSFETs, however, are normally on devices and require a control input greater than the pinch-off voltage to turn off. The paragraph also explains the drain characteristics of enhancement type MOSFETs, including the operation in the linear or ohmic region and the cut-off region. It highlights the importance of the RDS(ON) parameter, which represents the resistance of the MOSFET in the on condition, and how it affects conduction loss.

10:10

🔌 Basic Circuit and Considerations for Using MOSFET as a Switch

This paragraph outlines the basic circuit for using an n-type MOSFET as a switch, with the load connected between the supply and the drain terminal, and the control input applied between the gate and source terminals. It explains how the MOSFET acts as an open circuit when the control input is low and as a closed switch when the input exceeds the threshold voltage. The paragraph also discusses the selection of supply voltage and components based on the MOSFET's datasheet, including the threshold voltage and maximum continuous drive current. Additionally, it introduces the concept of the load line and how it intersects with the MOSFET's drain characteristics to determine the operating point.

15:13

⚠️ Gate Drive Considerations for MOSFET Switching

The fourth paragraph focuses on gate drive considerations when using a MOSFET as a switch. It explains the need for a series resistor between the gate and control input to limit transient current and protect the control circuit during the MOSFET's transition to the on state. The paragraph also discusses the importance of a pull-down resistor at the gate terminal to ensure the MOSFET remains off during power-up. Furthermore, it contrasts the use of n-type and p-type MOSFETs as switches, detailing the connection differences and the necessity of a pull-up resistor for p-channel MOSFETs. Finally, it addresses the importance of fast switching for high-frequency applications and the role of total gate charge in determining the speed of the MOSFET's transition between on and off states.

Keywords

💡MOSFET

MOSFET stands for Metal-Oxide-Semiconductor Field-Effect Transistor. It is a type of transistor used for amplifying or switching electronic signals and electrical power. In the context of the video, MOSFETs are discussed as voltage-controlled switches, which can be turned on or off by controlling the gate-to-source voltage. The script mentions that MOSFETs have advantages over BJTs (Bipolar Junction Transistors), particularly in high-frequency switching applications due to their lower switching losses and higher thermal stability.

💡BJT

BJT, or Bipolar Junction Transistor, is a type of transistor that amplifies or switches signals and power using both electrons and holes. Unlike MOSFETs, BJTs are current-controlled devices. The script contrasts BJTs with MOSFETs, highlighting that while BJTs are cost-effective for low drive current applications like driving LEDs, they require more base current for high drive current applications, which can complicate control with microcontrollers or logic circuits.

💡Gate-to-Source Voltage

Gate-to-Source Voltage (VGS) is the voltage difference between the gate and source terminals of a MOSFET. It is a critical parameter in controlling the operation of a MOSFET. The script explains that by controlling VGS, a MOSFET can be easily turned on or off, which is fundamental to its use as a switch. The video also discusses the importance of ensuring a constant voltage between the gate and source for the MOSFET to function properly as a switch.

💡Depletion and Enhancement Type MOSFETs

Depletion and Enhancement types refer to two different modes of operation for MOSFETs. Depletion mode MOSFETs are 'normally on' devices, meaning they conduct current when VGS is zero, while Enhancement mode MOSFETs are 'normally off' and require a threshold voltage to start conducting. The script explains that enhancement mode MOSFETs are typically used as switches due to their 'normally off' characteristic, which is more desirable for switching applications.

💡RDS(ON)

RDS(ON) stands for the drain-to-source resistance of a MOSFET in the on state. It is a key parameter that indicates the resistance of the MOSFET when it is fully turned on, affecting the conduction loss. The script mentions that a lower RDS(ON) is preferable as it reduces the power loss during the on state. The video provides an example of calculating RDS(ON) using operating voltage and current.

💡Conduction Loss

Conduction loss refers to the power loss that occurs when a switch, such as a MOSFET or BJT, is in the on state. It is a result of the resistance of the switch when current flows through it. The script discusses that the conduction loss in a MOSFET is dependent on its RDS(ON) and is a crucial factor to consider in high power applications.

💡Switching Loss

Switching loss is the power loss that occurs during the transition of a switch between the on and off states. The script emphasizes that switching loss is a significant factor, especially in high-frequency continuous switching applications. It explains that MOSFETs have lower switching losses compared to BJTs, making them more suitable for such applications.

💡Thermal Stability

Thermal stability refers to the ability of a device to maintain its performance under varying temperature conditions. The script explains that MOSFETs have positive temperature coefficients, meaning their on-resistance increases with temperature, which can help to reduce the drain current and prevent thermal runaway. In contrast, BJTs have negative temperature coefficients, which can lead to thermal runaway if not managed properly.

💡Gate Charge

Gate charge is the amount of charge required to fully turn on a MOSFET. It is an important parameter for high-frequency switching applications because it affects the speed at which the MOSFET can be switched on or off. The script discusses how the total gate charge can be used to estimate the switching time and how a lower gate charge allows for faster switching, reducing switching losses.

💡Logic Level MOSFET

A Logic Level MOSFET is a type of MOSFET designed to be easily controlled by logic circuits or microcontrollers, which typically operate at low voltages around 3 to 5 volts. The script mentions that if a MOSFET is a logic level MOSFET, it can be controlled directly by a microcontroller without requiring additional driver circuits, simplifying the design of electronic systems.

Highlights

MOSFETs can be used as switches with advantages over BJTs due to their voltage control nature.

MOSFETs turn on/off by controlling gate-to-source voltage, unlike BJTs that require base current control.

At low drive currents, BJTs are cost-effective for applications like LED driving, but require more base current at higher drive currents.

High drive current applications may necessitate a robust driver circuit for BJTs, unlike MOSFETs which are easier to drive with microcontrollers.

MOSFETs are more power efficient than BJTs in continuous switching applications due to lower on-state resistance.

Conduction and switching losses are key considerations when using BJTs and MOSFETs as switches, with MOSFETs generally having lower switching losses.

MOSFETs are preferred for high-frequency continuous switching due to lower switching losses compared to BJTs.

MOSFETs exhibit positive temperature coefficient, enhancing thermal stability, unlike BJTs which can enter thermal runaway.

Enhancement type MOSFETs are typically used as switches, as opposed to depletion type MOSFETs which are normally on.

The drain characteristic curve of enhancement type MOSFETs shows operation in the linear, ohmic, and cut-off regions.

RDS(ON) is a critical parameter indicating the resistance of a MOSFET in the on state, affecting conduction loss.

Basic circuit configuration for using an n-type MOSFET as a switch includes connecting the load between supply and drain, and applying control input between gate and source.

Microcontrollers can directly control MOSFETs, especially if the MOSFET's threshold voltage is low.

Selecting the proper supply voltage and components is crucial for ensuring MOSFETs operate in the linear region.

A series resistor is necessary between the gate and control input to limit transient current and protect the control circuit.

P-type MOSFETs require a negative gate-to-source voltage to turn on and can be controlled with microcontrollers through a pull-up resistor configuration.

Fast switching applications require consideration of total gate charge and the use of appropriate gate driver ICs to minimize switching loss.

The total gate charge is a critical parameter for MOSFETs in continuous switching applications, affecting switching speed and loss.

Transcripts

Hey friends welcome to the YouTube channel ALL  ABOUT ELECTRONICS. So in this video, we will learn  

how the MOSFET can be used as a switch. Now the  BJT can also be used as a switch. But the MOSFET  

has certain advantages over the BJT. So as you are  aware this MOSFET is the voltage control device.  

That means just by controlling the gate to source  voltage, this MOSFET can be turned on or off. And  

in this way, it can be used as a switch. On the  other end, this BJT is the current control device.  

That means by controlling the base current this  BJT can be driven either into the saturation or  

the cutoff region. And in this way, it can be  used as a switch. Now at the low drive currents,  

the required base current for driving the BJT  in the saturation will be lower. So for the low  

driving current applications like for driving the  Leds this BJT is very cost effective solution.  

But when this drive current is in ampere, then  the required base current to drive the BJT into  

the saturation will also increase. And in that  case this control circuit or the driver circuit  

which provides the required base current  should be able to provide the enough current  

to drive this BJT into the hard saturation.  So if you want to control the BJT as a switch  

using the microcontroller or any logic  circuit, then it is not possible for the  

very large current. On the other end, if  you see this MOSFET then it is the voltage  

control device. So just by ensuring the constant  voltage between this gate and the source terminal,  

it can be easily used as a switch. And  if the MOSFET is a logic level MOSFET,  

then it can be easily controlled even using the  microcontroller. But in case of the continuous  

switching and particularly at a high frequency  switching we also need to consider the other  

aspects of the gate driving. But we will talk  about it little later. But the thing is the MOSFET  

is easy to drive compared to BJT. The second thing  is for the continuous switching applications,  

these MOSFETS are more power efficient than the  BJT. So when the MOSFET is in on condition, then  

it acts like a resistor. And in the on condition  the drain to source resistance will be very low.  

So for high power MOSFETS, it is typically in  the milli ohms. So even at very high currents,  

if we see the conduction loss in the MOSFET, then  it is comparable or slightly higher than the BJT.  

Now when the BJT and the MOSFET are used as  a switch, then there are two types of losses.  

One is the conduction loss and the second is  switching loss. So the loss which you have just  

discussed is the conduction loss. That means  whenever the switch is in the on condition,  

then the loss across the switch is the conduction  loss. Now when the MOSFET and the BJT is used in  

the continuous switching application, then  the switching loss is very crucial factor.  

And for the MOSFET this switching loss is lower  compared to the BJT. So at high frequencies in  

the continuous switching applications this  MOSFET is more preferred over the BJTs.  

Moreover the MOSFETs are more thermally stable  compared to BJTs. The reason is the MOSFET has the  

positive temperature coefficient. That means  during the on condition or in the on state as the  

temperature of the MOSFET increases, then the on  resistance will also increase. And because of the  

increase in the on resistance the drain current  id will reduce and because of that the temperature  

of the MOSFET will reduce. On the other end the  BJTs have the negative temperature coefficient.  

That means as the temperature increases, then  this character current IC will also increase. And  

the increase in the collector current will further  increase the temperature. So if the generated heat  

is not dissipated properly, then it may lead the  BJT into the thermal runaway. That means MOSFETs  

are more thermally stable compared to BJT. So in  short the MOSFETs are easy to drive than the BJT  

and can work more efficiently at  the high switching frequencies.  

Now as we have seen in the earlier videos,  there are two types of MOSFETs. The depletion  

type and the enhancement type. But typically the  enhancement type MOSFETs are used as a switch.  

So for the depletion type of MOSFET these are the  transfer characteristics of the P channel and the  

N channel MOSFET. And as you can see, they are  normally on devices. That means by default when  

the voltage VGS is equal to 0, or when the  control input at the gate is 0 at that time  

the current is flowing through the  MOSFET. So when the control input is 0,  

at that time these depletion type MOSFETs are in  the on condition. And when the control input is  

greater than the pinch of voltage, at that  time only they are in the off condition.  

So because of this characteristic,  they are not preferred as the switch.  

On the other end the e MOSFETs or the enhancement  type of MOSFETs starts conducting, when the  

voltage VGS is greater than the threshold voltage.  So if the input is less than the threshold  

voltage, then it will remain in the off condition.  So these e-type MOSFETs are normally off devices,  

and this characteristic is more preferable  for using them as a switch. Now this is the  

drain characteristic of the enhancement type  of MOSFET. And when it is used as a switch,  

then it is operated either in the linear or  the ohmic region, and the cut-off region.  

So in the cut-off region when the control  input VGS is less than the threshold voltage,  

then the drain current ID is almost 0. And in the  on condition, when it is used in the linear region  

then the MOSFET acts like a resistor. And the  slope of this curve is the RDS(ON). So this  

RDS(ON) is the resistance of the MOSFET in the  on condition. For example for VGS is equal to 5v,  

if these are the operating voltage and the  current, then the ratio of this voltage and  

the current will give us the RDS(ON). And  as you can see as the voltage VGS increases,  

then this RDS(ON) will reduce. So this RDS(ON)  is one of the important parameter for the MOSFET.  

And the value of this RDS(ON) should be as  low as possible. Because as I said earlier,  

in the on condition of the MOSFET the conduction  loss of the MOSFET depends on this ON resistance.  

So this is the basic circuit of the n-type MOSFET  which can be used as a switch. So as you can see  

here, the load is connected between the supply  and the drain terminal, and the control input is  

applied between the gate and the source terminal.  So this control signal can be applied directly  

through the microcontroller, or it can be applied  using the separate driver circuit. Now when this  

control input is low, then the MOSFET will act  as a open circuit. And in this case no current  

will flow through the load. Now whenever this  control input is more than the threshold voltage,  

then the MOSFET will act as a closed switch,  and the current starts flowing through the  

load. So whenever the threshold voltage of the  MOSFET is less than 2 to 3 volt at that time this  

type of MOSFETs can also be operated using the  microcontrollers. Now as I said, when the MOSFET  

is in the on condition, then it should operate in  the linear region. That means for the given load,  

by properly selecting the supply voltage,  we can ensure that it operates in the linear  

region. For example let's say this RD is the load  resistance. And in the worst case assuming this  

VDS is equal to 0, the maximum current through  the load will be equal to VDD divided by RD.  

And whenever the switch is in the open condition,  or whenever this drain current id is equal to 0,  

at that time this voltage VDS is equal  to VDD. So for these two extreme cases  

we will get the two points. And by joining  these two points, we can get the load line.  

So the intersection of the load line with any of  these two curve, will give us the operating point.  

So let's say if we are operating the MOSFET at VGS  is equal to 5 volt, then this intersection point  

should fall in a such a way that the MOSFET  operates in the linear region. Or at least  

that is how we should select the load line. Now  if you see the data sheet of any MOSFET then the  

some important parameters and the important  curves are already given in the datasheet.  

For example for the MOSFET 2 and 7 triple zero g  the threshold voltage is between 0.8 and 3 volt.  

That means it can be operated using the  microcontroller. And here for the typical  

value of the VGS, we have been also given the  value of the RDS(ON). Now for this particular  

MOSFET the maximum continuous drive current is  equal to 200 milli ampere. That means in the  

worst case condition the current which is flowing  through the MOSFET should be less than this value.  

So let's say using this MOSFET, we want to  drive the 30 ohm load. And in the on condition  

the current through the load should be around  150 milliampere. That means whenever the VGS  

is equal to 5 volt, then the current through  the load should be around 150 milliampere.  

And with VDD is equal to 5v, it is possible to do  that. So with VDD is equal to 5v, and RD is equal  

to 30 ohm this will be the load line. That means  whenever the VDS is equal to 0, at that time this  

strength current ID is equal to 5 volt divided  by 30 ohm. That is around 166 milli ampere.  

And whenever this ID is equal to 0, then this VDS  is equal to 5 volt. That means this will be the  

load line. And the intersection of the load line  with the VGS is equal to 5 volt curve will gives  

us the operating point. So in this case if you  see the MOSFET is operating in the linear region.  

And the drive current is  typically around 150 milliampere.  

So in this way using the datasheet of the MOSFET  for the given drive current, we can select the  

supply voltage as well as the other components.  Now so far in our discussion we have directly  

applied the control voltage to the gate terminal.  But actually there should be some series resistor  

between the gate and the control input. So let's  understand why? Now so far we understood that  

the gate of the MOSFET offers a very high  input resistance at the low frequencies.  

That means the gate terminal hardly draws any  current from the supply. Right? But to turn on  

this MOSFET when we apply the input through this  control circuit without this series resistor,  

then without this series resistor the MOSFET  can draw a lot of current during the transient.  

And if this control circuit is not able  to supply that much of search current,  

then it can damage the control circuit. Because if  we see the gate to source terminal of the MOSFET,  

then in the equivalent circuit it will act like  a capacitor. So when we apply the input voltage  

through this control circuit or through this  microcontroller pin, then this capacitor suddenly  

tries to charge to the input voltage. And during  this transient it can draw a lot of current. But  

if we connect this series resistor, then this  series resistor restricts the transient current.  

For example if the MOSFET is controlled  using the microcontroller pin,  

and the maximum current which is supplied by the  microcontroller is 20 milli ampere at 5 volt,  

then the value of the resistor should be at  least equal to 250 ohm. So this will ensure that  

the maximum current which is being drawn from the  microcontroller pin is less than 20 milli ampere.  

Apart from the series resistor, the pull down  resistor is also required at the gate terminal.  

So during the power up of this control  circuit, this pull down resistor ensures that  

the MOSFET remains in the default  condition. That means whenever  

the input is 0, then the MOSFET should remain in  the off condition. Now so far in our discussion,  

we have talked about the n-type MOSFET. Similarly  let's talk about the p-type MOSFET. And let's see  

how it can be used as a switch. So as you can see,  the transfer characteristic of the p-type MOSFET  

is exactly opposite to the n-type MOSFET. That  means to turn on this p-type MOSFET, the voltage  

VGS should be negative. So if we want to use the  same control circuit or the same microcontroller  

inputs for this p-channel MOSFET, then this is how  we can connect the load to the p-channel MOSFET.  

So as you can see here the source is connected  to the VDD, and the load is connected between the  

drain and the ground terminal. And using the pull  up resistor the gate is connected to the VDD. So  

this pull up resistor ensures that, the voltage at  the gate remains at the default 5 volt, when the  

microcontroller or the control circuit is getting  powered up. So when the control input is high,  

at that time the voltage VG and the VS both are at  the same potential. That means in this case this  

VGS is equal to 0. So because of the VGS is equal  to 0, the MOSFET will remain in the off condition.  

Now when this microcontroller output goes low,  then this voltage VGS will become negative. And  

because of that the MOSFET will get turned on. And  due to that current starts flowing through this  

load. So this is how we can also use the p-channel  MOSFET as a switch. Now so far we understood that  

the MOSFET can be turned on or off using the  microcontroller. But this type of switching  

is the slow switching, meaning that once the  microcontroller pin goes to the high or low state,  

then the MOSFET will take some time to completely  gets in the on or off condition. So if you want  

to use the MOSFET for the continuous switching  application, and that too at the high frequency,  

then we also need to look for the other parameters  in the datasheet. So this total gate charge  

is the amount of charge which needs to be injected  into the gate terminal to completely turn on the  

MOSFET. And from the total gate charge we can  estimate how fast the MOSFET can be switched on  

or off using the given control circuit. So as you  are aware this charge Q can be given as I times  

T. Right? So let's say for a sum MOSFET if  this gate charge is equal to 20 nano coulomb  

and the maximum current supplied by the  driving circuit is equal to 20 milli ampere,  

then the time required to turn on this  MOSFET is equal to 20 nano coulomb divided by  

20 milli ampere. That is equal to 1 micro second  so for this type of MOSFET if we try to drive the  

MOSFET using the microcontroller, then this  is the typical time it will take to turn on.  

And if the value of the total gate charge is  more, then it will take more time to turn on.  

On the other end, if the gate driving circuit  can provide a maximum current up to 1 ampere,  

then the time to turn on the MOSFET will get  reduced to 20 nanosecond. And in this case it is  

possible to use the MOSFET for the fast switching  applications. That means for the fast switching  

applications, this total gate charge should  be as low as possible. And the MOSFET should  

be driven using the gate driver IC such that it  provides a sufficient current for the switching.  

The other advantage of this fast switching is  that the switching loss in the MOSFET will reduce.  

So as I mentioned earlier, for the  continuous switching application,  

this switching loss is very crucial parameter. So  when the MOSFET is in the off condition, then the  

voltage VDS is equal to VDD. And the drain current  ID is approximately equal to 0. So the power loss  

across the MOSFET is approximately equal to  0. Now when the MOSFET goes into the on state,  

then this drain current starts increasing. And  as soon as it reaches the on condition current,  

then the voltage VDS starts reducing.  So during the switching phase  

both drain current ID and the VD are on for some  time. And this triangular wave shows the switching  

loss during the on time. Now when the MOSFET is  in the fully on condition, then the power loss  

across the MOSFET is the conduction loss. And once  again when the MOSFET goes into the off state,  

then the voltage VDS starts increasing. And as  soon as it reaches to the VDD, then the drain  

current ID starts decreasing. So once again when  the MOSFET is getting turned off, then we are also  

getting the switching loss. So the area under the  curve of this two triangle is the switching loss.  

And with the faster switching the area under  these two curves can be reduced. And because  

of that we can also reduce the switching loss.  So this total gate charge is very important  

parameter for the MOSFET in case of the continuous  switching application. So I hope in this video,  

you understood how the MOSFET can be used as  a switch. And what are the factors we need to  

look into the datasheet to use this MOSFET  as a switch for the specific application.  

So if you have any question or suggestion, then  do let me know here in the comment section below.  

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