How to use MOSFET as a Switch ? MOSFET as a Switch Explained
Summary
TLDRThis 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
🔋 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.
🛠 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.
🔌 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.
⚠️ 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.
Mindmap
Keywords
💡MOSFET
💡BJT
💡Gate-to-Source Voltage
💡Depletion and Enhancement Type MOSFETs
💡RDS(ON)
💡Conduction Loss
💡Switching Loss
💡Thermal Stability
💡Gate Charge
💡Logic Level MOSFET
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.
If you like this video hit the like button and subscribe the channel for more such videos.
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