Application and Analysis of Switches I

Summary

This NPTEL lecture delves into advanced power electronics and control, focusing on switch analysis in power devices. It discusses switch characteristics, including ideal conditions and real-world deviations, emphasizing the importance of protection mechanisms against disturbances like high dv/dt and di/dt. The lecture also covers device physics, modeling, packaging, and thermal management strategies. It explores various power electronic devices, their applications, and the advantages of SiC materials. The session concludes with insights into snubber circuits for protection against overvoltage and overcurrent, setting the stage for future discussions on gate drive circuits.

Takeaways

• 🔬 The lecture delves into the detailed analysis of switches in power electronics, focusing on device physics, switching characteristics, and protection mechanisms.
• 🧩 It discusses the ideal characteristics of switches, such as zero power dissipation, infinite current carrying capability, and instantaneous switching, while acknowledging real-world deviations.
• 🔋 The importance of understanding and managing electrical and thermal stresses within power electronic devices is emphasized, including protection from high dv/dt and di/dt.
• 📈 The lecture covers various modeling viewpoints, including SPICE models and state-space models, which are crucial for simulating power electronic systems accurately.
• 🔩 Packaging considerations are highlighted, including thermal management, compactness, and device mounting strategies, which are essential for device performance and longevity.
• 🔌 The script mentions different types of power electronic switches, such as uncontrolled, semi-controlled, and fully controlled devices, each with specific applications and characteristics.
• 🚀 Silicon Carbide (SiC) devices are introduced as a new entrant in power electronics, offering advantages in power handling and efficiency over traditional silicon-based devices.
• 🔑 The role of gate drive circuits in controlling switches is discussed, including the need for proper current sourcing and pulse generation to ensure effective switch operation.
• ⚙️ Protection mechanisms like snubbers are explained, which are essential for limiting di/dt, dv/dt, and overvoltage, thereby enhancing the reliability and lifespan of switches.
• 🛠️ The lecture concludes with a preview of upcoming topics, including gate driver circuits and their significance in power electronic device control.

Q & A

What is the main focus of the seventh lecture in the NPTEL course on advance power electronics and control?

The main focus of the seventh lecture is the detailed discussion of switches in power electronics, including their physics, characteristics, protections, and modeling.

What are the key aspects considered when analyzing switches from a physics point of view?

When analyzing switches from a physics point of view, key aspects include doping levels, the type of material used (whether high or low band gap), and how these factors influence the device's features and fundamental switch operations.

What are the static and dynamic characteristics of switches that are important to understand?

The static and dynamic characteristics of switches include their turn-on and turn-off characteristics, which deal with both static and dynamic behaviors, as well as the device's IB (current-voltage) characteristics.

Why is it necessary to protect power electronic devices from disturbances and surges?

Power electronic devices need protection from disturbances and surges to prevent damage and ensure reliable operation. Protections like high dv/dt and di/dt protection, as well as thermal runaway prevention, are necessary to handle electrical and thermal stresses within the device.

What are some of the modeling approaches discussed in the lecture for power electronic devices?

The lecture discusses various modeling approaches including SPICE models, state space models, and the selection of appropriate simulation tools for power electronic devices.

How does packaging affect the performance and choice of power electronic devices?

Packaging affects the performance and choice of power electronic devices by influencing factors like compactness, thermal management, and device mounting strategies. Different packaging types, such as ceramic or plastic, can be chosen based on thermal runaway considerations and cooling systems.

What are the ideal conditions assumed for switches in power electronics?

In ideal conditions, switches are assumed to have zero resistance or voltage drop when on, infinite off-state resistance, instantaneous switching, zero power dissipation, and fully controllable on/off transitions without requiring any power to drive or control the switch.

What are the different types of power semiconductor devices discussed in the lecture?

The lecture discusses various types of power semiconductor devices including uncontrolled devices like rectifiers, controlled devices like power silicon diodes, and different categories of controlled switches such as regenerative, non-regenerative, and integrated packages like IGCT and intelligent power modules.

How does the choice of a switching device depend on its power ratings and frequency ranges?

The choice of a switching device depends on its power ratings, which are determined by the product of current and voltage it can handle, and its frequency ranges, which dictate the suitability for low or high-frequency applications.

What are the advantages of Silicon Carbide (SiC) devices in power electronics?

Silicon Carbide (SiC) devices offer advantages in power electronics due to their ability to handle higher power levels, potentially pushing the power ratings of devices like power MOSFETs into the range of IGBTs, and IGBTs into the range of GTOs, thus improving efficiency and performance.

What are snubber circuits and why are they used in power electronics?

Snubber circuits are used in power electronics to limit di/dt and dv/dt, protect against overvoltage during switch turn-on and turn-off, and reduce switching losses. They are essential for protecting switching devices from failure due to thermal, overcurrent, and overvoltage stresses.

Outlines

00:00

🔬 Power Electronics Switch Analysis

This paragraph introduces the lecture on advanced power electronics and control, focusing on the analysis of switches. It discusses the need to consider device physics, such as doping levels and material types, to understand the operation of switches. The lecture aims to cover the fundamentals of switch operation, including turn-on and turn-off characteristics, and the importance of protecting devices from disturbances and surges. It also touches on the electrical and thermal stresses within devices, the importance of modeling and simulation tools, and the different packaging and mounting strategies for power electronic devices.

05:04

🔌 Ideal Switch Characteristics and Ratings

The second paragraph delves into the ideal characteristics of a switch, including instantaneous on and off times, zero power dissipation, and fully controllable transitions between states. It contrasts these ideal conditions with the real-world specifications and ratings of switches, such as forward and reverse blocking capabilities, current carrying capacity, and switching speed limitations. The paragraph also addresses the need for protection against high di/dt and dv/dt, which can damage switches, and the importance of considering thermal limits and heat dissipation paths when choosing a switch.

10:08

📡 Classification of Switching Devices

This section categorizes switching devices into uncontrolled and controlled types, with further细分 into regenerative and non-regenerative devices. It discusses the applications of various devices like rectifiers, power silicon diodes, Diac, Zener, MOV, and their uses in protection and regulation. The paragraph also covers the classification of controlled devices such as BJT, IGBT, MOSFET, and the necessity of anti-parallel diodes for regenerative capabilities. It mentions integrated packages like IGCT and intelligent power modules, and the role of power conversions capacity and switching frequency in device selection.

15:10

🔋 Applications and Characteristics of Power Electronic Devices

The fourth paragraph explores the applications of power electronic devices like thyristors, GTOs, IGBTs, and MOSFETs across different power handling capabilities and frequency ranges. It discusses how thyristors are used in high-power rectifiers and welding machines, while GTOs find applications in high-performance motor drives and robotics. IGBTs are highlighted for their versatility in various power levels and applications like UPS, motor inverters, and electric vehicles. MOSFETs are noted for their high-frequency, low-power applications in regulators and power supplies. The paragraph also introduces SiC devices as an advancement over silicon, pushing power capabilities further.

20:12

🛠 Protection and Snubber Circuits for Switches

This paragraph discusses the importance of protecting power electronic switches from overcurrent, overvoltage, and excessive di/dt and dv/dt rates, which can lead to thermal failure or reduced switch lifetime. It explains the use of snubber circuits to limit these rates and overvoltage during switch transitions. The paragraph describes different types of snubber circuits, including RC, RCD, and reverse polarized snubbers, and their applications in various switch types. It also touches on the use of fuses, circuit breakers, and heat sinks as protective measures.

25:18

🏗 Gate Drive Requirements and Protections

The final paragraph of the script focuses on the gate drive requirements for switches, emphasizing the need for protection against high di/dt and dv/dt. It discusses the use of snubber circuits for thyristors, GTOs, and MOSFETs, and how these circuits protect switches during turn-on and turn-off. The paragraph also mentions the upcoming discussion on gate driver circuits and their features in the next class.

Keywords

💡Power Electronics

Power electronics is a branch of electronics that deals with the control and conversion of electrical power. In the context of the video, power electronics is the overarching theme, focusing on advanced topics such as the analysis and application of power switches. The script discusses various aspects of power electronics, including device physics, switching characteristics, and protection mechanisms, which are all crucial for understanding the efficiency and reliability of power systems.

💡Switching Characteristics

Switching characteristics refer to the behavior of a device when it transitions between on and off states. The script delves into the turn-on and turn-off characteristics of switches, which are critical for understanding their static and dynamic performance. These characteristics influence how well a device can manage electrical power without causing disturbances or damage, as highlighted by the discussion on dv/dt and di/dt protections.

💡Device Physics

Device physics pertains to the understanding of how the physical properties of a device, such as doping levels and material types (high or low bandgap), influence its performance. The script mentions that analyzing switches from a device physics viewpoint helps explain different features and the operation of the fundamentals of these switches within power electronic devices.

💡Thermal Runaway

Thermal runaway is a condition where a device heats up to a point where it cannot dissipate the heat effectively, leading to further heating and potential device failure. The script discusses the importance of thermal management in power electronic devices, emphasizing the need for protections like thermal runways to prevent such scenarios.

💡dv/dt Protection

dv/dt protection refers to the measures taken to protect power electronic devices from the damage caused by high rates of voltage change (dv/dt). The script explains that high dv/dt can lead to device failure, hence the need for proper protection mechanisms, which is a key aspect of ensuring device reliability and longevity.

💡di/dt Protection

di/dt protection is about safeguarding devices from the effects of high rates of current change (di/dt). The script mentions that high di/dt can also lead to device damage, and thus, it is necessary to incorporate protection techniques to manage these changes effectively.

💡Gate Drive

Gate drive is the mechanism by which power electronic switches are controlled. The script discusses the gate drive requirements for switches, emphasizing that while it is assumed to require no power in ideal conditions, in reality, it does require power and has specific current and pulse generation needs. Gate drive is essential for the proper operation and control of power electronic devices.

💡Snubber

A snubber is a circuit component or network used to protect power electronic devices from voltage and current surges during switching operations. The script describes various types of snubber circuits, such as RC snubbers and RCD snubbers, which are used to limit di/dt and dv/dt, thereby protecting devices from overvoltage and thermal stress.

💡IGBT

IGBT stands for Insulated Gate Bipolar Transistor, a type of power semiconductor device used for high-power applications. The script mentions IGBTs as a significant component in power electronics, used in a wide range of applications due to their ability to handle substantial power levels while operating at relatively high frequencies.

💡SiC

SiC refers to Silicon Carbide, a material used in the manufacture of power electronic devices that offers advantages over traditional silicon. The script discusses SiC as a material that can push the power handling capabilities of devices, allowing them to operate at higher temperatures and voltages, which is a significant advancement in the field of power electronics.

💡Thyristor

A thyristor is a four-layer, three-terminal power semiconductor device used for high-power rectification. The script mentions thyristors in the context of high-power applications such as HVDC power transmission and motor control, highlighting their role in controlling and regulating electrical power in various industrial settings.

Highlights

Discussion on the analysis of switches in power electronics, focusing on their detailed operation.

Exploration of device physics, including doping levels and material types, to explain different features of switches.

Explanation of the fundamentals of switch operation, including turn-on and turn-off characteristics.

Importance of protecting devices from disturbances, surges, and the necessity of protections like high dv/dt and di/dt.

Focus on electrical and thermal stresses within the device and the behavior of power electronic devices.

Introduction to different modeling viewpoints, including SPICE models and state space models for power electronics.

Discussion on simulation tools and hardware implementation approaches for power electronics switches.

Overview of packaging considerations, including thermal runaway and cooling systems for power electronic devices.

Emphasis on the ideal conditions assumed for switches, such as zero resistance and infinite voltage blocking capability.

Analysis of the power dissipation of switches, which is assumed to be zero in ideal conditions.

Discussion on the controllability of switch transitions and the limitations in real-world applications.

Introduction to ratings and specifications of switches, including forward and reverse blocking capabilities.

Explanation of the need for protection against high di/dt and dv/dt to prevent switch damage.

Discussion on the gate drive requirements and the power needed to drive and control switches.

Overview of the different types of power electronic devices, including uncontrolled, regenerative, and controlled devices.

Analysis of the applications of various power electronic devices based on power handling and frequency ranges.

Introduction to the advantages of SiC devices over traditional silicon devices in power electronics.

Discussion on the different types of power diodes and their applications in various power electronic systems.

Overview of the applications of thyristors, GTOs, and MOSFETs in power electronics and motor control.

Discussion on the protection methods for power electronics, including snubber circuits and overcurrent protection.

Conclusion and预告 of the next lecture focusing on gate driver circuits and their features.

Transcripts

Welcome to the NPTEL courses on advance power electronics and control, this is a seventh

lecture, today we shall continue with the application of the analysis of the switches,

so we shall discuss about the switches in detail, so based on this actually viewpoints,

so we cannot actually consider a physics point of view that is device physics that doping

level, kind of material we are using, whether a high band gap material or the low band gap

material and from there, we try to explain different features.

Then, explain the operation of the fundamentals switches of the devices, then we shall take

it a physics point of view, so that is called device physics and same way we talk about

a switching characteristics and its turn on characteristics, turn off characteristics,

deals with the static and the dynamic characteristics; IB characteristics, now required to understand

how also to protect the device from unnecessary disturbances, surges and the hazards, unnecessary

disturbances and surges and we require normal protections like high dv/dt protection, da/dt,

thermal runways.

So, focus on electrical and the thermal stresses within the device, drive viewpoint that is

actually emphasis all the behaviour of the power electronic device which can be applied

to the different kind of driver circuits, modelling viewpoint we have a different kind

of model, we have a spice model of the power electronic, so it is as state space model,

so we can write a; we can have a different kind of modelling viewpoint, what kind of

simulation tool should be used.

Based on that we have a different kind of approaches, so simulations and the hardware

implementation or hardware in the loop applications kind of thing, we have various kind of hardware

interfaces like RTDS, ((OPAL-RT)) (02:37), so there we try to find it out the solutions

of a few systems and we try to model the power electronics switches, so we will modelling

more realistically, so that is actually the modelling viewpoint.

And packaging; so, it comes under different kind of packaging, so because you want that

the device has to be compact, may be comes across in the ceramics packaging, in may comes

across the plastic packaging depending on the thermal runaway and different kind of

cooling systems and the compactness and so it comes at the device mounting strategies,

removal of the heat sinks and how you will dissipate the heat in different way.

And force cooling devices and the connection issues, these are you know different criteria

to choose a particular device because please understand that modelling of the viewpoint

is also important because once you before put in a hardware, we generally simulate a

system, so if it is not very authentically simulated, then actually the fragility of

this model is in a questions, so all those aspects has to be consider while actually

taking a consideration of a switch.

And when you are considering a switch we always assume this is the ideal conditions that no

resistance or zero forward voltage, obstruct resistance is infinity when it switches off,

no current flows, when on, conduct; it can carry infinite amount of the current, both

in forward and the reverse direction, it comes under some kind of mechanical switches like

SPST and all those things.

When off, infinite forward and the reverse voltage, so you have studied the forward breakdown,

reverse breakdown voltage but in ideal consideration of the switches, this is not being considered,

it can switch instantaneously that is also not the case because it has a finite turn

on time, we have referred to the reduction of the different device and we have seen that

different turn on and turn of time.

Instantaneous off and on time and from on, off states, so transition between on and off

states is basically does not take any time that is resilient, it is instantaneous phenomena,

power dissipation of the switches is 0, why; because when voltage is high, when it is blocking

the voltage, no current flows and when it is conducts current, there is no voltage stop

across it, so both conduction and the switch transitions losses are also 0.

On to off or off too on transitions of the switches are fully controllable, so we can

wish to turn on and turn off wherever is required but we will come in to such picture that even

if actually in a different kind of topologies in future, even if you are triggered, current

due to the direction of the current, switch will not take the conduction till current

through that other device goes 0, then only transitions comes into the picture.

But in ideal condition we assume that whenever I wish, I can turn it on, whenever I wish

I can turn it off and it require no power or to drive or control the switch, so power

losses in a driver circuit is 0, these are the ideal characteristics of the switch and

ultimately, we will see that all switch deviates from it and so we required to have; we have

to find a closer approximation to this ideal conditions while we are analysing and discussing

about the switches.

So, for this reason, we have few ratings and specifications, so this is what we are saying

that you know are its forward blocking and reverse blocking capability is infinity ideally

but actually you have a voltage rating that will pacify your forward and the reverse voltage

blocking capability, the switch should have ideally infinite current carrying capability

but the data sheet will prescribe the current rating of the device.

Transition from the switching on to switching off and vice-versa is infinitely small or

it is instantaneous, this is ideal characteristics but switching speed and the frequency will

have a limitation on it, same way di dt protection, so that is also comes into the picture in

transitions, so high di dt leads to the damages of the switches, so we require to protect

from it, same way; high dv dt also leads to the damage of the switches.

We require to give a protections, what we assume that that will ideal characteristics,

there is no switching losses but there will be a switching losses, so requite to calculate

it accordingly we have to choose the switch, gate drive; we have said that gate drive does

not require any power, actually it requires power and gate driver requirements are many

things, what should be the amount of the current sink into the system.

And how; what will the pulses can generated, these are the issues, same operating zone

of the devices which voltage and which current it can operate separately, so that does not

go for the thermal runaway and high square rating for the fuse and other devices, temperature;

different functional point have different temperature, you should ensure that it does

not causes the thermal limits and also the thermal resistance from the heat sink to the

body.

So, what are the actual path of heat dissipations, so these are will be specified we have seen

already in the data sheet, this data has been specified.

So mainly, switch depend on switching rating, that is which they handle power and the product

of their current and the voltage instead of their power dissipation rate, so higher the

power dissipation rate, we say that switch rating will be higher and we shall find that

you know thyristors is actually the first person in that category, first entity in that

category, consequently the major attractive feature in the power electronic is its capability

to dissipate low power or the no power.

So, there are different kind of loss, if it is on even though we assume that the power

losses across the switch is 0 but it is not 0, so thus we have a conduction loss and thereafter

when there is turn on and turnoff takes place, so there is a switching loss and also there

is a loss at the gate driver, so these 3 losses we have to actually accommodate, switching

loss comes with a turn on and a turn off losses and conduction losses is depends on sometime

it is ((TON)) (10:23) * I square for the MOSFET.

And other device is a constant voltage drop into the current, so from this discussion

we can come out to the switching device choices, one is semiconductor device, so it can be

uncontrolled and controlled, so under uncontrolled, it comes to the rectifiers and the accessories,

thereafter it comes to the power silicon diodes, short key diodes, fast recovery diodes and

there is a accessories like you know this is the used mostly in the protection and the

regulations.

These are Diac, Zinner, MOV, thereafter within a control, we can classified broadly into

3 categories; one is regenerative, so you want that power to be feedback from the load

to the source like for example, when you are using electric vehicle and you are breaking

or you are going to downslope, then power should fit it back to the source for the better

utilisation of the power and if you wish that regenerative braking from the we use the device

like SCS, TRIAC, GTO and if you are using because it is current control device.

So, as long as current as high, it can be a bidirectional and you can have a non-regenerative

that is BJT, IGBT and MOSFET, generally MOSFET with the body diode and thus also have a regenerative

braking capability.

And to make IGBT and BJT regenerative, we require anti-parallel diode parallel to it,

so and thereafter it can have an integrated packages that is IGCT, so the packages actually

the MOSFETs and the IGBT together, thereafter intelligent power module, PIC, this kind of

entities are also there.

So, you see that actually power conversions capacity and the switching frequency, so different

kind of power handling capability is been used, so far low frequency applications, you

can have a thyristors and where power handling capability as high has 100 of megawatt, so

it find its applications in HVDC, thereafter you got GTO, so while it is a little high

frequency and quite high-voltage or high-power, so for this is a high power tribes and relevance.

Thereafter, whole zone comes under IGBT, so IGBT is basically fitting you can see the

actually very big portion of the power electronics, so it can handle power to some level up by

paralleling hundreds of kilo watt, thereafter for this is an application is a UPS distributed

power supply, small motors invertor, small drive or medium voltage strength drive, electric

vehicle applications.

There after it comes, there after you go to the higher frequency and little less power,

then it comes to the MOSFET, here the bipolar transistor has seen a lowest duration nowadays

due to the advent of the IGBT, so generally a bipolar transistors is have phased out most

of the cases, MOSFET will have a very high frequency but low power handling capability

and that finds the application is switching regulator is MPS, VTR modules of the phones

and power ICs and all those things.

So, these are the power ratings and the frequency ranges that is available for the existing

device and what kind of device we will choose based on those criteria has been discussed

here and new entrant is basically the you know, this is the SI limit and the SIC and

SIC is a material instead of the silicon and SIC will have an advantage of; they are pushing

the power obtained to the power three.

So, if it is a power MOSFET of SIC, so that can come into IGBT range, so if it is actually

an IGBT can come to the GTO range and GTO can come to the easily the thyristor range,

so this is a advantage of the SIC devices.

So, we have discussed about actually power diodes and power diode you know we have classified

that is based on of its recovery time, so this is called pin diode.

And it is used in fast and the ultra-fast rectifiers, mostly in which the DC to DC applications,

isolated DC to DC or non-isolated DC to DC applications and the PV systems and thereafter,

we have a Shottky diode, this is used for the power rectifier mostly, in case of the

active rectifier where you are making AC to DC keeping ((THD)) (16:22) limit low and then

after, solar cell applications so that as a fast recovery.

Mostly, fast recovery diode is used mostly in SMPS, where actually switching frequency

required to be high and it is mostly use in a parallel with the MOSFETS.

Thyristors; mostly used in high power rectifier, rectification of high-power AC in high-voltage

DC power transmissions, all are actually (()) (16:56) those who are actually ((sub-urban tents))

(16:58), still they use actually use high voltage rectifiers, used to control the welding

of the machine mainly MTAW, metal tungsten arc welding.

And the gas tungsten arc welding, this kind of thing they are actually it finds the applications,

it is used as a switch for different devices and power regulators and the motor control

because it is uncontrolled device based on this actually, voltage polarity, it will conduct

and sorry, it is a semi-controlled device and for this reason quite actually high power

requirement is there and we can make the current leading then it is will automatically commutated.

And thus in this kind of applications you know, thyristors point is suitability, now

comes to the GTO, is uses in high performance drive system such as a field oriented control

scheme that is vector control for the high power motor drive, robotics, machine tools,

it is used for the traction application because of the lighter weight and it used in high

power inverters where GTO's cannot fit the power, they have to use in the DC drive and

fax devices and DC chopper, used in AC drives.

Or rectification operation of the high power application used in stabilised power supplies,

used in high-power induction heaters and used static VAR compensators or different kind

of fax devices.

MCT; mostly it founds an application in a circuit; high power circuit breakers, it uses

high-power application like high power conversions, their use for the induction heating, UPS system,

is also used for the convertor; DC to DC convertor, high power DC to DC convertor, mostly use

for this actually harvesting power from the offshore, wind plant or something, power handling

capacity can go as higher megawatt level.

Variable power factor operations are used as the force commutative power switches, now

IGCT is the new entrant, one of the application of the IGCT; it can handle around 100 times

or 1000 times more power than the IGBT, the same actually the frequency range, variable

frequency inverters is definitely a preferred choice for it because we can go to the high-voltage

applications and if we run at a high frequency since you have heard about the VYF control.

And you can track the switching frequency little higher then we can have a high-power

gearless operation that is one of the challenge we are facing, because once we have a very

high power drive, if you wish to go for the higher power rating, then torque compromises,

so keeping torque same if we wish to go to the high-power, high speed, so this kind of

application where IGCT has been preferred.

Drives and tractions, so it is a preferred choice and multiple IGCT can be connected

in parallel in series to actually increase the power handling capability of the line.

Same way, SiTh, we have discussed already used for the system of the energy accelerators,

so inverter with the soft switching that is actually we can make the switching losses

low or 0, that is called soft switching that can be ZVS or ZCS that mean, ZVS is 0, voltage

switching or ZCS or 0 current switching that can be easily achieved by this SiT and it

is one of the member that find very good applications for the current source inverter.

We will come across this voltage source and current source, different kind of topology

and we shall discuss about it with that the switches, pulse power inverter; so this kind

of application is quite natural for this in case of the pulse mostly in actually welding

kind of application now, let us come to the MOSFETs.

MOSFET used for the high frequency low power drives, motor controlled applications, there

are DC choppers, linear voltage regulators, power supply etc.

So, you can see that these are the different applications of the IGBT, I have seen that

IGBT fit into this actually day to day application quite well, so we have a modern elevator that

is having an adjustable spring type and these are run by the IGBTs, so power goes to 100s

of the kilowatts, there after you may have a inverter servo robotics, so that we use

for the different kind of mining operations mostly.

They have to arc machine or arc welding where we want to maintains a power quality better,

power supply is UPS, SMPS for a highly data bank kind of applications, transportations

for the railways and all those things and battery charger, electric vehicle, these are

whole area is of IGBT.

Now, let us come to the protections, we have talked about dv dt and di dt protections,

now let us and how you can achieve that; we achieved by snubber, due to the shortage of

time we shall take it out for the thyristors and this concept can be extended to the all

the switches we have talked about, the switching device and the circuit's component may fail

due to the following reason, so failure may come due to the thermal failure or this is

called thermal runaway.

Overcurrent and overvoltage, so these can be controlled by the supply site but you know

and also you know it may cause with the excess di dt and the dv dt, so that may actually

cause failure or reduce the life time of the switches and also excessive switching loss

while conduction, so we required to reduce this 2 losses mainly, by snubber directories.

Power electronics and switching device and the component can be protected from the overcurrent

by placing the switch will fuses or the MCB or now different kind of circuit breakers.

Heat sinks; over the different kind of cooling technique fins and fans are used to take out

the excess heat and what snubber will do; snubber circuit required to limit di/dt and

dv/dt and overvoltage during turn on and turn off of the switches.

So, there are different kind of snubber, so this is the device you know we required to

protect; to protect that device we require so many device, so anyway so this L will be

in a series with the device and this will ensure the protection over di/dt, it will

limit the di/dt protection and this RC rate work will come while turning on because you

know once actually, correct flows if it is an off and this is the actually, voltage across

VA is > VB so and you are triggering.

Then, this this will come for high dv/ dt protection and also while actually taking

off this capacitor will ensure that actually current flows through it through this R1 and

ultimately, it gets the capacitor charge in a reverse manner and it will ensure that really

over the dv/dt, same way we can have us different kind of circuit here, value of R1 is more

than R and mostly it is comes into the picture.

Because if you see that actually it is a RC network that is searching and this is the

time constant of the circuit and will ensure the time constant of the circuit in such a

way and that time stress across the switch does not comes, once capacitor is fully charged,

whole stress will come across the switches and if we used reverse polarity, inductor

will be in the same position and what happened here actually, this R has to be set beside

and R1 will be here.

So, it comes into the picture of this circuits while actually it did not turn off and it

is called a reverse polarised snubber circuit and it will actually protect for the dv/dt

protections and same way we have a unpolarised snubber circuit, so it can operate both the

connection it is with the, this is the RCD snubber, this is RC snubber and your time

constant will require to choose in such a way that stress dv/dt stress across the switches

get reduced.

Since it is unpolarised, it can operate for the bidirectional thyristors, so where snubber

finds its applications?

So, let us consider the simple chopper kind of operation and you have a high source inductance,

then you will have to provide the diodes as a snubber to have a you and since it is a

high and we can assume that load is a high current source, in this conclusion you know

you have to allow flow of current continuously and thus it will find this path like while

turning off, so this diode comes into the picture while turning off.

And similarly, power MOSFETs turn off, once the gate pulses is been withdrawn and negative;

little negative gate pulses, so ultimately you know the switches does not have any voltage,

so to short, so gradually it is actually getting up forward blocking voltage capability and

thus capacitor will be charging, so what will happen; since capacitors will start charging

this way, so automatically actually current through this MOSFET will go low and will have

a high dv/dt protection.

So, this is thyristors snubber what we have discussed here and so this is the RCT snubber

and it is used for the thyristors and this is the snubber used for the GTO, same snubber

and it can be used for the GTO, this is the turn on snubber and this will be the turn

off snubber.

So, we shall continue to the requirement of the gate drive, we have talked about the protections

and in next class, we shall talk from the gate driver circuit and we shall looking forward

to start about the different features of the gate driver circuit in our next class, thank

you.