Okay, so we are going to start the course on Power Electronics. So I am sure all of

you know what is electronics? Electronics is basically going to have semiconductor devices

which control electronic circuits. Whereas if you look at power electronics, you should

know the applications, basically the applications are right from mobile phone chargers which

is of only about a few watts rating to, if you look at industrial drives, which may be

of megawatt rating. So the entire range generally is covered by power electronic circuits. So

if we talk about industrial drives, they are of megawatt rating. Whereas when we talk about

normal electronic circuits, they are only of a few watts rating, but we are looking

at power electronics, which is consisting of both power which is high power circuit

as well as electronics which is low power circuit. So power electronics essentially

deals with power semiconductor device circuit. That is the circuits that are made up of power

semiconductor devices. Some of the power semiconductor devices are like power diodes, SCRs. We will

be talking about each of these devices later. Then you can say power BJT's, power MOSFETs.

So, all these devices, when you talk in electronics you will be talking about just BJTs or just

MOSFETs and you will be talking about diodes. Whereas here we are adding a prefix power,

so power diodes, power BJTs, power MOSFETs and IGBTs.

So all these things are going to be ultimately connected in different circuit form to make

either rectifier which is AC to DC converter or inverter which is DC to AC circuit and

so on. So we will be dealing with power semiconductor devices which are connected into different

circuit format and they will be used for different applications. Be it mobile phone charger,

be it a big rectifier or be it an inverter for industrial drives, that is what we are

going to talk about in this particular course.

Okay, so in electronics normally we handle 15 volts or 5 volts, nothing more than that.

So let me say, what is power electronics versus electronics. So electronics, you are going

to handle only 5 volts, 15 volts, I would say plus or minus 5 volts plus or minus 15

volts, 3.3 volts and so on and so forth. And as far as currents are concerned, they may

be microampere, milliampere, at the most a few amperes, not even 10 amperes it will be

generally less than that. Most of the time you will handle very, very small power levels

which will go of the order of a few watts or a few miliwatts nothing more than that.

Whereas in power electronics, normally the same electronic circuit may be made bigger

to handle higher current, cross sectional area should increase. To handle higher voltage

the length should increase. So if I increase the length, it will be a

high voltage device. If I increase the cross-sectional area, it will become a high current device.

So if I can handle higher currents and higher voltages, in that case I would call that as

a power electronic device. And when I put several devices or one or two devices in a

circuit, I call that as a power electronic circuit and it can handle generally controlling

an electrical machine or controlling maybe a transmission system, maybe controlling the

power that is delivered through the UPS uninterruptible power supply or switched-mode power supply

within your computer, all those things. Whatever circuits we see in these things are all power

electronic circuits because they handle a little larger power. What I mean by a little

larger power is power electronic circuits can handle right from a few watts to 10s of

or 100s of megawatts. So it can handle a wide range of power normally.

You are going to have right from a few watts to hundreds of megawatts, that can be handled

by all these power electronic circuits. Typical applications: Why should we study about it?

Unless there is some importance we should not be studying about it. So your mobile charger,

that is typically a power electronics circuit. If you look at the mobile charger, it is connected

to a 230 volts AC mains. If you look at the carefully it will say 90 volts or 80 volts

until 270 to 280 volts. So it is known as the universal adapter. So it will be able

to charge your mobile phone, whether it is in the US where it is 110 volts, right? Or

whether it is in Europe or India which is 230-240 volts, so it would be able to adapt

itself to any kind of AC supply voltage as long as it is within 50-60 hertz range.

India has 50 hertz and Europe has 50 hertz, USA has 60 hertz, so you will have different

frequency ranges. But from that 230 volts or 110 volts AC voltage, first of all there

will be a rectification that will be taking place; rectifier is a power electronic circuit

there. So it will rectify to DC, after rectifying to DC it has to step it down to a smaller

voltage, whatever be the mobile battery's voltage. Maybe it is 1.5, maybe it is 3 volts,

whatever it is, it will correspondingly step it down, no transformer of course because

DC voltage transformer cannot work on DC obviously, so there will be a step down converter which

is also a power electronics circuit. So if I try to look at the mobile charger,

I am going to have something like this. I will have first of all 230 volts, 50 hertz

or 110 volts, 60 hertz AC. This is now connected to a rectifier. The rectifier is going to

convert this into corresponding DC voltage, if it is uncontrolled rectifier with the help

of diodes, just diodes, then I am going to have very clearly 2VM by pi. All of you know

that expression, I suppose maybe in ELL 100 initially we had looked at some amount of

rectifier circuits. So we are going to get three, 2VM by pi approximately. So whatever

is the voltage that we get, it may be 0.9 times V rms roughly,Okay.

Now this has to be stepped down to the voltage that is corresponding to the battery voltage

of the mobile. So from here I may have to have a DC to DC converter which is again a

power electronic circuit. So I would say this is a PE circuit. This is also a PE circuit,

power electronics circuit. Now the output of this will go to the battery and it has

to be regulated. In case from 230 suddenly the voltage comes down to 180 volts in the

grid, still I should be able to give the same value of voltage to the battery, so there

should be some amount of adjustment. So very clearly there are several functions

that are associated with a power electronics circuit. One is regulation, so I have to regulate

the voltage, definitely voltage regulation has to be done irrespective of variation in

the input voltage irrespective of variation in the load, because battery may be fully

charged in which case the voltage will be close to whatever is the rated voltage. If

it may be very, very low charge level, then in that case the voltage is going to be somewhat

lower. See, if you talk about a 5 volts battery for example, it can go until 5.5 or 5.75 volts

when it is fully charged, slightly higher than rated voltage.

When it is completely discharged, it can be as low as 4 volts or 4.2 volts. So you can

have that much of variation. So despite those variations, the current that is going into

the battery to charge it, that has to be regulated, the voltage has to be regulated. So it is

not only voltage regulation, even charge or current regulation, both have to take place.

Second thing we may require is, we will have some kind of isolation required between the

input and output side. Isolation is you do not have to have a common ground like a transformer.

Think about a transformer. In a transformer if I am going to have this as the primary

and this as the secondary, this may be connected to the ground. This may not be connected to

the ground or it may be grounded separately. You will not connect the two together. Never.

Right? So the isolation takes place whenever I use

a transformer, two winding transformer not auto transformer, right? Auto transformer

is a single winding transformer. You connect primary and secondary together. So whenever

I use a transformer, I am going to have isolation between the primary and secondary. Similarly,

isolation will be possible when I use a power electronic circuit, probably I will insert

a transformer somewhere, probably I will insert something else. I will have to check what

are the stuff I have to insert. So these are two primary functions of a power electronic

circuit generally, regulation and isolation. Apart from that I would also convert probably

AC to DC or DC to AC or I may convert AC to AC with variable frequency. That is I give

a 50 hertz supply, I might like to have a 100 hertz AC output. So AC to AC conversion

is also possible and of course the last one which I have left out which is DC to DC. All

four types of conversions would be possible. So I might like to convert the frequency,

magnitude or both. So DC, I can say it is 0 frequency, right? So I can convert the frequency

or magnitude or both. Either any of them is possible with the help of a power electronic

circuit. So one example I gave you was the mobile charger,right?

Another example when we talked about induction motor for example, you guys had done a variable

frequency induction motor drive experiment in the laboratory. What was sitting inside

that box was actually a rectifier which was initially converting the 50 hertz AC into

DC. Subsequently there was an inverter which will convert that DC into any variable frequency,

AC. So that is a typical example of creating an adjustable speed drive. If I want an adjustable

speed drive, I would be able to use AC to DC converter and then the DC to AC inverter,

that will essentially enable me to get a variable speed drive or adjustable speed drive. So

this is another example, right?

So if I look at power electronics system in general, what consists of a power electronic

system? In a power electronic system, I am first of all going to have a source. That

source could be DC; it could be AC whatever it is. So, 50 hertz, 60 hertz or just a battery

which is a DC source. From here I am going to have a power converter circuit which is

actually the heart of my power electronic system. So it is going to convert maybe AC

to DC, DC to AC. Whatever it does it is going to have some functionality. Now this power

converter system will give an output. The output will be in the form of a particular

voltage and particular current, right? So I will have some voltage and some current,

so let me say them, I may call them as instantaneous values small v and small i. If it is three-phase,

of course I will have VA, VB, VC, IA, IB, IC obviously. So I am going to have voltages

and currents. Now I will have a load here. The load could be an induction motor. The

load could be a battery of the mobile. It could be like a keyboard, musical keyboard

which has you know in olden days we used to call it as an eliminator, so it is essentially

a charger. Basically, it is allowing you to convert AC into DC, right? So this is the

load. Now the load might require only 5 volts. It may require 400 volts and 50 hertz if it

is an induction motor, line to line voltage, it might require maybe 25 hertz and 200 volts

if I want to run it at lower speed. So I need to know the requirement of the load.

So what I will have here will be a sensor or a transducer. So what I will do is to sense

from here the voltages and currents. Maybe, if it is a battery it is good enough for me

to sense only the voltage and current. If it is an induction motor I need to sense voltage,

current as well as maybe torque and speed. So depending upon the kind of load, I may

have more than electrical quantities being sensed. So I would rather call it as a sensor

or transducer because transducer converts mechanical signal into electrical signal,

thermal signal into electrical signal and so on and so forth.

Invariably I would like to convert that into electrical signal because I am by and large

handling electrical circuits. So everything converted into electrical signal will give

me a lot of advantage in terms of processing them. So I would first of all like to sense

the mechanical quantities or electrical quantities or thermal quantities and so on. For example,

an air conditioning system, right? How does the compressor go off? Because it senses the

temperature, maybe you have set it at 25 degrees and the room has already reached 25 degrees,

so the compressor will go off. So for that you have to first of all convert the temperature

into an equivalent electrical quantity in all probability so that, the control system

realizes, yes, I have already reached the temperature so I can turn off the compressor,

right? So you will have basically a sensor or a transducer.

Now what comes out here is actually the sensed quantity in the form of an electrical signal.

Okay, now I may have a controller here or a control circuit. I may call this as a control

circuit, so in the control circuit, one will be a reference value. I may require 25 degree

centigrade I might require 5 volts. I may require 400 volts and 50 hertz. So the reference

value will be given as one of the input and the sensed value will be given as the other

input. Now the control circuit is actually the intelligence or the brain of this entire

power electronics system, that is going to now make a decision based on what kind of

inputs it is getting. It is going to make a decision whether I have to reduce the current,

reduce the voltage, increase the frequency. All those things it will decide.

So once it decides, it is going to give this to the power converter in all probability

through an isolation circuit. Because I would not like to mix the ground of electronics

or the control portion with that of the power portion because power portion will have a

thick conductor going to the ground. So the currents that are flowing are of the order

of hundreds of amperes, whereas electronic portion will have only milliamperes or 1 or

2 amperes. So if something goes wrong in the power portion, I do not want that current

to circulate through the electronics portion. If it circulates, it will end up burning the

entire electronics portion, so the grounds invariably have to be isolated.

So the isolation circuit essentially functions as a protection against burning the electronic

portion of the circuit. If there is a problem in the power portion, that will not affect

the electronic portion and apart from that I might require an amplification also. The

isolation is meant for you know decoupling the grounds of the electronic portion from

that of the power portion, whereas amplification is required because the power electronics

circuits are big, they will not work on milliampere. Even the base drive that you required for

a transistor power transistor, for every power device, I will add a power instead of saying

transistor, I will say power transistor. Okay, so if it is a diode, I will say power diode

because power diode means it indicates it is going to handle a larger power.

So if it is a transistor, if the base requires let us say 1 ampere, the amplification factor

probably is about, you know 10 ampere or 15 ampere maybe is going to flow through the

power circuit, but the base will be some fraction of that.

Collector current will be much higher. Base current will be much smaller. So maybe the

base current is 1 ampere, in all probability my electronic circuit, which is actually this

control circuit that may not be able to give even that 1 ampere. So I will need an amplification

circuit. So in all probability I will need isolation and amplification, both have to

sit in between the control circuit and the power electronic circuit. So this is the overall

structure of a power electronic system. So we will have a source, it will go to a power

converter circuit, which is going to convert the voltage magnitude and frequency as per

the requirement of the load. The load will function once it gets the supply.

So the load is going to give some output. So the voltage, current and output, everything

will be sensed. After sensing everything, we compare it with the reference values that

we want to achieve normally and then accordingly we take a corrective action until the error

becomes zero. This is the error, right? When I compare these two, so if I say that

this is v whatever star generally call the reference values as star, i star, v star,

omega star. Omega is speed, torque star whatever. So star is generally indicated for reference

values. Okay. And the actual values generally I specify as “Act” will be the subscript.

I generally say v actual, i actual, omega actual, Te actual and so on. Electromagnetic

torque actual So these two are compared, which actually comes out in the form of an error,

right? That error is processed by the controller circuit and that is what actually adjust the

voltage or current or whatever output of the power electronic circuits such that the error

becomes zero. This is the overall configuration of the power

electronic circuit. We are going to mainly concentrate on this portion, but as a power

electronic engineer, I need to know some amount of sensor and transducer working. If I do

not know, I cannot deal with power electronic circuit. I have to know how to isolate and

how to amplify. If I do not know, I cannot again design a power electronic circuit. So

I need to know some amount of analog electronics. Maybe not as much as Shauree knows, but I

have to know some amount of analog electronics, some amount of digital electronics, some amount

of sensors, some amount of transducers, large amount of characteristics of the load.

So if I am talking about a battery, I should know at least something about the battery.

If I have to control a machine, I better know what the machine can do for me,right? So I

should know the dynamics of the loads. I should know the dynamics of all the electronic circuits,

sensors, transducers, amplification circuits, isolation circuits and so on and so forth

apart from knowing the actual power circuit,right? Unless I know

I cannot control the power circuit. I do not know the power circuit, how will I control

it? So I better know all these things. And most of the times we model and simulate these

circuits before we really put it on the hardware. So modelling and simulation is another aspect

we need to know, how to model a power electronics circuit, how to make a control system configuration

for a power electronics circuit? So power electronics basically becomes interdisciplinary

because of all these areas involved. So I told you that analog and digital electronics,

and these days we use a lot of computers and microprocessors and digital signal processors,

so we need to know definitely at least something about the assembly language and you know other

internal machine language of the processes at least to some extent. So we should know

some amount of computer technology. Right? We definitely need to know to some extent

physics of the devices. You may not know, again as much as whoever taught you physical

electronics, but definitely we need to know how much of heat is going to be generated

within the circuit. If I do not know, then I will not be able to make sure that it does

not conk out. If excessive amount of heat is generated, then it will get overheated

and it will conk out and for dissipating the heat we use something called a heat sink.

This is a heat sink. You can see it looks like an insect. So many legs,right? This is

known as heat sink. Normally what we do is you can see that that is a hole, right? I

will circulate this. Not a problem. Generally to that hole, we will put the device and bolt

it together. This is a device. Small device. Okay.

It may be a transistor or thyristor, I did not check exactly what the device was. Maybe

an IGBT spoiled IGBT. So from the lab I just picked up. Okay, so this is a device, the

device as it is if I connect it to the circuit, it will generate a lot of heat and it will

conk out eventually. So to dissipate the heat, if I put a black body attached to it, its

body, then the heat is going to be transferred to the black body. Black body will radiate

it. To increase the surface area these legs are there, it is essentially increasing the

surface area, right? So typically for knowing how much heat will

be dissipated, I should know at least the physics of the device, pass it on. I am just

telling you that these two have to be put together so that it is bolted together. That

is a hole here. There is a hole here. They will be put together and bolted, right? So

I should know device physics to a large extent. I should know modelling and simulation and

I should know transducers, sensors. I should know the working of control systems. Of course

machines and drives go without say,right? because I am controlling most of the times

those things. I better know it,right? So power electronics happens to be interdisciplinary

in nature because of these things. Right? Now, apart from this, as we know when I use

a rectifier, it is converting AC to DC, right? So if I am having an AC supply here and I

put a simple diode here and here is my load, this is a half wave rectifier. Simple half

wave rectifier, only in the positive half cycle the current will flow, negative half

cycle current will not flow. So if I look at the current at this point it will have

only half of the sinusoid. The other half will be missing. What we want from the power

system normally is the currents and voltages should be as much as possible sinusoidal so

that it does not create a headache. I do not want harmonics. I do not want DC

because the machines are generally generating sinusoidal currents. They would like to supply

sinusoidal voltage and sinusoidal current. If I am asking them to deviate from that,

they can misbehave, so the power electronic circuits create automatically some amount

of misbehaviour whether we want it or not, when convert AC to DC with the help of a half

wave rectifier, I am automatically creating an aberration in the wave form. The wave form

is not anymore sinusoidal. It is non-sinusoidal. So there is something called power quality

we talk about. Power quality essentially deals with whether

the voltage is sinusoidal whether the current is sinusoidal, whether it is only 50 hertz,

not deviating from 50 hertz, whether the power factor is unity. All these things are generally

specified as the power quality indices. There are several power quality indices, one is

voltage, the second one is current. Whether they are sinusoidal in nature, whether they

are at unity power factor, whether they are at 50 hertz, all these things are examined

and said that if all of them are adhering to the normal standards, we say power quality

is very good. If they are not adhering to the standard,

then we say it is not good. For example our power system is supposed to be a very weak

power system because most of the times our demand is higher than the supply. The demand

is always larger than the supply because of which voltage sags, voltage will not be at

230 volts or 240 volts. It may come down to 180 volts, so the equipment which is expecting

230 volts, if it gets 180 volts, it is not a good thing for the equipment. 

So our

power quality normally is not good most of the times and added to that we put so many

power converters, so obviously power quality becomes worse. So we guys are the culprits.

Power electronic engineers are the culprits who make the power quality much worse. But

power converters definitely help us in controlling the voltage, controlling the speed, controlling

the torque, controlling any process. So obviously we employ a large number of power converters

these days. But we also essentially make the power quality bad, so power electronics again

comes to the rescue to improve the power quality. So on one side we create the problem. On the

other side we try to solve the problem. So power electronics is the root cause of the

problem. But some of the power electronics circuits we again employ to improve the power

quality. So we call them as power conditioning or power quality conditioning. For example,

SMPS and UPS what is UPS? Uninterrupted power supply will make sure that 230 volts, 50 hertz

is given to my computer, no matter what. SMPS make sure that plus or minus 5 volts, plus

or minus 15volts, 3.3 volts; all of them are given to the computer circuits whichever require

whatever voltage. So I am creating a problem with other power electronic converters. But

SMPS and UPS take care of these problems; they essentially insulate the actual computers

from these problems. They supply the actual value voltage and frequency that is needed

by the computer, right? So power conditioning is generally done by UPS, SMPS and such circuits.

Those are also power electronic circuits, right? So if you look at the application of

power electronics, it is plenty.

But before that, let us try to see what all different types of power converters are there.

As I told you in power converters, I can have AC to DC, which I may call as rectifier, right?

When I convert AC to DC, normally we will call that as a rectifier circuit, right? I

may have DC to DC, which normally are known as DC to DC converters in most of the cases.

But some people call it as choppers, chop they chop the voltage and then convert it

into another value of DC. What I am trying to say is, if let us say originally the voltage

is this, this is 20volts, right? I want only 10 volts on an average basis. I cannot make

20 to 10 obviously, if I have to do that, I have to use two resistors, say 10 ohms each,

drop off half the voltage in 10 ohm and other 10 ohm will have 10 volts.

I do not want to do that because it is lossy. So rather than using a potential divider,

if I try to do it this way, maybe I have 20 volts coming for T by 2 again ,0 volts coming

for T by 2 again, 20 volts coming for T by 2, 0 volts coming for another T by 2. What

will be the average value? 10 volts, right? Looks kind of strange, right? Because how

will the motor know, you are giving only for half the time. It will start rotating. That

is what you might think but this I will do at kilohertz or higher frequency, hundreds

of kilohertz whereas motor has a huge time constant. Right? Mechanical time constants

are generally very large. It is going to take a while for the motor to realize that, okay;

I have to pick up speed quickly because they have given me 20 volts. Right?

And they have not given me any voltage, let me just come down to zero speed. It is not

going to happen because the inertia is going to take a while before really the motor respond

to the variation in the voltage, because of which the average value seems to be 10volts.

So this is what is DC to DC conversion. For example, we chop the voltage for 50 percent

of the time. We apply the voltage for 50 percent of the time, so w e call this sometimes as

chopper,right? This is DC to DC conversion. We will have

AC to AC converters as well. So I can do the same chopping in AC. Imagine, maybe I have

a switch. Okay, so if this is my original AC voltage, I keep the switch on for only

some duration and again I keep the other switch probably in the opposite direction on only

for some duration. What will happen? RMS value will decrease. If I have it on for longer,

I am going to have a higher RMS values, if I have it on until here for example,right?

so which means this entire thing will become my voltage,right?

So I can vary the magnitude by controlling an on-off switch. I can simply have an on-off

switch in positive direction as well as negative direction. So imagine I have one diode like

this, another diode like this. But if it is diode, I cannot control. So I have to have

another gate terminal, the gate will open the conduction or close the conduction. So

generally, we call this terminal as gate, this terminal as anode and this terminal as

cathode, three terminals, right? So the gate will allow a device to be turned

on or turned off. So, I will be able to delay the conduction. It can start at zero or it

can start at 90 degrees or it can start at 130 degrees depending upon when I give the

gate signal. So the RMS value can be definitely adjusted. So this is a typical AC to AC converter

circuit. Okay. But please note the frequency is not changing, if it is 50 hertz, it will

be still 50 hertz at the output. It will not be anything different.

So transformer is like AC to AC converter. The frequency does not change, but I can reduce

the voltage, right? Transformer is like an AC to AC converter, only thing is transformer

is very bulky. It is really really bulky because you have iron core and windings and so on

and so forth whereas power semiconductor devices generally are very small, they are really

miniature. What I circulated you can see, that is about 10 ampere and 400 volts device.

It is a real small miniaturized configuration. The heat sink occupies more space than the

device itself normally. Right, so miniaturization if it is required, power semiconductor devices

are the solution, really.Okay. So, AC to AC converter is sometimes known

as AC chopper. There is one more type of AC to AC converter, which will be, we will be

dealing with, which right now I am not going to explain, which is known as cycloconverter,

really dealing with both these circuits. Okay. Last but not the least, DC to AC. DC to AC

is generally known as the inverter, so inverters are used in plenty of applications. For example,

I told you induction motor speed control, you will need an inverter. At home many of

you guys will be having an inverter, especially the power outage is very frequent in some

places they will require inverter, right? So inverters really find application, UPS

has an inverter. SMPS might have an inverter inside. So there are n number of applications

where inverters are used. So what we will do during this course is each of these circuits

we will be looking at with different configurations. For example, if I am looking at AC to DC,

it can be single phase AC, three phase AC. Half wave rectification, full wave rectification,

right? So there are different configurations we will be dealing with, we will be looking

at different cases like resistance load, RL load, RLE load. E is a battery, or if it is

a DC motor drive, back EMF, right? So we will have to look at all those things.

So I am just telling you for example for the rectifier; single phase AC, three phase AC,

half wave, full wave, R load, RL load, RLE load, RC load and so on. So many things we

will have to look at,right? So we will be looking at each of these circuit configurations

with variety of inputs and different kinds of outputs we want. For example, DC to AC

I may require single phase AC, three phase AC,right? So we will be looking at all those

circuit configurations and how to control them,right? and finally what are their applications.

And how to implement the hardware, at least I will give a small hint.

I will have to at least give you a little bit of idea as to how to implement the hardware

as well,right? So this course is about all these converters, their control configuration

and their implementation aspects and finally application. This is what it is,Okay. So we

will start this course with the devices first because you guys have not really studied devices

except the physics of the device as such. We will be talking about three different kinds

of devices kinds of devices. One is completely uncontrolled devices diode. For example, if

I forward bias, it will conduct no matter what. If I reverse bias, it will stop conducting.

I do not have any control. It just will conduct no matter what if you

forward bias it. So uncontrolled device typical example is diode. We will call it as power

diode because it is going to handle a larger power, that is all. Otherwise it behaves exactly

like the normal diode. Normal silicon diode if you say 0.7 volts is the voltage drop,

if it is a diode of 6 kilovolt rating for example, it will have a larger voltage drop

because it will be longer. So it is like a little larger resistance, as simple as that.

Otherwise the behaviour is essentially the same, there is hardly any difference between

an electronic diode and a power diode. There are devices which can be controlled during

turn on, which cannot be controlled during turn off. Once you turn it on, it is as good

as opening a tap, that is it, you cannot close it. It will keep on conducting. So those devices

are known as semi-controlled or it is only on controlled, off not controlled. You cannot

control the turn off. Typical device which we will be looking at will be SCR or silicon

controlled rectifier which is also generally commonly known as thyristor.

And what I showed here, this is the thyristor. It will have three terminals, an anode, a

cathode and a gate. In the gate if you give a signal, it will start conducting. You cannot

turn it off, unless it is reverse biased. When it is reverse biased, and the current

goes to 0, it will automatically go off. And the other device again when you give a signal,

it will start conducting when the negative half cycle comes because negative half cycle

this will be negative and this will be positive. So it is forward biased, so it will start

conducting,right? So that also has to go off, only when it is automatically reverse biased.

So these devices are good in some applications, not in all applications. Definitely, these

are semi-controlled devices. So in these cases on is controllable, off uncontrolled,Okay.

The third type of devices is fully controlled; it can be controlled during turn on as well

as turn off. For example, a transistor, you give a base drive, it will conduct; you do

not give a base drive, it will not conduct.Right? So it can be turned on and turned off as per

your requirement. So both on and off are controlled,right? Both will be controlled normally. And those

devices are the most useful generally because you can control everything about them. But

unfortunately the ratings of these devices are still limited. A lot of research is going

on in devices because of this reason. The research is only concentrating on mainly four

factors. One factor is to increase the rating, power rating. Voltage rating should be increased,

current rating should be increased. Ideally, we would like to have infinite rating.

You apply any amount of voltage, any amount of current but we are not yet there clearly.

So the first and foremost is to increase the voltage as well as current rating or overall

power rating. The second is to make sure that the drop across the devices are minimal. How

much ever we may say these devices are close to ideal, there is hardly any resistance,

then diode conducts, it is only 0.7 volts but every 0.7 volts matter,right? So we would

like to as much as possible reduce that forward voltage drop if it is possible. And the forward

voltage drop multiplied by the current carried by the device, that is the power loss across

the device. That is what is heating the element that particular

device. And that is what is the reason for the device conking out ultimately. So you

have to put big heat sinks and so on. So if I can reduce the power loss across the device,

nothing like it. So the second effort is generally towards on-state losses or on-state resistance

reduction. And the third effort generally in many of these devices is towards high frequency

of operation. So if I look at that, I cannot say which is the highest priority and which

is the lowest priority.

So research essentially concentrates on power rating increase, loss, rather on-state loss,

reduction. The third one is frequency of operation. How much ever we may say that it turns on

in microseconds, it turns on in nanoseconds, we want to bring it down to zero literally.

Until now it has not been possible. If we are able to reduce the turn on and turn off

time to really really minimal, we must be able to go to 10 power 12 hertz or even more.

But right now we are able to go until megahertz, not really beyond that. So if the frequency

of operation of the device is increased, that means turn on and turn off is very very quick.

But turn on and turn off can be quick only if the charge carriers can be emitted quickly

and charge carriers can recombine quickly. Recombination essentially causes the off-state

of the device. And emission of the charge carriers causes on-state of the device. So

this requires some finite time. Maybe nanoseconds, maybe microseconds but still, so a lot of

research is going on towards this reducing the on and off time of the device which will

enable it to operate at higher and higher frequencies,right?

So frequency of operation whether it can be increased further, this is the other effort.

And last but not the least, nobody is making devices for charity. Everybody is business,

they are essentially doing business. So if you can bring down the cost of the device

nothing like it. People would go for your device if you bring down the cost,right? Taiwanese

devices are really really low in terms of their cost. So if we are given say from Fuji,

Mitsubishi and say international rectifiers which is in America and Taiwanese device,

many of us go for Taiwanese device if it is not a very critical application because it

is generally lower in terms of cost.Right? So the fourth one is cost and size reduction.

So these are the major focus of many of the device research and we are essentially seeing

giant leaps almost every decade. When I was working with my M. Tech project and PhD project,

I finished my PhD in 1992, we could use only thyristors nothing else was available. Then

came power transistors, then came power MOSFETs, then came IGBTs.

Insulated gate bipolar transistor, IGBT, then now IGCTs have come out, integrated gate commutated

tyristor. Anyway I will give you different devices, what all we are going to study in

the next few classes. So apart from all these devices what is available, although diodes

are uncontrolled, they are also used extensively in many applications although they are uncontrolled

because they are the cheapest and they are available at the highest power level.

So wherever I want a very high power rectifier, generally diodes are the best choice. Then

come thyristors.Okay. That is the way normally we go for. So the choice of the devices depend

upon what is the frequency of operation you want, what is the kind of power level you

want,right? and what is the kind of size limitations you have and so on and so forth. All those

things decide really what kind of devices you are going to choose for your particular

application,right?I will not really go into the details of application now because it

may not make sense now. We will try to devote a couple of classes towards the end for applications

of power electronics. I only told you a few applications what you

know about already so that you realize power electronics is an important course, it is

not you know something that padna hai kya kare, not that kind of attitude please.Okay.

So let us try to first of all look at the topics that we are going to cover and roughly

how many hours that we are going to devote in each of those topics. So I am hoping so

that today I will complete introduction, I do not expect it to spill over beyond 2 hours.

So I am saying 1.5 to 2 hours, not more than that.Okay.

Then we will have to look at power devices, what are all the different power devices,

how are they having you know specific characteristics, why they are having that kind of characteristic,

to some extent we will look into the physics. I am definitely not a device physics person,

so I will not be able to really deal with that in great detail. But we will try to look

at basically how the structure of the devices, why the ratings are limited, why maybe on-state

losses are high or low and such things. So we look at devices, their characteristics

and so on. I think it should take anywhere between 4 to 5 hours because I am not really

going to deal with it in great detail.Okay. So I should be able to finish it within this.

After this we will start off with rectifiers, so in rectifiers we will have to look at half

wave, full wave, uncontrolled, fully controlled, semi-controlled, that is we can use thyristor,

we can use diode, a combination of thyristor and diode, R load, RL load, RC load, RLE load.

All those things we have to look at. So it is going to take a while. I expect that

it should be between 8 to 10 hours. After this I would like to deal with AC to AC converters.

So we will look at AC voltage controller and cycloconverters both. But these are not dealt

with in many of the books because they have specialized applications. So I am also thinking

that I will not concentrate on this to a very large extent. So hopefully it should be over

in 3 to 4 hours. I do not want to really dwell on this for too long.Okay.

Then comes DC to DC converters. So in DC to DC converters, I just showed you 20 volts

can be reduced to 10 volts but we also have the opportunity of increasing from 10 volts

to 20 volts which is known as boost converter. What I showed you just now was a buck converter.

So we might look at boost converter which is a step up, buck converter which is a step

down converter. We will look at a converter which can do boost or buck which is step up

or step down converter. So these are the basic configurations of converter

we look at. If time permits, I would like to go for a few more converters which are

used extensively in power supplies. Power supplies generally require isolation, because

which there will be a transformer in between. So when you insert a transformer, the behaviour

of the converter becomes somewhat different. So we will have two configurations, whichever

has isolation we call them as isolated DC-DC converters, whichever do not have transformer

which will not be isolated from the input to output we call them as non-isolated. So

I would like to touch upon isolated converter as well although we will concentrate more

on non-isolated converters. So I expect that if I include isolated converter,

I will required at least 8 hours, maybe I would say 7 to 8 hours we will required for

DC to DC converter.Okay. Then comes DC to AC inverter, DC to AC inverters mainly we

will be talking about. I have a DC voltage and I want an AC voltage. Maybe single phase,

maybe three phase, maybe I have R load only, maybe I have RL load or I may have an RLE

load like a motor because that also have a back EMF. So we will be looking at all those

configurations in single phase and three phase. So this will require more duration for us

to deal with. So let us say 9 to 10 hours if I am talking about voltage to voltage,

DC voltage to AC voltage conversion. If I talking about DC current to AC current conversion,

it is a different ballgame altogether. So let me see if I have time, I deal with the

current source inverters. If I do not have time, I will ignore current source inverters

for now but let me see how the time goes, how many hours are over according to this.

Take a higher number, 2 plus 5, 7, 17, 21, 21 plus 8, 29, 39.

So I have three more hours, I think I will require those three more hours for applications

because I think if I do not tell you the application it does not make much sense. So I will have

to have some applications. In fact, there is more to power electronics than this, but

I am only uncovering some portions so that you would be able to proceed further if you

are seriously interested in a particular topic. So I would say applications may be 3 to 4

hours. And this includes problem solving as well. That is why I am a little worried, how

we are going to deal with it, let us see.Okay. Yes, any specific questions. Yeah.

Evaluation policy? 

Evaluation policy, before that let me give you the reference books. Do not tell me you

are not going to read any books. You better, so as far as reference books go all the books

are titled power electronics, so I do not have to write anything else. Most of them

will say power electronic devices, circuits and applications. So the title is fundamentally

power electronics. It may be devices, circuits and applications will be the smaller subtitle.Okay.

There is one book by M. H. Rashid, this is from Pearson, this book at least older edition

had some mistakes. So I would like you guys to study this with a little bit of caution.

I know where the mistakes are, I will tell you. As I go along, I will tell you if there

is a mistake in this book in a particular chapter. But the latest edition I have not

looked it, I have to look at that as well. Older edition had some mistakes in rectifier

as well as AC voltage control, so let me take a look at this and then tell you.

So this is one of the reference books which is easily available, not a problem. The second

one is N. Mohan et al. There are three two more authors. Okay. This is from John Wiley.

This book does not have any mistakes but this is like a typical foreign author book which

does not have many problems to work out. You might have one or two examples that are done

and it is a little of higher level. Sometimes it is a put-off for a beginner like Fitzgerald

in machines. OK. Fitzgerald is very well written but it is a little higher level for a beginner.

So Rashid, Rashid is a little lower level whereas Ned Mohan is a little higher level.

So this is from John Wiley. Another book which is pretty good is by Daniel W. Hart. This

used to be some Tata McGraw Hill, now I think Tata has dissociated themselves from McGraw

Hill, so it is McGraw Hill India, that is what it is. So this book is written in a very

very simple manner. And he has done a pretty good job of all the circuits but he has done

an especially good job in DC to DC converters. He has done a very very good job in DC to

DC converters,right? So this book is pretty good. And this is from McGraw Hill India,

so it is not very expensive. It is like 300, 400 rupees something like that.Okay.

There is one more book, Issa Batarseh, I do not know how you pronounce this.Okay. Maybe

a South American author, this book is very very good for DC to DC converters. He has

also given others, but DC to DC converters is dealt with very very well. In fact, anybody

who works on SMPS and all, I generally ask them to study this book first to understand

DC to DC converters thoroughly. This is from John Wiley. And of course you might have heard

of this book, there is one by Erickson. Erickson is also from Pearson. This is also from Pearson

Education. There are further several books available

but if you actually primarily refer to this book that is good enough. But if there is

something I am dealing with from some other book, I will let you know. Right? Fine. So

these are the reference books.