 I will just I will just tell you why polyelectronics is so important and how polyelectronics can be used to extract the power from the solar cell and how to feed it to the grid. Before that before that I will just tell you some of the popular course that came in IEEE papers this what it is we now live in a truly global society in a highly automated industrial front with economic competitiveness of nations in future to technology will dominate it was somewhere in 2001 or 2002 IEEE industrial electronics computers and power electronics the form of providing the intelligence as to what to do and the latter the means to do it. So, power electronics is being compared with with computers and I cannot think of life without computers. So, now it is up to you to compare and as to why it is so important. Name any equipment you will find a small power electronic equipment there name any equipment model equipment you will find a small power conversion equipment. Second one modern computers communication and electronic systems get their life blood from power electronics another very strong statement modern computers communication and electronic system and get their life blood from power electronics and finally, third there are large number third solid state electronics is the first solid state electronics brought in the first electronic revolution whereas, solid state power electronics brought in the second electronic evolution. Let us see why power electronics so important. Now, let us talk about energy everyone talks about energy conservation afterwards I will come why it is for solar globally 87 percent of energy from fossil fuels 60 percent from nuclear remaining from renewables world has limited fuel yes it was projected it is projected do not ask me whether these figures are right or wrong let us not ask it does not matter oil is expected someone said in the morning also oil is expected last in the for 50 years oil for 100 years natural gas for 150 years and coal for 200 years the question that is being asked is the wheels of civilization will it come to an halt at the end of 22nd century it does not matter none of us will be around to see whether it is right or wrong, but we owe some one thing for the generation to come what is that we have to make a better place to live for the generation to come what we can do extend this period for that what do we need to do reduce the per capita consumption sorry what do we do conserve it or use it efficiently fine see I just give a small figure power of steam input whatever all that input power that is consumed bulk of the load is bulk of the load is either fan or pump type of loads fan or a pump type of load where load characteristics is proportional to square of the speed TL load torque is proportional to square of the speed bulk of the loads are of this type fans and pumps of course lighting is very small. So, see the losses if the 100 percent is the input here bulk majority of the losses at the generating end at the steam and finally, when it comes here it is only what is the input what is it remaining hardly 16.3 100 is the input one is input 16.3 percent is remaining. So, measure of the loss at the input side. So, why when the measure of the loss at the input side why worry about saving at the load end. So, my answer to is if I try to say 1 kilowatt here I will try to save 6 kilowatt of energy at the input if I try to save 1 kilowatt input at the 1 kilowatt of energy here at the load end I am saving 6 kilowatt of fuel at the input. So, input of 100 kilowatt of fuel energy output is around 20 kilowatt of fuel why spend much effort on motor or whatever the equipment when the loss at the front end. Answer is every kilowatt of loss saved in the process drive 6 kilowatt of fuel energy is saved on the front end can we can power be renewable yes. So, use electric energy very efficiently increase the conversion efficiency it is estimated that 15 to 20 percent of electricity consumption can be saved by by extensive use of power electronics 15 to 20 percent. See I said bulk of the power is being consumed by electric motors majority of the my induction motors majority of the my induction motors and these are constant speed motors. Variation of speed from no load to full load is approximately very small the variation of what is the variation of speed of induction machine from no load to full load slip is of that of 3 percent no load slip is 0.75 full load slip is 3 percent for a small motor for a high power motor is still less still less from no load full load slip could be of the order of 1.5 to 1.75 percent. So, for all practical purposes for an engineer all the motors are constant speed motors. Now, I said bulk of the load is either fan or a pump where in TL is proportional to TL is proportional to square of the speed. You want to suppose having said that I want to control the air delivered or the water delivered discharge how do I control it? How do I control it? By throttling throttling by throttling I may be able to control the output discharge, but then what am I doing? Input power to the motor remains approximately constant it does not change there is no significant change in the input you may be able to get the output. So, there is practically there is no savings practically there is no saving. So, see here TL is proportional to omega square therefore, power is proportional to omega cube or cube. So, if the ratio of W 1 to W 2 it is half if I want to reduce the discharge. So, P 1 by P 2 is 1 over 8 this is the third law of affinity that is what they say. Are you with me? So, if I want to vary that control the discharge I can throttle it is there is no input there is no reduction in the input power. Another one is how do I how can I regulate reduce the discharge of a pump? Another one is reduce the speed itself reduce the speed itself or speed of rotation of the fan itself. Unfortunately, these are constant speed motors, constant speed machines. So, I need to only one thing is yes I have to change the speed of rotation itself how do I do that is the problem most of them speed of the machines are determined by the supply frequency itself. I need to change the frequency of the supply that I am feeding to the motor. I think one question that I want to ask you we have been told that say for example, induction machine for a fan type of load for a fan type of load there is a wide variation in speed does change over a wide range are you with me just change over a wide range. Frequency I am not changing the frequency is still 50 hertz are you with me therefore, synchronous speed has not changed, but then fan speed has does change when it when I keep it in minimum speed it rotate with a very very low speed. So, what must be happening we have been told that s times the input is dissipated as heat s times the rotor input is dissipated as heat there is a induction machine theory speed does change over a wide range why according to me if this theory is correct it is a highly inefficient operation why if that is true why fans are so popular why fans are so popular why fans are so popular if s times the input is dissipated as heat and I know that speed does change over a wide range here fan will be blowing hot air when I kept in one, but it does blow cold air what must be happening why the fans are so popular why the fans are so popular if s for this theory it is a very highly inefficient operation highly inefficient operation s times the input is dissipated as heat s is the slip synchronous speed is constant I am not doing anything to the supply frequency that speed is lowering what is the reason what is the reason what is the reason you may be changing the reducing the voltage fine, but the synchronous speed is determined by the supply frequency synchronous speed is determined by the supply frequency I have not changed the supply frequency therefore synchronous speed has remained constant speed is varying rotor speed at full speed it rotates a very high speed and at low speed at low speed slip is very high so s times the input is dissipated as heat and see the number of fans there are installed in a high rise building what could be the power loss in the rotor how do we save power where are we doing pulse width modulation we are not doing pulse width modulation we are not doing pulse width modulation load torque is reduced s times yes there is everything is right s times the input is dissipated as heat, but then at low speeds input the motor itself has come down another thing is what are we doing as this gentleman said we are keeping the frequency constant voltage has reduced so what is happening in the machine what is happening in the machine V the coil V is equal to E that is equal to 4.44 F into phi m into n this is a very popular equation I have reduced V frequency is held constant so therefore which quantity will come down flux will come down if flux comes down what happens with the core losses reduces reduces constant losses are coming down so if the constant losses are itself are changing so therefore there is a possibility that you can operate the fan at a maximum efficiency condition maximum efficient condition is constant losses is equal to variable losses variable losses variable losses so it is all this possible because of the power electronics so you need to know the theory the core losses remains constant so that is what is the theory yeah all of you know may be why when I think when I put on the air conditioner or when I when the when the refrigerator turns on there is a momentarily there is a there is a reduction in the intensity of the bulb that is because of the inrush current what are the various consequences because of the inrush current there is a voltage drip may be another one is life of life of the cable cable stress because that current has to flow through the cable another one that current has to flow through a transformer also life of the machine also comes down how do I read how can I address all these problems how do I reduce the inrush current how do I reduce the inrush current yeah soft what do you mean by soft starting soft starting in the sense I need to reduce I need to apply a low voltage as well as I need to reduce the frequency itself somewhere I need to have or somehow I need to have a frequency converter so in in Japan or in the US 70 percent of the air conditioners use variable speed operation 70 percent of the air conditioners use variable speed application so they have address all the limitations of course your initial investment has gone down my another question to you that I am asking this why today all of us are when it comes to solar why of all of us are asking DC conversion and DC to AC conversion and MPPT talk to any person who is working on solar system side you will say that you need to have a DC to DC converter and then again a DC to AC converter and an MPPT any one system these are the three parts are there all of us are saying the same thing is it possible for us to take a different route are you with me what contest I am talking in a see there is a saying if all of us are saying the same thing in fact nobody is saying anything so why there is why there is no different school of thought why cannot we use directly DC energy that is available from the solar take any equipment take in a that AC we are getting is converted into DC converted into and again may be take in a computer a laptop that is what I said variable air conditioners now 70 percent of you the variable speed variable speed means AC to DC and again DC to AC future lighting is people say it is LED lighting where it requires a very low voltage low voltage DC low voltage DC low voltage DC this fan should have been replaced by permanent magnet by permanent magnet long long back I do not we have not replaced so far for various reasons if that happens there is a significant saving in energy in the fan again there is a AC to DC conversion so take any equipment you see this AC is being converted into DC so if I have my if I am having my own generation if I am having my own generation one panel in the roof top why do I need to convert this DC to AC transmitted and again back to back to DC why is this can we think differently and can we can we use this energy bit more efficiently see the problem DC to DC conversion DC to AC conversion again again AC to DC conversion three stages of conversion can I do it with one stage of conversion overall efficiency is NETA 1 into NETA 2 into NETA 3 three stages of conversion can I can I avoid it and have only one stage of conversion I am asking you that question please please give a hand thought do not like everyone like everyone else let us not talk about DC to DC DC to AC MPPT thing different how another thing is I want to make my product very compact light weight how do I make how do I make I do not know have you I think most of you must have might have used might have used might have at least try to lift a regulated power supply in the lab regulated power supply in the lab 3 amperes 30 volts 3 amperes 30 volts what is the total wattage 3 into 33 into 30 180 watts what is the weight of that power supply and all of us might have I do not know sometime might have lifted that PC a Pentium PC which is we have it on a there the power supply could be of the order of 275 to 350 to why it is so light and why the regulated power supply that we are using in the lab and why it is so heavy to to see any technology that has to be popular it has to be lightweight and it has to be compact are you with me it has to be lightweight it has to be compact we have I just give an example the inverter I am from a small village we have a small inverter 300 V a inverter I have I find it bit not it is bit heavy but I saw the other day when small advertisement 600 V a inverter a person is holding that inverter why it is so light and why the inverter which is being used which is sold in the rural areas why it is so heavy what could be the reason what could be the major component there is is a 50 hertz transformer transformer. So, if your product is to be lightweighted you cannot use or you cannot operate the magnetics at 50 hertz you have to operate the magnetics at at a very high frequency very high frequency same thing here see here V is equal to 4.4 for f and 5. So, V into I is the V a rating for a for a given V a rating V i how do I reduce n tends to as f increases as f increases n comes down n comes down n comes down. So, as n comes down copper and therefore, the size comes down but then what price that I have to pay what price that I have to pay what price that I have to at what price whatever that you do in life you have to pay a price nothing comes for free. Yeah as as frequency increases as frequency increases what happened yeah core losses increases core loss increases. So, for a very high frequency I do not think we can use the conventional conventional which core ferrite core ferrite core you may have to use going for a air core the moment I use air core what will happen 2 coils are coupled through air as a medium number leakage increases leakage increases leakage increases coupling coefficient of coupling comes on leakage increases. So, now how to take care about the leakage flux these are the issues we have to address how to address we will find out as we go along. So, high frequency conversion is always desirable yeah I have a transmission line fine suppose I have a high high power solar inverter I have to feed the power to the grid what do I do what are the precaution that I need to take care forget about the finer details final details forget about the finer details what are the precautions what do we need to do I have to take care about solar power inverter I have to connect it to the grid what are issues synchronization in the sense yes we all of us synchronize our generator to the grid by adjusting the frequency of rotation what do we do here for the inverter I have to connect that inverter to the grid I have to connect inverter to the grid and mind you generator output voltage is fairly sinusoidal are you with me generator into the voltage is fairly sinusoidal inverter output voltage cannot be a sinusoidal it could be a square wave as well how do I connect this just 1 megawatt inverter or whatever the power that is there how do I connect it to the grid what all what do I need to do what did what did you do in when we connected to the grid phase frequencies right frequency and then magnitude of voltage in the inverter what all things something similar concepts are here how do I how do I so what do I do what all things are required how do I ensure that inverter frequency as well as as well as the grid frequency is the same how do I how do I ensure how do I ensure I will ask you these questions I will not answer this as we go along let us see whether we will find an answer or not on Friday evening let us see we will try to find answer to most of these questions one way is I have to feed this 1 megawatt power to the grid what do I do what all things what all precaution do I need to take care now the question is that I want 1 megawatt inverter is already installed there solar insulation is lower evening time can I do something now can I use this inverter for some other for some other purpose because solar insulation is low afternoon time there are solar insulation was full inverter was supplying 1 megawatt power to the grid evening time or in the night time do I need to do I allow this inverter to remain idle or can I can I use it for some other purpose see in the night time what happens is all the loads are coming to the grid most of the loads are most of the loads are inductive in nature most of the loads are inductive in nature except for lighting most of the loads are inductive in nature high or not yes ok so therefore transmission line or our distribution network is loaded by a highly inductive load what happens voltage falls voltage falls that is the reason that is the one of the GPO from a rural area you know the life there the fluorescent lamp does not blow when it is supposed to glow it blows may be around 2 o'clock in the night or 7 to 9 o'clock it will not blow because voltage is so low how can I improve this how can I improve the voltage in the at the far end of the transition or distribution network one second if you use a capacitor what will happen if I use a capacitor what will happen see the problem here I connect a capacitor ok I said in the in the when the system is highly loaded voltage at this point comes down you all agree isn't it for it at that time system requires requires a high leading current when the voltage is low it requires high leading current but then when the voltage is low capacitor I C I C is V divided by X C so as the voltage comes down as the voltage comes down leading current supplied with a capacitor also comes down but then system requires a higher current higher current basically it is a there is a capacitor is also known as an unreliable friend when the system wants more leading current it will not supply it can't supply it can't supply but then I have an inverter here which supplies 1 megawatt to the to the grid during normal installation now in the evening there is no sunlight ok grid requires grid requires leading current or reactive power support my question to is can I use this inverter to supply supply leading current to the grid that is the question that I am asking see I just can't install 1 megawatt inverter and see in Mansoom Mansoom there is no probably basically there is no no no sun at all no sun at all can I can can I try to get some revenue from the from the utilities for supplying leading current or during over voltages can I suppose of the suppose the transient line voltage is high now what do I need to do light load condition voltage could be high voltage could be high so I have somehow I have to somehow I have to what do I need to do yeah I need to connect an inductor draw Q can I use this inverter to draw Q instead of connect to a separate inductor see these are the question that I am asking you will see yeah if you talk to any power system man will say transmission light AC lines transmission are fully loaded you can't add extra power most of the lines are loaded now you are installing wind farm you are installing solar power what is the effect on the grid what is the effect on the grid can I arbitrarily set up a power plant and power connect the power to the grid it is already overloaded what do we do what do we do you follow what I am saying you just can't install high power solar plant or you can't install high power high wind farm because you have to extract the power and feed it to the grid there has to be a transmission line these transmission lines are loaded unfortunately these are see the power flowing through a transmission line is determined by determined by voltage at two ends and the angular displacement between them b1 b2 divided by x into sin delta delta cannot be though theoretical value is 90 degrees this delta is of the order of 30 degrees 30 degrees you can't schedule the power in AC line you cannot are you with me you can't say so much of megawatt of power has to flow through this line it is completely determined by the circuit conditions completely determined by the circuit conditions voltage at two ends angular displacement is fixed around 30 degrees divided by the reactants and you are setting up a power plant there what do we do what do we do what do we do can I can I use my pyroelectronics to to improve the power that is flowing through the transmission line it may be all related to so these are the question that so that is what can I is it possible to increase the power through the line securely without without affecting it energy from alternative sources historic energy simple our four fathers used wind power wind and those days our four fathers now we are using so called today's energy scenario is use oil colon nuclear and future energy sources that again renewable looks like really is getting completed are you with me four fathers how did what they use they used they used wind power hydro looks like and here see the problem here solar is DC yes it has bulk of the if I have to feed the power to the grid it has to be converted to AC and again this power has to be connected to the grid there are synchronizing issues line may be overloaded wind power as the speed of the wind changes what happens the rate at which the rotor is being rotated in the magnetic field also changes therefore therefore voltage induced as well as the frequency changes come on as the speed changes frequency changes voltage also changes grid requires constant voltage and constant frequency constant frequency what do I do what do I do yeah power electrons I know but then what wind solar is output is expensive but decreasing I should not be saying the cost today in solar this one anyway the fact cheapest environmental clean wind energy now provides more than 30 and 30 megawatts of power around the world in India installed kept installed capacity I do not know about what exactly the power that exotic no no data is available what is the definition and goal of power electron is the technology associated with efficient conversion and control of electric power by power semiconductor devices is the definition and the goal of power to control the flow of energy from electrical source to electric road okay I said power power electronics is being compared with computers okay why it is so popular any technology that is very popular it is two things efficient and reliability should be very high it has to be the conversion efficient it should be as possible and it should be it should be reliable size weight and cost should be low okay everything fine the need has a good measure of success of any technology efficient if the moment I say efficiency is high see power loss is low cooling requirement comes down are you with me the moment cooling requirement on packaging density can be increased size comes down size comes down okay so how can I how can the circuit change the voltage yet dissipate less power yes dissipate power input voltage could be solar voltage could be low grid requires 440 at a distribution level fine afterwards I can set it up but yet yet dissipate low power how is that possible what are the circuit elements circuit elements pass the circuit resistance inductance and capacitance if you want to be efficient it should be high you cannot use resistors your solution cannot have a resistive component it has to be only L and C okay now there are of course active devices I have transistors MOSFETs and and IGBTs IGBTs okay yeah power loss in the device in the BJT is either is VCE into VCE into IC okay it is minimum during on-state during on-state power loss is minimum if I if I saturate the transistor VCE sat into IC okay it should be very low okay but then if I saturate it I may be able to reduce the losses what happens to the frequency of operation come on wake up I said to improve the efficiency we have to reduce the losses here okay in the device that are taking place I can reduce the losses by saturating the BJT okay if I saturate the BJT what price I am paying come on my turn of time increases my turn of time increases the device is being driven into deep into saturation we will see sometime later turn of time increases okay the moment I turn of time increases therefore therefore your turn of losses and I cannot increase the frequency of operation okay so there is a problem if I operate in active state our losses increases if I saturate it if I saturate it your frequency of frequency operation comes on so question that I am asking can we take a middle path are you with me neither operate in active nor in saturation what is that what is that be active come on if I operate in active there are on-state losses are high isn't it that's what happens in a linear power supply linear power supply why it is so heavy why what is the heatsink mounted outside there these heatsinks are not mounted outside in a power supply that is used in a computer come on am I speaking first no power supply that is used in a computer there are no heatsinks are mounted outside the power supply that is used in a laboratory heatsinks are mounted outside so what is my conclusion there must be a lot of losses are taking place in in in the lab but not that may pass that much of power is being best dissipated there so it looks like in the regulated power supply power semiconductor device must be operated in active active mode there is a reason we see into IC is high this is my conclusion gut feeling it could be right or wrong I don't know we will see but in a in a computer power supply devices are sorry I can't see the heatsink outside looks like looks like losses are low so that means you must be operated somewhere and it is lightweight not very heavy so they must be operating at a at a at a high frequency high frequency how are they operating high we found here that if I saturate the device I can't operate in saturation how are they being achieved let's see I have to operate at a very high frequency I have to reduce the size but as of now it is not very clear as to how exactly to do this because to reduce the heatsink I have to saturate it if I saturate it frequency of operation comes down is there a way out looks like looks like there is a way out for high active element should be operating either in saturation region or cutoff in addition use of only else and elements input this is a simple example that I have given input is 30 volts output is 5 volts what do I do input is 30 output is 5 I can put a registry network I square R losses or what is being done in the lab VDC is 25 output is 5 rest of the voltage I will drop across this BJT or I will use switch mode power conversion I will try to reduce the average voltage of the load average voltage applied to the load initially I will apply full voltage and after some time I will apply a 0 voltage. So, power electronics is the reason why this power is just extensively used in motor drives power supplies lighting high frequency induction heating electric welding active filters active filters power transmission bulk power transmission electric vehicles to process from non conventional energy source and we are here and most of them are interrelated and why it is so popular power electronics primarily due to the advances in power semiconductor devices fast devices are available that is the reason there is a significant improvement or advances in power electronics in power electronics coupled with fast processors cost of dedicated chip circuit configuration control and estimated. So, if you say that I am working in power electronics you need to you are you should be reasonably good in almost all the areas in power all the areas in electrical engineering you need to have good circuit theory then then circuit theory fast semiconductor devices how to operate those fast semiconductor devices so I need to know little bit of analog and digital are you with me I need to operate that switch. So, analog and digital control theory control theory and fast process again digital so little bit of everything is there in power electronics these are the significant events in the history of power electronics the concept of Volta came in 1783 first rectification in 1830 rectification effect of copper oxide selenium rectifier 1876 1896 single phase rectifier bridge 1896 more than 115 years still that circuit is so popular I do not think anyone can ever challenge this circuit single phase bridge single phase bridge so popular there is a let us think differently can we use this solar energy in a different way let us not take the same path DC to DC DC to AC MPPT let us not split our hair in all those people let us not walk the same old path the people will take a different path you may land in a ditch or you may land in a I am telling you take a chance you take a chance and it is worth it and I am here to push you to take a different path not the same path 1897 three phase bridge circuit 1901 invention of glass mercury rectifier 1848 invention of transistor 1948 so became first industrial revolution and they said power electronics brought in the second industrial revolution that is what 153 germanium power diode 154 silicon power diode 1957 pyristors now pyristors up to 6 point blocking we can block the voltage up to 6.5 kV they are available as of now we can block one device blocking 6.5 kV 6.5 kV one device yeah little bit of I will start with power semiconductor devices if you have any questions feel free let us have a dialogue not monologue feel free why there is free drop silence what is the problem I am not going to do anything say something no problem anyone any questions any counter views feel free sir nothing do not take or no point having a dialogue something what is that go ahead any questions yeah now I will start with power semiconductor devices what are the important characteristics we need to know and how to operate that we will see today and tomorrow power semiconductor devices is the heart and soul of modern power electronics and is used at switches it is used as switches what is the property of an ideal switch what is the property of an ideal switch fine in the sense it should be able to block any voltage should be able to carry any current on straight losses are 0 off straight losses are also 0 on straight and off straight losses are 0 both when switches should be 0 you should be able to withstand any voltage across it this is an ideal switch but then it has power dissipated in the switch when is off or on both should be 0 that is the condition so basically operation is along this graph on straight turn on time should be as small 0 ideal switch turn on time should be 0 turn off time is also 0 okay voltage across the device voltage across the device when it is on it is 0 and current flowing through the device when it is off it is 0 so basically this is the path and basically it is therefore it is unstable unstable unstable okay so characteristics of the device are practically very close to the ideal switches off straight current is is not equal to 0 but then a very small current that flows when the transistor is transistor is on worry when when its transistor is off a very small current does flow okay and when when the transistor is on voltage across the voltage across it is very small but definitely it is not equal to 0 power loss in off straight and on straight is not equal to 0 T on is not equal to 0 T off is also not equal to 0 it takes a finite time to switch from on straight to off to another therefore therefore the switching losses switching losses occur when when when it is turned off current has to flow current has to fall down to 0 and voltage across it rises from 0 a very low value to value to the supply voltage so the product V and I is finite V and I is finite that is dissipated as heat that is dissipated as heat okay power cut so therefore there is a power loss so heat sink is required cooling requirement so what are the different types of power semiconductor devices basically there are three types there are three types one is control devices one is uncontrolled and third one is semi-controlled let us not discuss about semi-controlled we will discuss we will discuss about control devices and uncontrolled devices okay uncontrolled is the basically a two terminal device I am talking about only diodes and I am telling you why this is very important what are the important parameters that you need to consider while designing an inverter what are the important parameters that are required in a BJT while designing in a DC to DC converter or a DC to AC one is one is very obvious average current and what is second what is the second important parameter that you need to yeah it is the voltage that it can that it should withstand okay it depends on the circuit apology it depends on the circuit apology okay in the sense suppose I am using a full verification what is the voltage rating send it up this is V this is also V this is also V what is the voltage rating of this what is the voltage it has to block what is the voltage it has to block or fine in the sense I will just see here what is the voltage rating of this diode what is the voltage rating of these two cases what is the voltage rating of this diode and what is the voltage rating of this diode it is V here it is 2V here 2V here 2V here because capacitors in the positive of capacitors charge to V in the negative when this voltage reverses voltage across a diode is diode is 2V so you very careful in the circuit apology input as well as output both cases are important what voltage the diode has to withstand another important device that is being used in DC to DC and low power DC to AC is MOSFET okay very popular in DC to DC very popular in DC and very is used in low power low voltage in water as well MOSFET okay generally low voltage high current device MOSFET is metal oxide semiconductor field effect transistor fast device is a majority carrier device majority what about BJT BJT is a basically it is a minority failure device okay fine by the way see if you see here the structure here gate source and the drain okay we have N plus P and again a N structure are you with me N P and again N so if I start from source and if I reach drain both there are two diodes connected back to back I start from source reach the drain the P N and again a again N P two diodes connected back to back how is the conduction possible there are two if I see the structure here there is N plus N P and again again P N N plus and N plus and N minus indicates that it is highly doped and it is lightly doped where is the N itself is the P and N how is the conduction possible how is the conduction possible both two diodes are connected back to back if there is as though there cannot be any flow between drain to source how the conduction what is the limitation why BJTs are where we are replaced by maybe MOSFET why BJTs are not being used now what is the problem what is the problem with the BJT basically it is a current control device are you with me it is a basically a current control device what happens what happens the so called gain as the power rating increases as the power rating increases what happens to the gain or what happens to HFE HFE is may be high for a small transistor SL 100 gain may be very high suppose a power transistor what is the gain gain is low gain is low 8 ampere BJT gain could be of the order of 10 to 12 8 ampere BJT gain is of the order of 10 to 12 so how much is what is the IP that is required what is the IP that is required to turn on the BJT see here the problem 8 ampere BJT HFE could be of the order of 10 to 12 how do I reduce the turn on time of a BJT how do I reduce the turn on time of a BJT I will give a high pulse current that current is could be of the order of 1.5 times the steady state current 1.5 times the steady state current so 8 ampere BJT HFE is of the order of 10 to 12 so IB at steady state base current could be of the order of 1 ampere 1 ampere could be of the order of 1 ampere and during starting I need to give a 50 percent high current so current is could be of the order of 1.5 ampere every time when I turn on current could be of the order of 1.5 ampere so basically it is not a small signal transistor I cannot use a SL100 to drive this transistor so I require a small power transistor to drive another power transistor got the problem because basically the current control device so I cannot use a small signal transistor to drive this I need to have a another one small power transistor to drive another power transistor so this what about the MOSFET MOSFET is a voltage control device voltage control device I said here two diodes are connected back to back so looks like there is no flow between DSN possible but then here there is a SiO2 layer SiO2 layer I think something similar to something similar to capacitor capacitor with a different different epsilon R or whatever so when I when I apply a suitable voltage between gate to source what happens gate is insulated from rest of the device no steady state current only displacement current like a parallel plate capacitor parallel plate capacitor so when you apply a suitable voltage here a channel is formed I say a channel is formed and current starts flowing from ring to source a channel the moment I say channel it can be a bidirectional flow water is flowing through a pipe or flowing through a channel I can reverse the direction of flow so MOSFET is basically a bidirectional device depending on it the way you bias it unlike in a BJT unlike a BJT IC is always positive IC is always positive here I can so when the MOSFET is conducting and is on a channel is basically it behaves like a purely resistive element purely resistive element so you can reverse the direction of flow direction of flow of current so you do not need to use a separate MOSFET to reverse the direction of current one MOSFET is sufficient it depends on how you drive it are you with me so it is a basically a voltage control device voltage control device no steady state current so turn on time depends on the rate at which you charge the charge the gate to source capacitance remember the turn on time depends on turn on time depends on the rate at which you charge the gate to source capacitance see that is the problem I will repeat it it depends on the rate at which you charge the capacitance between gate to source how do I first charge it at a minimum possible time sir I want to charge this capacitor minimum possible time what do I do is the RC circuit I have to charge it as well what do I do R should be the moment I if I make R 0 what price you need to pay the initial pulse current given take so to reduce the turn on time I need to reduce the charging that R if I reduce R that source has to supply the peak current so during this this is the region where we operate in in power electronic device power electronic equipment only a losses here this is an active mode power see another advantage in another advantage in BJT is you can try to reduce the on state losses by saturating by saturating the BJT you can try to reduce the on state losses MOSFET once it starts conducting it gives like a channel and it is on state losses are I square into RDS on it is not VC sat into IC it is it is ID squared into RDS on RDS on so on state losses are slightly higher in a MOSFET compared to a BJT but then MOSFET is a you require a very low power to drive it so that is the advantage on state channel the resistance we are like a resistance RDS on RDS on delta is by delta ID BJT has a substantially lower voltage drop than a MOSFET another thing is while operating a device you need to always ensure that you have to operate in something known as a safe operating area safe operating area for a BJT there is four parts whereas for MOSFET there are there are there are only three zones one is limited by voltage current junction temperature junction temperature in a BJT there is a fourth zone what is known as the secondary breakdown secondary breakdown that is why is what is my secondary breakdown is resistance the resistance coefficient is in a BJT is is negative whereas whereas in a MOSFET it is it is positive what happens what happens if the resistance coefficient is is is negative temperature as a temperature increases resistance decreases so what happens so thermal runaway will happen so it is basically difficult to parallel it and that is the because of the region that is what is known as secondary breakdown in a BJT whereas whereas in a MOSFET it is a positive temperature coefficient so paralleling is easy and therefore the the secondary breakdown region is also absent in the MOSFET so these are the three areas secondary breakdown region is completely absent AB is drain current at a steady state CD is the maximum BDS drop that the device can sustain this is the maximum power dissipation imposed by RDS on paralleling is these are the various parasitic capacitance main important is drain to source drain to source is the most important parameter this parameter this capacitance you need to know while designing your gate drive circuit how exactly to design a gate drive circuit what are the issues I do not know whether I will cover this now but then we will be covering in your final final lecture in December so looks like BJT has certain advantages MOSFET has certain advantages what could be an ideal device what could be an ideal device what is the advantages of BJT on state losses are low on state losses are low what are what is the advantage of but what is the limitation there it is a current control device current control device so therefore base requirements are high base requirements are high what about MOSFET MOSFET is the voltage control device gate power requirement is less gate power requirement is less but then but then RDS on is RDS on is high what could be an ideal device ideal device may be best of looks like I can combine can we combine the best of MOSFET as well as as well as best of best of BJT so IGBT insulated gate bipolar transistor insulated gate bipolar transistor input is MOS output is output is BJT output is that is known as IGBT device may be tomorrow we will start I will do that I will do discuss diodes as well then may be other other important devices that we are using DC to DC as well as DC to AC that is about it today if you have any questions feel free MOSFET is bidirectional is it both the way it is controllable or one way it will not be uncontrollable. No controllable you have to apply the proper gate signal gate signal most of the characteristics only lie in the first quarter only yeah that is the reason in the sense if you want to reverse the direction like in a I do not I will do it tomorrow or suppose in a buck converter basically buck converter DC to DC basically I have a diode here and a switch is it not how does IL change in this how does IL change what is the wave how does IL look like what is the minimum value of IL minimum value of IL is 0 are you with me now if I replace this diode by a MOSFET I can make that current IL to be negative I can make the IL to be negative I current can this current linearly increases say then I can reverse the current also possible if I use a MOSFET how we will see I will explain it is possible so my performance of the buck converter improves if I do this synchronous converter synchronous sorry in the grid connected system supposes that grid is turned off but the energy is being transmitted if anyone is working on the other side you may get electrocuted no how will you avoid this problem I do not know say that again see if anyone is working on that EB line I know I can understand these are the issues to be addressed these are the address so the back flow will be there no back flow will be there no no in the sense these are the issues to be addressed these are the major concerns whether the standards on what grid generation a domestic user wants to pump power to the grid so still there are no standards for that standards in India may not be there but then it is a normal use one in the US or in Germany or in Australia you just do feeding the power to the grid no a domestic user domestic user yeah I can possible but then it is not yet come to a continue it will come soon which one yeah but then where in India it is not applicable so far there are where do you have the system where where a single user feeding small amount of power to the grid standards are there international standards are there IS may not we do not may not have an IS standard you mentioned that one power transistor to drive another can you please I gave a simple example in the same 8 ampere transistor gain is 10 to 11 10 to 12 so steady state current steady state gain is of the order of of the order of 1 ampere are you with me okay but to to reduce the turn on time I would need to pump in a slightly higher current may be 50 percent so around 1.5 times that is required again see these ratings are at 25 degrees 40 degrees again I have to derate it you follow what I am saying all these ratings are data sheets are at 25 degree what is the total ambient you design a product for 40 degrees that minimum may be okay so again this all this HFE will fall come down reduce so steady state IB that it may be required is of the order of 1.5 ampere can you pump what is SL under current rating 500 milliampere 0.5 so what will you do so I need to have a 2 ampere BJT to drive 8 ampere BJT so I need to have a that means I need to have a BJT drive one BJT driver another BJT is a low voltage high current BJT to drive another any other question synchronous converter I will see tomorrow I will see instead of using DC to DC converter from a solar panel output why not use it direct why not that question that I have posed you people but if we use LED lighting if we use LED lighting that lighting would not be sufficient and that may cause some diseases the emission of that may or you are saying may or will I have heard that one of the professor was speaking I do not in what context that professor was speaking let me not comment but then one thing is for sure future future lighting is LED that is what that is what the literature says everywhere the fan should be redesigned not means the new coming fans should be DC fans and all that is what I feel it should be permanent micro BLDC motor yes that is what I feel so for the existing one anyway AC should be used and for the new fans the DC fans should be designed that is what you are telling now what I am saying is I do not know how many of you have traveled in Bombay locomotive once upon a time the fan was not working you take a comb and rotate and it will start working those are the good old days now I think all these fans all the fans in Bombay locomotive they are sorry in suburban train they are BLDC fans Bresla DC motor fans permanent magnet base DC fans so there is so it is a basically synchronous machine synchronous motor it is a basically synchronous motor operated in Bresla DC mode so you require a DC supply and an inverter so what I also what I feel that these fans highly inefficient fans should be replaced by replaced by this BLDC fans and if that happens yes you require so most of the loads are there is a convert for efficient conversion for efficient conversion there is this AC is converted to DC so my question is why are we saying the same why all of us are saying the same tune can you use this energy directly no there are lot of issues that has to be to see whatever that you do in life you have to pay a price that is what I am trying to tell yes there are other issues grounding grounding is today as of now it is very well defined for AC the standards for DC grounding is as of now they are not available grounding issues then how to handle the the capacitance between the panel to the ground if I use the DC transmission there are other issues there are lot of issues there but then unless until we start working they will not be addressed those problems will not be addressed my question to is why cannot we directly use DC when there is this AC is being converted to DC it is possible especially when you are when you have your own generation when you have your own generation may be when you have to transmit the power yes because of transformer and all it is not very convenient if you are making your own generation why not why not thank you see you tomorrow.