 from the department of electrical engineering IIT Bombay for another 10 days or so, I will be discussing power processing and conversion of solar energy. So, what are the various power electronic interface required to process the solar energy? Before discussing the content, I would like to share the proceedings of one of the IEEE meetings. One of the issues that was identified in power system was a large penetration of power electronics into power system will happen. The main transmission grid will not be affected. Third point is power electronics development will be in distributed generation and in loads. I will repeat power electronics development will be in distributed generation and in loads and major technological challenge will be to integrate distributed generation sources into the grid. See I am emphasizing these two points. The power electronic development will be in distributed generation and in loads and major technological challenge will be to integrate distributed generation sources into the grid. What are the three levels that exist in power electronics? Basically, there are three levels. What are they? One is components, second is circuits, third is systems. The major technological driver are if you see the history of power electronics, more specifically being the semiconductor devices. If at all there is a significant improvement or progress in power electronics, it is due to the progress in power semiconductor devices. I will just show you the slide. See the significant events in 1783, the concept of semiconductor came in by Volta. In 1830, the rectification effect of copper oxide. In 1876, the first selenium rectifier was invented and 1896, single phase bridge rectifier circuit by Pollock and till date this invention is unchallenged. Single phase rectifier circuit is still popular. It is more than 115 years or so, still is being unchallenged. In 1897, came in the three phase bridge circuit. In 1901, invention of glass bulb mercury rectifier came in. 1848, invention of transistor that triggered the first electronic revolution. 1953, germanium power diode, 1954, silicon power diode, 1957, thyristor. Now, as of now, blocking voltage capability of 5.6.5 kV devices are available. I will just show you some of the devices that we use in power electronics and I just want you people to think what effect it had on the size of the power electronic equipment. You might have seen this three-legged devices, but this is a basically a MOSFET. See the size. This is a 4 ampere, 400 volts. In other words, theoretically it can block 400 volts and maximum current it can carries 4 amperes. And I have mounted them on a common heat sink. And that heat sink though all the devices are mounted on the same heat sink, they are isolated by using this isolation tape and they have fitted through. So, they have fitted to this heat sink through a screw and the screw is a plastic sleeve is being used to isolate this heat sink from the body of the heat the MOSFET. So, if I do not use this isolating tape, then I have to use a separate heat sink for each MOSFET. The next device that came in is basically a one leg of a voltage source inverter. There are two 30 ampere devices and they can be used in power electronics which can block a voltage of 200 volts, they are connected in series. I will repeat two 30 ampere devices which can block 200 volts, they are connected in series. This is a collector of one device. This is the emitter of the second device, emitter of the first device and collector of the first device, they are shorted together and brought in here. And these two pins are used to supply the gate drive. See the size, there are two 30 ampere devices can block 200 volts. Another beauty is that I can use one common heat sink to fit the these modules. Basically, this surface is completely isolated from the collector of these two devices. So, I can mount any number of devices in this heat sink. I do not need to use a separate, I do not need to use this isolating tape or a screw through a plastic sleeve to insulate this heat sink from this device. So, from this it has been graduated to this module. To drive this module, I require a separate a gate drive circuit. The size of the gate drive circuit we will discuss at the appropriate time. After that came in an intelligent power module IPM both size is the same, but then here I said this module requires separate gate drive, whereas this module does not require the gate drive circuit. Everything is built in, I have to just feed in the supply and the signals directly from the controller. Whereas this module, what do we need to do is the signals from the controller has to be processed, they have to be isolated, various issues we will see. All these things are not required here. So, there is a significant reduction in size of the power electronic equipment from this three leg devices to one leg of an inverter, to two leg of an inverter and IPM. Again this is one leg of an inverter. From there I will show you this is a basically a 75 ampere 200 volt three phase intelligent power module. So, in this module there are three such legs are there, three such legs. All thus require only one leg of an inverter. Using the control signals directly from the controller, you do not need to have a separate isolating circuit or the gate drive circuit. 75 ampere, 1200 volt device, see the size. So, the reduction in size that is possible as of now compared to the 1980s is really significant, more about it I will discuss. So, the significant improvement of the progress in power electronics is because of the availability of high power devices, the way they have been packaged that is the driving force. The second point is as a three level first is components is basically more specifically semiconductor devices. The second is the circuits and topologies, they have reached much attention and this area is more or less is saturated, more or less saturated. Third is now the systems needs to get more attention, more attention. By the way let me tell you when the power electronics is an enabling technology. So, performance control and system integration issues will have will become very important. And reliability cost and efficiency will be very important for consumer acceptance. So, if at all you have to sell the power electronics equipment in the market, first it has to be reliable, it has to be cost effective and third one should be highly efficient. How to achieve all this, we will see. All of us know solar panels are coming, this project mentioned that for solar panels will be integrated in the construction of the roofs of the buildings. Now, the question that I am asking is how to process this energy, output of the all of us know that output of the solar cells is DC, but then both the voltage as well as current change depending upon the operating point. So, these are the characteristics of the features of the solar cell and what do we require? We require most of the loads require a single phase 50 hertz AC voltages of the order of 230 volts. The domestic load industrial loads could be the voltage could be different. So, and the power level depending upon the application. So, depending upon the application or depending upon the power level circuit configuration of the polecton interface need to change. So, how do I choose or suitable power circuit configuration to meet or to handle the required power level that we need to find out. Now, if there is grid we can feed the energy to the grid or when there is no solar alright, solar insulation I can extract the energy from the grid. What if the grid is not available plus the solar is also not available, what do I do? Do I need to store it in a battery? If I have to store it in a battery, what is the best way to charge the battery? We will study about it. What is the most efficient way to charge the battery? If the grid is available, can I feed this power to the grid? If yes, assuming that grid can accept this power, what are the issues that has to be handled? Can I just cannot connect my inverter to the grid? What do I need to do? All these issues we might have studied for a synchronous generator while discussing the machines in a undergraduate level, we have studied and we do teach them in the course. Now, when it comes to connecting this static device inverter, what are the various issues do I need to take care? We will study and by the way while feeding power to the grid or even if at all if I have or while designing an equipment, that equipment should satisfy certain IEEE and IEC standards. What are those standards? We will study during this course. So, in my introductory lecture, I have asked you few questions. They are here depending upon the power level, what is the circuit configuration to choose? If I want AC, what do I need to do? Because output of the solar is continuously changing. First of all, I have to convert this DC to AC. If I have to store it in a battery, how efficiently will I can I store and how do I extract that power from the battery without compromising on the life of the battery? If the grid is available, how do I feed this energy to the grid? What are the issues and what are the various standards that power electronic equipment should satisfy? Finally, I would like to ask you instead of converting this DC to AC, my question to you is can I directly use this DC? Because if you try to understand the power conversion process, even today most of the equipment what they do is whatever the AC that they are receiving, first it is being converted to DC and this again this DC is being converted to AC or it is being converted to other voltage levels. Say for example, PC, why do I need to convert this solar energy which is output is DC to AC and feed it to a computer and that computer again converts this AC to DC. So, can I directly feed in this DC to a computer? Most of the air conditioners and refrigerators in the developed countries they use variable speed drives for energy conservation. So, what they do? Whatever the AC that they are getting first is converted into DC and this DC is again inverted to feed a variable speed drive. So, if this AC is being converted to DC, my question to you is why convert solar energy to AC? So, thereby see there are two conversions are involved, can I improve the efficiency? We are talking about energy conservation, can I try to save this energy? So, last lecture or last two lectures I will be discussing can we directly use DC. So, these are the question that I have asked let us see as we go along, can we find an answer to this? If you cannot find maybe I will encourage you to find even after the course. So, what are the various topics to be covered? First I will cover the power semiconductor devices, basically I will cover diodes BJT, I will tell you why am I covering BJT though they are not very popular to appreciate MOSFET and IGBT I need to understand BJT. So, I will cover diodes BJT MOSFET then IGBT. So, these are the diode MOSFET and IGBT they are used, they are very popular in power electronic equipment that I use to process the solar energy. I am discussing BJT in half a lecture also to appreciate the advantages of MOS and IGBT then I will depending upon the power level we will discuss various DC to DC converters then DC to AC converters various circuit apologies how to regulate the output voltage because input will continuously changing if I have a large installation can I feed the power to the grid, grid connected solar inverter what are the issues. So, these are first three topics are very broad and general in nature do I need to do any modification when it comes to solar to process a solar or that will discuss in power electronic interfaces for solar whatever type. So, what modifications do we need to make from these two topics we will study here and finally DC micro grid micro grid in this I will try to emphasize or I will try to tell you the advantages of directly transmitting DC and that is being directly used by our. So, these are the topics that we will be covering in the next 10 days or so I will start with very first topic power semiconductor devices characteristics and their ratings very first device is before that these power semiconductor devices are that are being used as switches these are hot and soul of modern power electronic equipment. Now, before if I am using them as a switch what are the ideal what are the properties of an ideal switch they are when the switch is of the current should be 0 they should be able to withstand any voltage across it both positive as well as negative third is when the switch is on voltage across the switch should be 0 and you should be able to pass any current through it these are the characteristics of an ideal switch second is the voltage across the device is 0 when it is on power dissipation power dissipated in the device is 0 or when the device is open current is 0. So, therefore, power dissipation is again 0 in other words power dissipation through a ideal switch is 0 and losses switch can be turned on and off instantaneously in other words T on and T off should be 0. So, this v i plane if I plot current is 0 device is blocking the entire voltage it could be anything when it is turned on voltage across it is 0. So, if I see here product is 0 product is 0 or in the v i plane should be able to handle anywhere T on and T off 0 even the loss that is taking place while turn on and turn off what are the characteristics of practical devices and how they are or what is the gap between the ideal device and a non-ideal device. Practical switches are very close to ideal switches in the off state current is approximately 0 of course, there is a voltage the upper value of voltage which it can withstand I told you I showed you some devices I showed you the MOSFET 400 volts maximum it can block is 400 volts. I showed you one leg of inverter I said 1200 volts it can block 1200 volts in the off state voltage across the switch when it is on it is not equal to 0 of course, it is very low compared to it is rating a 1200 volt device when it is turned on voltage across it could be of the order of 2 to 2.5 volts. So, this voltage is very low compared to it voltage rating. So, voltage across the switch is 0 when it is turned on. So, power loss in during off state as well as in conduction mode is finite. So, they are very small more over the device take finite time while turn on and while turn off. So, in other words area under this curve when it is being turned on is finite area under this curve is finite when it is being turned on and when it is being turned off. So, there are loss of taking place when the device is being turned on and off. So, here is the characteristics currents fall slowly when it is being turned off and voltage rises slowly. So, area under the curve is finite. So, there are switching losses at any time during the operation the operating point should lie within the what is known as the safe operating area safe operating area. So, here I have shown you there are three zones. So, some devices like BJT might have four zones what are they we will see here the maximum current it can block maximum voltage it can block and this limit is the power dissipation. So, power dissipated is finite. So, this limit is the power limit. So, these are the properties of an ideal switch I will start discussing the very first device what are the various types of devices basically there are three types of devices one is an uncontrolled switch basically is a two terminal device diode depending upon the circuit condition if it is forward bias it starts conducting and current is limited by the circuit on or off determined by the circuit or straight of the circuit where it is connected. And there is a second type of device is a semi controlled switch basically the three terminal device anode cathode and gate why it is semi controlled take for example, thyristor I can turn on by applying a positive gate signal to the gate when it is forward biased. But then having turned on we cannot turn it off using the gate signal I have to reduce the current flowing through the device gate has no control I can use the gate only to turn it on. So, hence the name semi controlled switch and third one is a controlled switch again the third terminal three terminal device example is a BJT or a MOS I can turn on or I can turn off by applying a suitable gate signal a suitable signal to the base of a transistor or a gate of a MOS coming to a diode two terminal device what are the I am not going to discuss about the device physics what are the important parameters that are required in a while designing the power electronics what are the various parameters all of us know V a k should be positive for small signal diode it should be pointed for power diode it could be of the order of 1.5 volts or so what are the important parameter that is required in a diode one is a voltage rating fine and the current important parameter is the reverse recovery time and the reverse recovery current why it is so important I will tell you later. But let us try to understand what exactly is the reverse recovery time and reverse recovery current the minority carriers which are there require certain time to recombine with the opposite charge and to get neutralized. So, even when the see the profile of the current the diode that current it was carrying some steady current it becomes 0 and the direction of current reverse now see here current flows from anode to cathode at this current flowing from cathode to anode. So, peak of the reverse recovery current is I R R and this is the reverse recovery time T R R is the reverse recovery time it is time between 0 crossing to 25 percent of the peak of I R R this is T R R why it is very important I will tell you later T R R is the first initial 0 crossing the diode current to 25 percent of the maximum reverse recovery current and let me again emphasize during T R R negative current flows through the device current is flowing from cathode to anode of course, current can flow only when the circuit is complete therefore, this current some of the device may have to carry this current. So, the device has to carry this current. So, we have to choose its current rating appropriately and this T R R decide the maximum frequency of operation. So, the important parameters are average forward current reverse blocking voltage T R R generally this is very important when we are using in a high frequency conversion circuit in a DC to DC or DC to AC when you switch at a very high frequency T R R is very important and I square rating I square R rating is the short term surge energy that it has to handle. I will just give an example here assume that capacitor is completely discharged is connected to a AC source and I am closing the switch and assume that you are closing the switch or you are closing switch when the instantaneous voltage of this input is at the peak. This capacitor is completely discharged voltage across it is 0 and at that instant voltage at this point is V m. If I am using 230 volts 50 hertz it is 230 into root 2 comes around 300 volts also and see the current this diode has to carry. So, all this data sheets this parameters are well explained in the data sheet I will encourage you to go through the diode data sheets. So, what are the various types of diodes? One is the rectified diode or the slow diode where T R R is not mentioned generally these are used to rectify a 50 hertz AC T R R is not very important. As of now 6 kV 400 ampere diodes are available fast recovery diodes basically they are used in high frequency application in DC to DC converters DC to AC see the volt current rating here it is of the order of 5 kilo amperes whereas, fast recovery diodes are of the order of 1.1 to 1.5 kilo ampere are available T R R could be of the order of 0.1 microsecond. So, when I am switching at a very high speed I need to know T R R because diode has to third one is short key diode very fast diode they are very low on straight voltage drop see the rating where they are used I will tell you at the appropriate time voltage rating could be of the order of 100 volts current is around 300 and last one is silicon carbide diodes ultra low power loss. Now you may say that voltage across the diode is 1.5 volts and see see the current if the current rate current that is flowing is 4500 ampere see the on straight power loss 4.5 ampere into 1.5 can I reduce this. So, silicon carbide devices diodes are ultra low power loss fast switching highly reliable, but then everything comes to the package they are bit expensive only time will tell of the cost of the falling rate of the cost. If I compare even the solar energy in 1970s or 1980s was very expensive and yesterday the secretary say the honourable minister said that the unit as the grid tariff has come down to 7.5 rupees. So, therefore, you never know even the silicon carbide diodes the cost of the diodes may come down as the number increases definitely the price has to come down that is about the diodes I just discussed about the parameters what are the important parameter that required for circuit design and what are the various diodes and when do we need to use them second one is the BJT let me tell you one thing not very popular it was very popular it was popular in early 90s or late 80s or so, they are being replaced by IGBTs they are not to understand IGBT or to understand must I need to understand BJT first what are the limitations of BJT how they are addressed in IGBT we will see it was invented in 1970 in 1975 300 volt 400 ampere joint transistor by Toshiba for 1948 by basically NPN transistors are being used of course, voltage rating is determined by the thickness of the collector range I am not going to device much about discuss much about the device physics I said this power semiconductor devices are used as switches. So, in which mode will I operate this BJT the ideal characteristics are when it is closed voltage across the device should be 0. So, if it is not 0 the voltage across the device should be minimum. So, that can that happens in a BJT when I saturate the transistor the question that I am asking is in order to reduce the on state losses I need to saturate the transistor in other words I have to operate the transistor in saturation what term what price am I paying what will happen to the turn off time of the BJT. So, generally BJT is not operated in saturation it is operated somewhere in between saturation active what is known as a quasi saturation why so I will tell you in saturation all of us know the current is given by I C is equal to V C C minus V C Z divided by R V C Z and at that time I C is not equal to beta into I B by the way I said for a high voltage BJT the gain is very low low signal BJT say SL 100 can have a gain of the order of say 500 or so, but then a power BJT say B U 500 8 D rate the 8 ampere 500 volt BJT gain is of the order of 8 to 10 the gain is very low how do I how do I increase the overall gain all of us know that by using or connect them in if I use a Darlington pair I can improve the current rating. So, what is the Darlington pair circuit configuration is shown here so what is the overall gain the overall gain is collected I C is given by I C 1 plus I C 2 and what is I C 2 I C 2 is given by beta into I B 2 and what is this I B 2 I B 2 is nothing but the emitter current of emitter current of BJT 1 that is I E is nothing but I C 1 plus I B that is nothing but beta plus 1 into I B 1 I can safely say that if beta is high I C is approximately equal to I E. So, if I substitute in this equation I get I C is given by this equation. So, the overall gain is now beta 1 gain of the first transistor gain of the second the product of. So, the gate draw requirement reduces significantly if I use only one transistor it is beta times I C is equal to beta times into I B I said beta is very low power BJT. So, this I B is relatively higher how do I reduce it I will use a Darlington pair one of the solution is used Darlington pair. Now, I need to what is the relationship between this current and this it is given by I C is equal to. So, now the gate draw requirement reduces significantly. So, what are the problems here you cannot saturate the output transistor you cannot operate this transistor in saturation. So, overall losses have increased on set losses I use this pair to reduce the gate draw requirement the price that I am paying is overall increase in on set losses. I said at any given point the operating point should lie within the safe operating area. I showed you this of area has three zones one was voltage rating current limit and third one was the power limit BJT has four limits what are they A B steady state current admissible I C steady state B C maximum power it can withstand C D it is a secondary breakdown what is secondary breakdown we will see both is the voltage limit there are four zones the secondary breakdown is there in BJT. What are the problems in a BJT apart from having a secondary breakdown we know that it is a minority carrier device. So, what is the minority carrier device it has a negative temperature coefficient. So, what is the problem in having a negative temperature coefficient as the temperature increases resistance comes down. So, when I connect two devices in parallel and if those devices have negative temperature coefficient then there could be a thermal runaway. So, what happens in a BJT is basically the minority carrier device has a negative temperature coefficient. So, what happens is assume that at some part of if a becomes hot it starts carrying more current. So, as it starts carrying more current becomes hot. So, when it becomes hot temperature it has a negative temperature coefficient resistance decreases. So, it starts carrying more current. So, this is going to be a thermal runaway and device will fail. So, that is the problem of having a device which has a negative temperature coefficient paralleling is difficult. When do I need to parallel when I want to increase the current capability I need to current the two devices in parallel and I cannot use a BJT in this case. Now, how do I turn on and turn off a BJT the efficient operation requires efficient operation of a BJT requires very efficient gate drive as well how fast I told you that turn on and turn off should be very fast. I need to operate the device in quasi saturation and the last point is how do I protect this device all of us know most of our electrical equipment we protect them using a fuse by the way we cannot use this fuse to protect this pass semiconductor device. Let me assure you nothing will happen to the fuse fuse will not blow, but then your semiconductor device will blow. So, we need to or we need to design a gate drive circuit which has a feature of turning off the device when its current exceeds a certain a predetermined value. So, it has a built in short circuit protection. So, these are the features a efficient gate drive should have faster turn on faster turn off operate the device in quasi saturation and finally, inherent short circuit protection. How do we design and implement all these features in this circuit? What to do to reduce TRR time? See one second I do not want to or this is not my area of expertise you are saying that how to reduce a TRR time this is does not belong to my area of expertise I do not know is a device design of fabrication time a pass semiconductor designer does this does not fall into my area of expertise. I know having given a TRR how to use it what should be how to incorporate this in the circuit design may be you can ask Chetan or or or some other power semiconductor not my area of expertise how to reduce TRR. Thank you sir TRR is a is a is a device parameter. Sir you are stating that minor minor BJT is a minority carrier device. Is a minority carrier device? Can you explain why we have we have dealt it as a bipolar device is a minority carrier device what is the collector current made of see what what do you mean a minority carrier device? Are you referring it with respect to the thermal no no you can have a current due to the majority carrier device and a minority carrier device in a in a p n junction or if it is a n if it is a n p n transistor and a p n p transistor what is the minority current due to the majority carrier and a minority carrier by MOS is a majority carrier device why is the n p n is known as a minority carrier device current due to the flow of electrons or current due to holes. Okay you are referring to only n p n. Yeah I am talking about n p n n p n is a very popular. Baravati please go ahead with your question. How to decide a maximum operating frequency of diode from TRR? See circuit design we will discuss at the appropriate time how to use TRR how to use IRR I will tell you at the appropriate time it is only is not only the TRR time of the circuit of diode will determine the frequency we need to switching frequency will depend on the power level the voltage level and other devices as well when we do the complete circuit design we will see. But what is TRR and what is IRR you need to know that is all at this stage please try to understand what is IRR and what is TRR what is the direction of current what is the direction of IRR that you need to know that is all how to use them we will see may be when we are designing buck converter or boost converter we will see what will how to use this parameter not at this point I told you this IR this current flows from cathode to anode but then circuit has to be complete then it has to flow through some device. So that device has to carry this IRR so how to choose the device rating we will see and how to use this the TRR also we will see it is not only TRR will that will determine your switching frequency a switching frequency will be governed by the power level is that ok power level and type of device if a 100 or 200 watt power supply may be switching at 500 kilo hertz a 1 mv a power level drive is switching at around 500 hertz 500 to 550 hertz. So we will see how to decide on the switching frequency I am just discussing as of now the importance per device parameters that you need to understand how to use them we will see later. NIT Calicut please go ahead. Why the isolation tape is required when using the karma insulation at least why? The question why isolation is required. When using the karma insulation. The question why isolation is required if the potentials of all the collectors or potential of all the drains is the same then you do not require I will repeat if the potentials of all the drains is the same then you do not require the isolation say for example I will just draw once what is the this is the collector what is the potential of this collector always at the DC bus positive DC bus what is the what is the collector potential of this device it is floating floating in the sense when this is on when I close the switch say when I close the switch this collector gets connected to positive DC bus when this when this switch is open and when this is closed this gets collector gets connected to this negative DC bus. So collector potential is continuously changing of this device whereas the collector potential of this is always at the DC bus. So how can I mount these 2 devices on the common heat sink now? I cannot if the collector potential is the same then I do not require if the collector potential is floating yes you require see more about it I will discuss how to use an oscilloscope in power electronics circuit one has to be very careful all these practical issues during the tutorial we can discuss good that you ask see all the question right now this design issues I do not want to discuss may be at the appropriate time I will tell you how to use an oscilloscope is also one has to be extremely careful because same here potential is continuously changing floating potential. So 2 channel oscilloscope I have how do I use it one has to be extremely careful the question the answer to your question is if the collector or drain potential is the same I do not need to isolate it if they are different yes see here in this bridge there are 2 devices this is the collector of first device collector of the second and the emitter of first device is collected is brought in here but then this surface is completely isolated this surface is completely isolated. So I can mount any number of devices on a common heat sink which is not the case in this case which is not the case because collector this is the drain and if the potentials are different I have to use separate heat sink or I have to isolate it that is all. Thank you. Jalgao. Yes can you develop smart DC motor to our to skip a battery and inverter. Say that again in what contest are you asking? Your first question use at starting of lecture you ask one question AC we have to losses we have to face losses in AC to DC it is AC while inverting. So and battery also can you skip battery and inverter if you use smart DC motor in almost all applications like water pumping, fridge etcetera. Oh now let me let me repeat you mean to say that I will use a DC motor which is being directly fed from solar panel. What do I say S or no see solar and that will be really cost effective solution. No no cost there is nothing like cost effective. Can you develop DC motor? You have to pay a price somewhere. What will happen if I the moment if I connect solar panel directly to a pump what will happen? You are operating point see assume that assume that you have the sufficient power when I start a DC pump what may happen then it will try to draw a large current. So as the moment you start drawing large current your operating point will change now are you with me. Your operating point is going to change it will so happen that voltage will become very low because machine you are assuming the moment I turn on a DC motor without any power electronic interface directly connected to the solar panel motor by default it will start drawing a current the starting current the moment I draw a huge current it may be equal to your short circuit current of a solar cell the voltage will be very low. So it may not be possible. So you may have to use a power electronic interface between a solar cell and the DC motor.