 Yesterday we discussed we found that for any product, any technology that has to be popular efficiency should be high. It has to be compact, light weight and low cost. Low cost may be cost reduction. I do not want to discuss, but then can we try to address these issues compact size, light weight and high efficiency. We said we know that size of magnetics I can reduce by switching at a high frequency. But then every time I switch a device, there is going to be turn on loss and turn off loss. So, as the frequency increases, this losses also increase. So, I need to control the junction temperature. Are you with me? If the junction temperature increase about limit, the device fails. One way to use may be the heat sink and increase the cooling. Are you with me? The cooling and heat sink. But then the moment I increase the cooling requirement by using a heat sink, my size increases. Of course, there are very other force type of coolings also, my complex increase. That is generally force cooling, the water cooling and all they use for high power inverters. Now, one issue is can I reduce, can I reduce the switching losses turn on and turn off losses or can I eliminate them. Eliminate quote unquote, not strictly eliminate, can I reduce substantially that turn on and turn off losses, conduction losses I cannot. Voltage across the device and conduction losses. So, that question I need to address. How do I reduce the switching losses? And second is how do I reduce the size of the overall inverter. See unfortunately, input voltage is DC. If I, if I want to feed it, feed that power to the grid or if I want to use it, all the equipments are rated for 230 volts, 230 line at the phase voltage, line is 440. If I have to feed that power to the grid, I think a crude method is what is the crude way. I have a DC, invert it, whatever that I get, then use a suitable transformer and strap it up and, strap it up and connect it to the grid. The moment I am saying there is a transformer, what frequency it has to handle? 50 hertz. Magnetic, 50 hertz magnetic size increases, cost increases. Are you with me? Second way is I may have a low voltage DC, can I boost this DC voltage? I will boost the DC voltage to a suitable value, invert it and then directly we use it to reduce the size. So, there are two ways, one is boost the DC voltage, somehow I have to eliminate that, I have to eliminate 50, the 50 hertz transformer. Of course, unless until you want an isolation. If you want an isolation, there is no way but to use a 50 hertz transformer. Assuming that you do not want an isolation, I want to reduce the size, reduce the size. What do I do? Is there a better way? You will see these issues. One is compact size, efficiency and what all issues do I need to take care while connecting here in water to the grid connection? Grid connection will spend substantial amount of time. So, before that today I will just briefly finish. What are the important parameters that I have to consider while using a diode? Apart from average current rating, voltage rating is all fine, which is the important parameter when I am using a diode in high frequency application. It is the reverse recovery current, reverse recovery current. Reverse recovery current, we all know that current in a diode flows from anode to cathode, but then while it is turning off, while it is turning off, see here this charge, whatever the trap charge has to be recovered. So, reverse recovery current is a very important parameter, because that current flows from in the reverse direction and some other device when it is transferred on may have to carry this current. The reverse recovery current of the diode, of course, for the current to flow circuit has to be complete. So, that current may have to flow through some other device. So, while turning on that current may increase its rating or I may have to suitably design this number, suitably design this number. I will, if not today, may be in those 20 lectures in the month of December, we will address this issue. How exactly to calculate the current rating and how to address this reverse recovery current? It is in calculation. So, very important parameter, this is a very important parameter in high frequency diodes, high frequency, reverse recovery current, especially 50 years, 60 to DC 50 years, these are non issues. Unfortunately, as the frequency of operation increases, that frequency increases, this reverse recovery current also increases. So, it is a very important parameter and this is reverse recovery current and this is reverse recovery time, reverse recovery time and the initial surge current. How do I, how do I, how do I suppose assume that, see we do not assume that I have configured this circuit, I have connected a diode. Can I go and arbitrarily close the switch? Gentlemen, a simple circuit, diode capacitor, circuit is complete, can I arbitrarily close the circuit? What may happen? Now, feel free, it is a dialogue, come on, high also, high in rush current, it, high in rush current depends on, I am not saying that it depends on where do you switch on the AC cycle? Where do you switch on the AC cycle? You need to be extremely careful while switching the capacitor circuits, while energizing the capacitor circuit, while and you should be extremely careful while switching of a inductive circuit. See, these are the issues, these are the issues. So, depending upon the AC cycle, arbitrarily if you close, there will be huge in rush current, that current has to flow through the diode, so these are the parameters you need to see. The short time surge energy that the diode has to withstand, I square T rating, short time, you need to see it depends on the size of the capacitor and the input voltage, this is the parameter. We will cover this in detail, if not today, sometime later, TRR, these are the important, rest all are, all of us know, we generally, we do not take this is, what are the type of diodes, I put a rectifier diodes, load diodes, non issues, fast recovery diodes, invariably they are used in high frequency, high frequency switching in even PWM, wherever there is a switch frequency is high, I need to go in for fast recovery diodes, fast recovery diodes, short key diodes and new diodes that are coming in our silicon carbide diodes, ultra low power loss, but then they are very expensive, as of now they are very expensive, cost may come down, cost will come down, silicon carbide. So, for inverter and DC to DC, I have to go in for, I have to go in for a very high frequency, I may have to go in for short key diodes depending upon that or fast recovery diodes, fast recovery diodes. I discussed yesterday, now let me briefly discuss the difference between BJT and MS, current control device, the voltage control device, minority carrier device, majority carrier device, as a secondary breakdown, no secondary breakdown, parallel link is easy, difficult parallel link is easy, on straight power loss is low, it is basically a resist, channel it is formed and therefore the current flows, channel. So, when during the flow of current, the flow of water, it only faces the resistance, so I square R, I d s, R d s 1, turn off time is higher, turn off is a very fast device. So, an ideal device could be combining MOS and a BJT, input is a MOS, output is a, output is a Bifolo junction, so BJT is, there is what is known as a cool MOS also, on strength resistance is very low, conduction losses are very low, cool MOS, IGBT, 1980, invented by again an Indian, Jantabaliga, 1983, insulated gate, bipolar transistor, prior to advent, I have to add a BJT at the output, so what do I need to have, I need to have a P stage, which was not there in MOS, what am I losing by connecting, putting a P stage, it forms a drain, so what will happen, when the channel is, how does, whatever the holes are, come on, what is the flow of, how does the, what is the principle of operation, we had no P stage in cool MOS, sorry in MOS, now we have a, we have a P plus, P plus, the moment I putting a P plus, what am I losing, what am I losing, that N stage was, suppose if there is no P stage, I have only N plus and P whatever, I am putting enough P stage, what will happen, what can happen, in terms of blocking the voltage and turn of time, yes, it comes in direction, now it cannot block, negative voltage, P junction is out biased, holes are injected into N region, some electrons combined with the holes, see come, some electrons combined with those remaining holes are collected at the source, J 1 can now block negative voltage, J 1 can block now negative voltage, initially in MOS N, the moment N is negative, what will happen, which junction can block a negative voltage, here J 1, J 1 can block, J 1 can block negative voltage, come on, I have a diode, if I can, if I make P negative, then it is reverse bias, it can block, now it can block negative voltage, see I will just give a small incident that happened when I was in Kanpur, at that in 1994 or something, yeah, 1995, we imported the GTOs, there are two types of GTOs, what are they, symmetrical and asymmetrical, anode short, asymmetrical GTO is also known as, also known as anode short, which cannot block negative voltage. And one of our resource caller started connecting into AC to DC circuit, AC to DC converter, GTOs are just coming in those days in India, it cannot block a negative voltage, went on blowing the devices, at the time even we did not know, literature was not available, so you should be very careful as to which device to use, if the input is AC or when the excitation is DC, you just cannot arbitrarily put a, take a GTO or IGBT and connect it in a circuit, no, should be extremely careful, anode, there are symmetrical devices, asymmetrical devices, an ideal device is, is an SCR, I am telling you, it is a very ideal device, very rugged, unfortunately I cannot turn it off, you cannot turn it off, there is nothing like an SCR, I am telling you, it has all the characteristics, it requires just a pulse, all these devices are continuous gate drive, see the moment I use a continuous gate drive, gate drive design is going to be difficult because I have to isolate, see the problem, I even need to isolate your control circuit from power circuit, are you with me, one way to isolate from controller power is to use the small high frequency transformer, I can use the transformer provided there are only pulses, the moment I use a continuous gate drive, the transformer will may, transformer may get saturated, so flux, sorry, the pulse may not be, may not be produced identically in the secondary, so next way to, however, provide an isolation is to use optocoupler, opto-isolation, the moment I use opto-isolation, at the secondary state there is a transistor, I need to have a, a separate voltage for biasing, biasing, so in, so first in a 3 phase inverter I will address those issues, there are 3 phase inverter, there are 3 devices on top and 3 devices are bottom and all the signals are applied with respect to, what is the reference point, take a half bridge, S1, S4, switch, this could be a MOS or IGBT or whatever, what is the reference point for S1, everything is with respect to its, its source or an emitter, all this in the lower half, all the reference point for all 3 devices is the same in the lower half, whereas on top if the reference points are at a different potential, different potential, so if I use an opto-isolation I need to have a separate supply, ground is not the same, reference point is not the same, are you able to understand or no, reference point is not the same, so I need to have, how many power supplies, how many power supplies, 3 power supplies here, again these are dual power supplies because to turn on it requires a positive, to turn off it requires a negative, so dual power supply 3 and a one common power supply of higher current rating, hello, reference point is the same for lower half, I can, I can afford to use one common power supply of higher current rating, again dual power supply, so I need to have a separate power supply, designing a power supply is going to be another issue there, so I am coming by, so SCR is a very ideal device, unfortunately it cannot turn off, now J1 can block, can block the negative voltage, J1 can block the negative voltage, in the structure why do you require, see there is n minus and n plus, n minus and n plus, for normal operation do you require this n minus, do both the regions, what will happen, what will happen if I remove n minus, n plus, why do I require n plus and n minus both, both are n, is not it, both are n, what may happen, I will just have n minus only, what can, the moment I saying n, what do you mean by n plus and n, what the plus sign indicates, highly doped, what may happen, there is no n plus, n minus only, this junction may block, I said negative voltage, I said it looks like p plus, there is a p and n, so it can block, it can block, it can block negative voltage, but then what is the magnitude of voltage it can block, it depends on what, it depends on doping, it depends on doping, I have a highly doped junction and a lightly doped junction, operation may not come on, operation may not except for on times operation may not effect, but then what is the magnitude of voltage it can block, does it depend on doping level, gentlemen it depends on doping, now what will happen, I said operation may not effect, because I can, I can remove one then, what will happen, if I highly doped junction voltage rating will increase or decrease, I R D it will decrease, it will decrease, it will decrease, lightly doped junction it can block a higher voltage, lightly doped junction it can block a higher voltage, higher voltage. So, now you see, now if I have to use an IGBT in the AC circuit or if I have to use a IGBT in the DC circuit, do I need to see, what all things do I need to consider, see I am telling you, you need not have two junctions here, need not, sorry two layers, not two junction, two layers, I can just have p plus and n minus, so if I have this, this may, this cannot or this will the reverse voltage, reverse blocking voltage rating of this device is low, low, so it is not a symmetrical device, not a symmetrical device, not a symmetrical device, so all these issues you need to take care, so what is known as a punched through and non punched through, some have only n, just n minus layer, it is a non punched through IGBT, if both are present these are known as a punched through, punched through IGBT and NPT, NPT, NPT, non punched through and punched through and the reverse voltage is applied, J 1 should block negative, due to heavy doping on both sides, this V is low, so punched through IGBT has a negative, low negative voltage blocking capability, a non symmetrical IGBT, non punched IGBT is a symmetrical IGBT, see here NPT and NPT, same, p plus n minus, what happens, the moment I modified the structure something must be happening there, what price am I paying, what am I paying, what price am I paying, n minus, what happens, what did I say, when the moment I turn it, some holes in electrons are combined here, now for turn off complete, when the device gets complete it turned off, they have to recombine, recombine, n minus implies less number of electrons, so what will happen to the turn off time, turn off time increases, increases, increases, increases, so what is important, see the turn off process, what is known as the current tail, we have a, see the problem IGBT has a MOS at the input under, under, under BJT, so MOS is a very fast structure, it turns off, but then now I have an, this one, this MOS current has come down, but then during this time, what is known as the current tail, current tail, voltage across the device has increased, a small current continues to flow, because device completely turns off and all the charges, a very small current flows, but then voltage across the device has substantially increased, so there is going to be a substantial power loss, so power loss during the current tail period, so this has to be minority carriers, everything the journal disappears, MOSFET blocks, the quickly ID drops, minority carriers in n minus layer gradually recombine, ID decreases slowly, ID decreases slowly, what is known as the tail current, it was, this period should be small since losses are high, points through IGBT has a smaller tail time, smaller tail time has n plus layer almost similar to, so again, so your heat sink design calculation, because loss are taking place substantially during, during turn off and current tail period, TF, so all these parameters are given by the manufacturer the data sheets, so while designing the inverter you have to take care of, so a safe operating effort in IGBT, same as the MOS, yeah, I do not know how many of you have seen the, the inverters in the 1980s, 1990s and 2000, what could be the major modification, the, the improvement, I said improve size, what, how the sizes come down, yeah, in the sense initially we had a small transistor, I had to connect so many transistors in parallel, unfortunately, unfortunately paralleling was, paralleling was difficult, because they are negative temperature coefficient, then slowly, slowly the, then started coming one leg of an inverter, one leg of an inverter, there are three terminals, one is plus, this point and these are the three terminals, 1, 2, 3, corresponds to 1, 2, 3, okay. Now I need to apply gate signal, gate is applied with respect to its source, so these two are, these two pins for one IGBT, these two for, these two for other IGBT and one common heat sink, one common, I can mount, I can mount this, you know, you know, heat sink, one more, I can mount all three in one common heat sink, see originally I had the small, small transistors, the TO3 package, body was collector I guess, right, okay and collector potential of, see the, see in this collector, this is collector, collector, okay, potential may be the same here, where I was collector potential here, different, so I need to mount them separately, are you with me, you can't, you can't, you can't come out, connect or you can't mount all the transistors in a common heat sink, those technologies improved, I could connect all of them in one common heat sink, common heat sink, but then see here again I have to apply the signals with respect to source and emitter, source and emitter, okay, their potentials are, the potentials of these two points are different, okay, the reference points are different, so I need to have a separate gate drive, come on, I need to have separate gate drive with isolation, you follow what I am saying, so I said I need to have six power, three power supplies of one current rating and one power supply of higher current rating or have six power supplies of same current rating, now see the improvement further, all the devices entire bridge, entire bridge, entire bridge and these are the control pins, now here I need to apply I can directly connect controller signals here, I don't need to give the gate, you follow what I am saying, output of the micro controller I can directly connect here, isolation is built in, isolation is built in, now I need to have only one power supply at the control side or low voltage side, I don't require a separate six power supplies, a dual power supplies at the secondary side or the output of the optocoupler, I can directly connect, directly connect, see the reduction in, reduction in the size of the equipment, compare it with the TO-3 package, compared to TO-3 package, compared to TO-3 package, expensive, the thing is of course you can't use it, they will first go, if one device fails you have to, you have to, you have to just discard it, current sensing, everything is the intelligent power module, power module driver circuit, protection circuit, over temperature protection, over current protection, over voltage protection, I did not know how many of you, no sorry I am not underestimating you, the issues how do I see over current protection, how do I provide that over current protection, suppose I am using a BJT, how do I provide over current protection, how do I protect the, can you tell me the, see the research that has gone through, research that has gone through here, to make the compact size, to reduce the size, reduce the size, reduce the size, now can you tell me how do I provide, suppose I am asking you provide a short, over current protection for a device, how do you do, what is the over current protection, what is, no relay will not, by the time relay operation device will be gone, you cannot, relay fuse and all are, they cannot be used, what do you need to do, you need to sense the over current and immediately you have to shut down, shut down the gate signal, so you have to sense the current, now how do I sense the current, now the moment I use the whole current see the cost, I will sense the voltage across the device, see the moment the voltage, the current flowing through the device increases, what will happen, VC will starts increasing, the device tries to come out of, come out of saturation, initially you have saturated it, now nothing has changed, IB has still remained the same, you have not changed the base drive, because of the fault current has starts increasing, IB is the same, IC starts increasing, so what happens, device will starts come out of saturation, so the moment, so it goes and enters into active mode, the moment it starts going active mode voltage across the device increases, so by that time if you say my relay is going to operate, you can see the smoke coming out, take it from me, so what you need to do is sense the voltage across the device, immediately shut off, see the complexity in a gate drive circuit, the gate drive circuit, if your gate drive circuit is perfect, I am telling you your device will not fail, till date I do not think you have a full proof, full proof gate drive circuit that is available, designing a gate drive circuit is not easy and if you make a good gate drive circuit I am telling you, take it from me, 99 percent your device will not fail, device will not fail, so this current is built in here, the over current protection is built in here, so that is the reason why one of the ways I am telling you, let us think differently, let us not think in a routine way DC to DC, DC to AC and this one, can we think differently and try to try to try to reduce the cost, we should target those areas, I am telling you, see the size reduction in size that is possible, made possible by, capacity the rating here is 75 amperes 1200 volts, each device, see if this device is again 1000 volts, 1000 volts is 75 amperes, one leg, 6 devices of 75 ampere, 1200 volts, 1200 volts, quite bit expensive yes, I had noted what price we imported, we imported 2 years ago, it might have come down by now, cost is not the issue today, payback period or reliability and another today, see we are, I hope you are targeting the rural area, if you are designing an inverter to the rural sector, what should be the concern, reliability, I do not need to, please forgive me, I do not need to very sophisticated equipment, it should work in the rural area, because if it fails, the person may not know what to do with that, there is no infrastructure that is available to repair, D rating may not help sir beyond a point, beyond a point, what is the, which is the weakest link in, I do not all of you know, if I will come through, it is an inverter, what is the weakest link, maybe I, what is the weakest link in a, a voltage source inverter, I will do it later, what is the weakest link in a voltage source inverter, I hope all, what is the weakest link capacitor, electrolytic capacitor, so the people are saying today, can I do away with this capacitor, can I do, the moment I say, because we have capacitor, now it has become, I have to change the power circuit configuration itself, then I have to go back to a current source, there are only two types I guess, voltage source and a current source, the weakest link is capacitor, weakest link is capacitor, you may say, okay I will derate it, capacitor, see one of the student is working, maybe I will ask him to talk to you for half an hour on how to improve reliability, people are talking about 20 years, because solar, life of solar panel, Chetan may say that 20 years, inverter also should last for 20 years, I do not, does it make sense, it does not make sense to me, I do not think I will use 20 years old technology, will you, I do not know, I do not know, these are my views, we are in a by back period, use and throw within 2 years, is not it, once upon a time when I brought a premier Padmini, it was supposed to last for the, for the entire generation, now we, in 3 years we, in a middle class person also changes this, refrigerator, once in lifetime purchases, is not it, once upon a time, not so long ago, got the refrigerator, buy and keep it for at least 25 years, premier Padmini ambassador, all those, nowhere, so why are we talking about building an inverter for 20 years, will I use a 20 years old inverter, in who will I, will I use a 1980, when inverter no, it is highly inefficient, no you see, state of the art is available, we are in a by back period, by back era, I do not know, these are my views, but then people are still talking about improving the life and it should last for 20 years, people are talking about reducing the capacity size, yes, we are making an attempt, I do not know whether it will be successful or not, so if, if people want it to last for 20 years, I think we need to at least inch towards the direction, try to improve the reliability, mean time failure, yes, any questions, so my device part, I have moral has covered, MOSFET, IGBT divorce, how to operate them, these three, if you know for solar, and there is one more very popular topology, where I can use it, a grid connection, grid connected inverter, a single phase grid connector, where I can use an SCR, if time permits, I will discuss it, SCR base inverter, what is the voltage profile, when the, when the device is being turned off, what could be the max, it depends on the, see if there will be a stray inductance, are you with me, when I turn off the device, when I turn off the device, dA by dt is, dA by dt is negative, so LdA by dt is negative, so voltage across the inductor can change instantaneously, so net voltage across the device is much higher than the DC link voltage, from here it appears as if the maximum voltage rating of the device here is, here is, when this is closed, what is the voltage across, that voltage has to come across this, so it appears as if, it is the DC link voltage, which is not the case, it depends on your inductance, the stray inductance and I that is, how the DC link is made, we will show you in the inverter, we show the inverter, it is a sandwich type, you have to reduce the inductance of the DC link, are you with me, you have to reduce, it is not a just a conductor, a copper strip, no, no, this is, this is one copper strip, maybe this is another copper strip, how they are sandwiched to reduce the inductance, we have to reduce the inductance, you have to reduce the inductance, how it is reduced, you can, we will show you the inverter, we will show you the practical inverter, how the DC link is, yeah, any other question, no, see, let me, let me correct my statement, I said, one way to provide isolation is transomers, right, and if the transomers may get such, may, I am not saying that transistor will, continuously if you give, yes, now you can't design a transomers, you can't design a transomers, you can't design a transomers, see, we say that transomers is a differentiator, suppose if I give this pulse to this transomers, what do I get here, what do I get here, is a differentiator, is it not, why, because it is, we accept that in this zone, transomers is saturated, now you can't design a transomers such that it will not dissolve, it is possible to design a transomers where in I get a reasonably good waveform at the output, it's possible, it's possible, how do I avoid, one question that I am asking, how do I avoid transomers getting saturated, what do I do, I should not allow the transomers to get saturated, yeah, provide an air gap, provide an air gap or should not allow the flux to increase, isn't it, so provide an air gap, now the moment I provide an air gap, yeah, so that leakage and all will come into it, not an issue, transomers designs for isolating, transomers are being used in gate drive circuits also, but then it is, these are, so shall we wait for, not the ones which are available in the market, pulse transomers.