 I will start inverters. I will try to increase the phase. There are basically two types. Input is both are being used. Both are being used in solar. The reason I will tell you the input to the inverter is a voltage source. Either it could be a battery or or voltage could be a charge capacitor. Basically, voltage source inverter is a two quadrant. I am using a capacitor. Voltage cannot change instantaneously, but current can change instantaneously. So, this current in the DC link, I am calling this part as the DC link. Current in the DC link can reverse. So, I need to connect the devices accordingly. Current in this part can reverse, but then voltage cannot reverse. This voltage cannot reverse because voltage cannot change instantaneously. So, that is the point to be considered. By the way, the popularity, the one of the issues that are being raised in any meeting is what is the life of your inverter? Because solar cells or the semiconductor people seem to be saying that the life of solar cell is 25 years. Can your inverter last for 25 years? Now, one of the weak links, it was reported that that is the capacitor, electrolytic capacitor. So, my question to you is or my suggestion is can you come out with the inverter without using this electrolytic capacitor? Now, there are other elements also there. Even the switches also can fail, but then the past experience or if you refer the literature, what they say is this electrolytic capacitor seem to be failing very often, especially when the temperatures are very high in the desert or something like that. So, this is the limitation. Can we have a inverter without this capacitor or a very small capacitor? Do not use an electrolytic capacitor. Do not use an electrolytic capacitor. So, another one is if I can use a capacitor, can I use an inductor? Yes, the moment I connect an inductor, this is almost a current source. By connecting the voltage, by varying the voltage at this point, I can try to control the current. I can try to keep the current approximately constant. Now, then sorry, but then I have a large inductor here. Current has to remain approximately constant. Current cannot change instantaneously in the capacitor. So, therefore, I need to use the devices here which can carry the current in only one direction, but then voltage at this point has to change. Voltage across the inductor can change instantaneously. So, I need to use the devices which can block voltages in both directions. Remember, we discussed MOSFETs, we discussed IGBTs, we discussed BJT as well. BJT, NPN transistor, base emitter junction is highly doped. So, reverse voltage blocking voltage is very low there. Even in IGBT, there are two types, punch through, non-punch through. So, asymmetrical IGBT cannot block the negative voltage. I can use them here because this voltage cannot change, but then current has to change. So, I need to have a device here which can carry current in both directions, but should be able to block the voltage in only in one direction. It need not block the negative voltage, whereas in a current source, because of this inductor, this current cannot reverse. So, therefore, I need to use the device which can carry current in only one direction, but then it should be able to block the voltage in both directions. This voltage could be negative as well. Why? If I have to decrease the current, voltage across the inductor has to be negative. So, it may have to block a negative voltage as well. So, basically these two inverters are two quadrant converters, two quadrant. In the V I plane, they are two quadrant converters. Voltage source inverter is voltage always positive, current could be either positive or negative. A current source inverter, current is always in the direction of positive, voltage could be either positive or negative. So, I will quickly do, most of you might have done this basic block in a voltage source inverter. After doing the voltage source inverter voltage control, then we will see the synchronizing with the grid. There was question that was asked in very first lecture. How do I connected? How do I feed the power to the grid or how do I synchronize? May be in other three or four lectures, I will cover the topic. So, be attentive. Now, inverter is bit, see DC to DC converter is relatively easy. Input is also DC, output is also DC. DC circuit analysis is relatively simpler. Now, we are doing DC to AC. Input is DC, output is AC. May not be very straight forward, this try to understand, but it is not very difficult either. Half bridge, temporal half bridge. See I said current can reverse here. So, I need to connect or I need to use a device which should be able to carry current in both directions. So, S 1 is a controlled switch and D 1 is the diode. So, because of these two diodes, voltage minimum voltage at this point is limited to how much? So, 3 volts or so. I cannot reduce this further because of D 1 and D 2. Assuming this power diode voltage is of the order of 1.5 volts or so. That is what if you see this, the module that I had shown you that 30 ampere, the diode drop is of the order of 1.5 volts or so. So, this minimum voltage could be or is 3 volts here. It cannot be negative. This voltage cannot be negative because of these two diodes. Basically, in the V I plane, this is how they work. I will quickly draw open S, sorry close S, DC to AC. Close S, point A gets connected to this point voltage 1.5 volts voltage applied to the load is V DC, sorry V DC by 2. I am assuming load to be R L, R L load voltage applied to voltage applied is DC, current will exponentially rise. Do not allow it to saturate. It may be DC to AC, but then time constant tau and frequency T you choose accordingly. Because here for T by 2 seconds, you are applying positive V DC by 2. After T by 2, open S 1, close S 2. You are applying minus V DC by 2. So, current will fall, current will rise in the negative direction. But then let us see what happens. This is how it looks S 1 on, S 2 off, S 1 on, S 2 on here. Sorry, this should be S 2 on. I will just show you in the white board the transparency is missed. S 1 on, S 2 on. This has to be T by 2, this is T. So, if you want to have a 50 yard supply, T by 2 is 10 millisecond and this T by 2 is another 10 millisecond. So, 20 millisecond one cycle. So, you have S square wave output. I am calling it as AC because average value is 0. I am applying this S constant DC voltage to an R L load. Current will change exponentially at S 2, S 2, S 2, S 2. At steady state, current waveform look like something like this. In the first cycle, it has, it will start from 0. Then it will decay and at steady state, current will look something like this. So, S 1 on, S 2 on. What I mean is I am applying a gate drive to S 1. I am applying a gate drive to S 2 here. But if you see voltage and current, see in this region, voltage is positive, current is negative. Voltage is positive, current is negative. Positive direction of current is from A to B, sorry, B to A, positive direction of current is B to A. So, if you see here, positive voltage, voltage applied is positive. So, definitely this point has to be connected here. Only then voltage applied is positive, but current is negative. So, therefore, this current is flowing through diode D 1. Voltage applied is positive, current is negative. Voltage, you can apply positive voltage to the load only when either S 1 is conducting or D 1 is conducting. So, this point has to be connected here, but then current is flowing in the reverse direction. Looks like it is being carried by D 1. In this zone, both S 1 and voltage is positive, current is positive. So, S 1 is carrying current. Similarly, here voltage applied is negative because S 2 is on. S 2 is on. That can happen. And voltage applied is negative. Voltage applied is negative, current is positive. So, current is flowing from B to A. Voltage applied is negative. That can happen only when S 2 is carrying current. Voltage applied is negative, current is positive. And in this zone, both voltage is negative, current is negative, current is negative. S 2 is carrying current. See, it is already time. I will stop here. I will take few questions. The question is, can you please explain why there is a peak current in case of short switching? It depends on the power circuit configuration, DC to DC converter. I will suggest a book, power electronics by Lander, power electronics by Lander, L A, L A, N D, E R. There are soft switch, DC to DC converters, voltage, 0 voltage switch, 0 current switch. They are explained very well. And you will find that there are peak currents. The currents in the sense, currents is sinusoidal. And see, one has to explain, one has to go through the power circuit and start explaining from basic. It is not going to be very easy. If you insist me to cover the soft switch converter, I will. Give me some time. I will. I thought I will just try to tell you what are the possibilities. That is all. Explaining the peak current is not going to be easy. One has to draw the entire circuit and explain, start from scratch. It will take an R or so. It will take. So, if you, for more clarification, go back and read this book Lander, Power Electronics. You will find it. Yes, it is, is it because of energy storing element? Yes, there are L and C, resonant current. So, that resonant current forms a sinusoidal power and there is a peak current because, because come on, it has to start from 0. Initially, it was instantaneous value and it remained constant there. Now, it has to start from 0, slowly rise. Average has to be same. So, definitely I will have a peak current. Sir, how can we design a low-cost inverter E 1 with 1000 rupees for using a, sir, how can we design a low-cost inverter E 1 within 1000 rupees? It is possible. What is, what is the power rating? It is not a 12 volt. What is the power rating that you are talking about? The component cost is not much. Component cost is not much. Today, inverters are bit expensive maybe because one is there, because of the failure rate and they have to replace it and servicing, they may be, because of this reasons cost may be high. If you just see the component cost, component cost, I am not talking about the labor or whatever component cost, you should be able to design a solar, so solar inverter that too. I do not know what is the voltage rating that you want. What is the power rating? You just cannot, a device inverter, the rating of the inverter is V a voltage into current. Solar model is 12 volts maybe. So, what is the output that you require? That depends. Now, if you want, so in principle, it should be possible. It should be possible. It should be possible, not a very sophisticated product, but then for your home, you can design, make it work. Now, if it does not work, if there is a failure, you know where it is and you can rectify it. 1000 rupees, it should be able to design a good inverter. In recent years, I am seeing a new concept called synchronous converter, where diodes are replaced by MOSFETs to reduce losses. Using the topology, can we reduce the switching losses? Synchronous converter, yes, DC to DC converters, yes, you can, you can, you can. In principle, there also they, it is almost like a soft switch converter. The question is, in recent years, I am seeing a new concept. It is not a new concept, it is quite old. What is known as a synchronous converter? Two MOSFETs are being used. Two MOSFETs are being used. Now, you turn on this MOSFET when this diode is on. So, voltage across it is 0. Now, I have reduced, I have eliminated the turn on losses. In principle, yes, these are some sort of a soft switch, not entirely, because diode is not soft switch, diode is not soft switch because the moment I turn off this, current has to flow through diode initially, initially. So, entire losses are not eliminated here. Please, what are the advantages of phase shift full bridge converter? Gentlemen, I will explain to you when I integrate them. I did tell you in beginning of my lecture that we have to do certain modification when I integrate the DC to DC as well as inverter. Have patience, I will. Now, if I say phase shifted, now people may ask what is phase shifted? So, everything I will explain to you. Do not worry. At least one day I have kept, a minimum one day I have kept that is 2 and half hours or so for these questions and these issues. In a voltage source inverter, the capacitor we use causes unreliable. So, can we use a proper protection to avoid? So, there is a question in a voltage source inverter, the capacitor we use causes unreliable. So, can we use the proper protection to avoid this problem? What protection, sir? What protection? The weak link that is what the literature says that electrolytic capacitor. So, people are working or the need of the need of the R is today, inverter without electrolytic capacitor. I am saying electrolytic capacitor, you may you can use other capacitors. Now, do not say that, can I use a super capacitor? So, if you can come out with an inverter nothing like it, up to which level we can get to increase the DC, increase the DC converter and inverter efficiency? Oh my God, up to which level, what is the maximum, maximum efficiency that we can get in a DC to DC converter and inverter efficiency? See the German inverters, they are people are talking about 98 percent efficiency. That cannot be the part load, that cannot be the part load efficiency. My gut feeling is, gut feeling is this must be the peak efficiency. So, my sincere request to you is, see we cannot compete with this efficiency figures. Abroad people have started, I do not know how many thousands of manas they have spent in improving the efficiency of the order of 98 percent or so. And that cannot be taken up by an academician, it has to be taken up by an industry. My, what I feel is today for the need of the R of the country is, can we, that is what even the, our honorable minister told, can this technology go to the rural area? I do not know how many of you are from rural area, I am from a rural area, you know the life there, if the inverter fails, there is, I do not think the person from that village will take it for repair. The need of the R is, can we make a good reliable inverter, efficiency may not be an issue, may not be an issue. So, my sincere request to you is to divert all your energies in designing a good reliable inverter. A good reliable inverter, efficiency need not be comparable to what they are available in the international market. For us, it, it, I do not think, so now please do not quote me out of contest. Let us not get into this figures of claiming our inverter efficiency is so and so, I do not think we should be doing that. Please, these are my views, my views. Government column, Salem, what do you? Yes. May the soft switching suitable for all kind of PWM techniques? Whether soft switching is applicable for all PWM switching, it, it, it depends, it depends, it depends, certain resonant, it depends on what sort of a, what sort of a soft switching that you are using. Now, see here when I am using a DC to, I am just giving an example of a voltage source inverter being soft switched. There are various types. What they do is, here there is a constant DC, here there is a bridge inverter in between. They connect L and C and make the voltage to oscillate and switch the device at these points. This is known as the resonant DC link. Now, you have to switch the device at these points alone to eliminate the losses. Now, this voltage is twice V DC. Now, this is V DC, this is twice V DC. Now, you cannot use the conventional PWM technique because conventional sinusoidal PWM technique says, switch the inverter when sign intersects with the triangle. But then, resonant DC link says, you have to switch it here. So, you have to use a separate PWM technique for resonant DC link. Now, there is another technique, what is known as a quasi resonant, quasi resonant. Temporarily, you will make the voltage 0. Temporarily, you make, here it is continuously oscillating, here temporarily you make voltage 0. Here, you can use any PWM technique. So, soft switching, in soft switching there are various class and whether conventional PWM technique can be used or not, you need to see. In resonant DC link, it is also resonant, it is also a soft switch. Conventional PWM cannot be used. Quasi resonant, now you have to create a short in the DC link that you do when this intersection takes place. Yes, it is possible. Here, it is not possible. So, there is not a unique answer to a question. Is that clear? Thank you, sir. In the inverter input, you have shown that input is VDC by 2 and VDC by 2. That can be achieved using, let us say two capacitors I think you will be using. My question is, when you use two capacitors, you have to go for voltage balancing. Suppose, if I use a fly back DC-DC converter, where the output, we have two transformers, yesterday you told that you can have multiple outputs. So, the two secondary you can have and you can have two outputs. In that case, voltage balancing of the capacitors without that directly I can connect it to the inverter. Sir, now about capacitor balancing, let us not worry now. We will see later. My question to you is half bridge I have used a center tap. Sorry, half bridge I have used a center tap. Whether this center tap is required or not, just for convenience sake I have used it, we will see. Center tap it is required. As we go along, we may not use this half bridge. We will see. Let us not worry about capacitor balancing as of now. I know capacitor balancing is an issue. The moment I use a center tap, capacitor balancing is an issue. Now, let us go step by step. If you use full bridge, I do not require center tap. If I use a three phase, I do not require a center tap. Now, see the problem in half bridge. Here VDC by 2, VDC by 2, I get VDC by 2 and this is VDC by 2. Now, if you want to have 230 volts RMS, peak is 230 into root 2. If the square wave, this peak is 4 by pi into VDC by 2. Now, what should be to get this 230 into root 2? What should be VDC by 2? Calculate. Instead, can I use full bridge? Now, there are only two devices are conducting. Now, there are two devices are conducting. So, this is just a building block. That is how I used half bridge center tap. The moment I use center tap, there is a capacitor balancing. Yes, will go step by step and I am not able to, I did not understand your solutions, whether using a fly back converter is possible or not. See, I do not want to comment. I never understood, but one thing you remember. If the solution is simple, you might be losing somewhere. That is the law of nature. So, I do not want to comment on your answer or your solution. It may be possible, but then we will see if I am doing multilevel inverter for high power. If the time permits, I will do three level inverter, three level multilevel inverter capacitor balancing is an issue. How they have tackled the issue? How they have tackled that problem there? I will try to explain and we will see at that time whether this also issue can be addressed or not. That is what I was asking, sir, about the fly back, output fly back if you are connected. Capacitor balancing can be problem. I think it can be reduced as per your yesterday lecture. Is that possible really? What is the solution? What is the solution? The fly back converter that you have said, sir, you can have multiple output also is possible. You can connect two transformers in the secondary. Secondary have two winding. So, we can have two VDC by VDC or two VDC by two, two VDC by two. So, there I think directly you can use the supply without using any capacitor. Is it possible? Because the transformer here, there itself balancing takes place, I think. Might. Is that correct? I am wrong because directly you are interfacing. Might be possible. Why I should say it is not possible? It has to be studied in detail. It might be possible. It might not be possible. Now, you tell me, even in multi level, I do not whether I should be going to multi level or not. I have to balance the voltages. What do I need to do? I have to take some power from here, dump it here. If this is, if this is discharging and this is, sorry, if this is charging and if this is discharging, I have to dump, I have to remove some power from here and dump it from here. I have to use a separate DC to DC converter. That is one way to balance it. So, something similar may be possible using fly back. You are saying that I will have one primary and another secondary. Now, here all sudden done, all sudden done, it may be difficult to regulate all the voltages. It is, all of them are related to some turns ratio into d divided by 1 minus d. And depending upon the load on one winding, you are trying to control the other winding. So, it may be difficult to control, regulate all the voltages. I just told you the possibilities, but I have never discussed the control issues, control complexity. Then we will see whether fly back will be a better solution or using a separate DC to DC converter, transferring the power from one capacitor to another capacitor is easier or not. So, I do not want to say whether your solution will work or not. Might, might not, if it is, yeah, that answer. Sir, like in a DC to DC converter, we have a dedicated control circuits. Similarly, for the inverters also, do we have the dedicated? Yes, sir. Yes, answer is yes. Before, before, before completing a question, I will answer. Yes, it is available. I will give you. Yes, sir. Yes, sir. I will suggest. Do not worry. Sir, thank you very much, sir. I will. Why electrolytic capacitors are restricted? Then what is the alternative capacitor? That you need to find out, man. Why electrolytic capacitors are restricted? I told you the literature says that it is a weakest link. The literature says that electrolytic capacitor is the weakest link. Now, do not say that even the devices can fail. Yes, I also know that devices can fail. The literature says what is the alternative capacitor? That is what I told you. Solar people, the semiconductor people are saying their panel is 25, can last for 25 years, can you design an inverter for 25 years. So, you should be worrying as to how to improve the, how to improve the, or how to increase the life of your inverter. You come out with a solution. What are the alternative capacitors you just see? You need to use a very smaller capacitor because size is all sudden done. Electrolytic capacitor size is very small compared to its for a given micro farad. If I compare with the capacitor or AC capacitor, size increases. They want a smaller size, no electrolytic capacitor. Sometimes I feel it is going against the law of nature. I do not know. These are my views. For reliable operation of DC to DC converter, can we use base converter itself like buck boost or else? Can you go to advanced converter life? Sir, I do not know. For reliable operation of DC to DC converter, can we use base converters like buck, boost? Sometimes you may not be able to use. Sometimes even if you want to use, it may not be possible. It may not be possible. What if the ratios are too high? You want to, you have 12 volts and you want to convert it to 230 volts AC and you want to boost the DC voltage. It may not be possible to use boost, single stage boost. Now, you have a choice to use three or four stages. Then the reliability cost, I told you already. This I had answered in the beginning of my lecture. I do not want to repeat it. Thank you. See you later.