 may be I think we have not even gone to that level, have not even discussed the closed loop control of a DC to DC converter. Now battery charging, that is a conventional battery charging, constant current, constant voltage, trickle charging. These are to be very honest with you, I do not think the techniques which you are asking, they are generally used in switch mode power conversion. I do not think they are relevant in solar power conversion, how to improve the response time. Now see my, please do not get petrope, we talk about MPPT, my response time I have improved. I am asking you a question, fine I have not improved my response time, I am a bit fluggish, what am I losing? Be very honest, let us be very critical, what am I losing? If my response time is lower by say 10 millisecond, what am I losing? In the sense is it something fuel is burning there in a tsunami to recover and response time? And another thing is, is the sun installation changes so instantaneously? No please do not get petrope, my due respect to people who are working in MPPT, but for me according to me they are splitting your hair, there are other issues to worry about. Now do not get, how to improve the response time? These are all very important issues may be in drives and where 20 millisecond is lot of time, are you with me? Say for example 20 millisecond, see when the locomotive is going in this one, pantograph jumps, right? 20 millisecond, suppose if there are disconnected, your pantograph gets disconnected by 20 millisecond, entire 1 MVA power or megawatt power the capacitor has to supply, are you with me? If the capacitor voltage falls by 70 percent, your control circuit trips saying that is under voltage, these are very important issues there. I do not think this so called improving response time or those things are issues in solar. If those issues have to be addressed, may be give me a slot, I will come and talk to you. Otherwise you come, may be sometime later we will discuss, are you with me? Now please do not get better, let us concentrate on the issues which is going to make a small impact rather than, now please do not, this is not the answer to your question. Trying to response, improving my response time by using a MPPT this one time, for me according to me these are non-issues, fine you can improve your control algorithm response time, all fine if you get the publication out of you and if you are happy, go ahead. Yeah, yesterday I asked you what are the question that I have asked, forgot about the question, you will answer, what are the question that I had asked? The question that I had asked was the so called MPPT gets extract the maximum power or whatever the power that is available from the solar cell, it dumps in the capacitor. Now that power has to be, power has to be fed to the grid, how do I determine how much power to feed, which control loop, where is the control loop which will tell you this amount of power has to be, it has to be online, are you with me? It has to be online, like a MPPT that operating point goes on changing, the so called if I use a hill climbing method, every sample it calculates the power compares to the previous, if it is less it changes operating point, are you with me? It may be the, it is maybe sampling time could be as small as 1 millisecond or so. Now similarly, here I have to feed the power to the grid, how much of power to feed, how do I determine, who will tell you, who will tell you, that one question we need to answer, okay. Another thing is as we have discussed a two level inverter, a conventional voltage source inverter, a conventional voltage source inverter, voltage output voltage varies from plus V dc to minus V dc, plus V dc to minus V dc. So we found that, we found that the dvp dt is, dvp dt is very high, very high, okay. So there is a lot of stress on the device, leave alone to your load, okay. How do we address, as the power level increases, see as the power level increases, my DC link voltage may have to be, may have to increase the DC link voltage, okay. So under that condition, is this two level inverter, the conventional voltage source inverter, is it suitable or can we modify the structure, can we modify the structure? Okay, I am telling you, I will just ask you a question. What the PWM inverter does? There are large number of pulses, large number of pulses and we told that, if I apply these pulses to an RL circuit, initial portion is linear, okay. So somehow I get a sinusoidal current. Now assume, instead of having this sort of waveform, if I have, do I need to have, I have just drawn the half a cycle, somewhat symmetrical, okay. Do I need to have, is there a need for large number of pulses? Come on, if I waveform something like this, if I waveform something like this, which is better? First one is better or the second one is better? Second, it looks, if there are more number of, it looks approximately a, it is approximately a sign, approximately a sign. There are more than, more than two levels, hence the name multilevel, hence the name multilevel. So you may have to, you may have to change the power circuit configuration itself. I am telling you, I told you that in a DC to DC converter, there is a, there is a DC to AC inversion and again we are converting it, okay. In order to, in order to reduce the filter size requirement, this is one solution, this is one solution, use multilevel. I told you that half a converter, full wave converter, three phase converter, capacitor requirement or filter requirement is minimum in, in three phase, three phase. So similarly, if I have to reduce the, this is the solution, stress on the device if I have to reduce, multilevel is a solution. Multilevel is a solution in a grid connected inverter as well, to feed the power to the grid for high power, the multilevel inverter V h0 is either plus or minus V dc by 2 or V a x is V dc or 0, V a x, okay. As the power level increases, V dc has to increase. See another thing, as the power level increases, V dc has to increase and switching frequency has to come down. I told you 250 watts switching frequencies of the order of, this gentleman said around 200 to 400 kilohertz. For 1 AMV inverter, switching frequency reduces to, so around 550 to 600 hertz. So see, so the moment the switching frequency, the, the frequency of the predominant harmonic in a two level inverter, frequency of the predominant inverter is approximately equal to switching frequency. Come on, in a sinusoidal PWM technique, sine wave is compared with triangular wave, okay. If the triangular wave frequency, the, the, the, the frequency of the triangular wave will determine switching frequency. Are you with me? If the frequency of the triangular wave is 550 hertz, if the frequency of the triangular wave is 550 hertz, what is the harmonic spectrum? What is the harmonic spectrum? It has a fundamental component whose frequency is same as the modulating wave. Come on. If this side did not cover thinking, what is the sinusoid? These are sine versus triangular, okay. Fundamental frequency is, fundamental frequency is equal to frequency of the modulating wave, okay. Are you with me? If that is 50, this is also 50, okay. If, if this is FS, switching frequency, okay, the harmonic spectrum, what all, what is the frequency of the predominant harmonic? For a sinusoidal asynchronous PWM, approximately, plus or minus 1, yeah. See, there are two side bands somewhere here, somewhere, what is the frequency? If I call this as F triangle, this is, and this is F sine, this is approximately F triangle minus FS and this is F triangle plus. So, switching frequency of the triangular wave is 550 hertz. So, frequency of predominant harmonic is around, around, around, that is relatively low. It is less than 11. Are you with me? Less than 11th harmonic. So, we have problem in a two level. As the frequency, as the power level increases, as the power level increases, the DC link voltage increases and switch frequency comes down. If the switch frequency comes down, frequency of predominant harmonic also comes down. That will affect your, that will affect your THD. So, you may have to design a separate L and C output filter to handle this, okay. So, is there a solution? Is there a solution? Solution, instead of having two levels in a BA0, increase the number of levels, number of levels. As the number of level increases, VN becomes approximately a step sine wave. It looks approximately sine wave now. Are you with me? After all, sine is what? There are large number, infinite number of steps. Only then I can get a smooth sine wave. So, fire, convention three, predominant harmonic is low. Instead, modify the inverter structure more than two multilevel inverter, three level inverter. There are various types of multilevel inverter, diode clamp, flying capacitor, okay, type or cascaded. I will discuss diode level, sorry, diode clamp and the cascaded. Cascaded is bit attractive for solar. I will tell you, okay. This is also quoted. It is a three level inverter. Just see the structure, okay. This is one leg of the inverter. This is one leg, this is one leg, one leg of an inverter. Conventional BSI, one leg of inverter. There are only two devices, two devices, two devices. See here now, this is one device. Voltage source inverter always requires a switch in parallel with, parallel with the diode. This is one device S1, S2, S3, S4, okay. Are you with me? We did use center point of the DC link as the reference point. Are you with me? I told you that is not important for three phase. Half bridge it is required. Here it is required. This is the center. You have to split the entire DC link into two. This is the center point. Are you with me? And see the connection. I have to use two additional diodes. I have to use two additional diodes for a three level. As the level changes, power circuit changes, I will tell you. It looks bit scary, but then it is not, okay. At any given time, three level inverter, at any given time, N minus, this is the general rule, N minus 1 switches are on in each leg. At any given time, N minus 1 switches are on in each leg. In each leg, only one switch is on there, N minus 1. N is the number of levels. If N is equal to 3, 2 switches are on. That is S1, S2, either S1, S2 or S2, S3 or S3, S4. S3, S4, okay. S1, S2 on. What is VAX? X is the middle point. S1, S2 on. S1, S2 on. What is VAX? VAX is the entire DC link voltage. S1, S2 on. These two are on. This point, A is gets connected to plus VDC. So, VAX is, is, is VDC. S1, S2 on. So, voltage source inverter current as, current will be bi-directional. Positive current as well as, if the current is positive, it will flow through S1, S2. Are you with me? The current is negative. It will flow through, flow through D1, D2 and capacitor. Are you with me? The prevailing diodes of, prevailing diodes of S1, S2. S1, S2, S1, S2. So, voltage is VAX is VDC. S3, S4 on. Sorry, S2, S3 on. S2, S3 on. What is VAX? S2, S3 on. S2, S3 on. Just see this point, either it gets connected here or it gets connected here. It is a short. So, VAX is, start from here, capacitor. Now, current could be either positive or negative. It can flow in or flow out. The voltage source inverter. Look, look. Or if it is flowing in, do not worry, here is the circuit. Place it on. Positive direction of current, positive direction of current, negative direction of current. And last one is, last one is S3, S4 on. S3, S4 on. VAX is 0. VAX is 0. Positive direction of current is through, through D3, D4. Positive direction of current. Negative direction of current is through S3, S4. This is for three level. Number of capacitors required are N minus 1. N minus 1. Number of switches required are 2N minus 1. As the number of switches increase, look, as the number of levels increases, number of switches also will increase. So, four per phase, 2N minus 1. Number of diodes that are required, I do not want to give you right now. Maybe sometime later I will tell you. From here it looks as if there are only two. You will see whether it is two or something else we will see. Any questions? Number of diodes, look, I put a question mark. Four level inverter I will just show you. Then I will not discuss so in detail. Three level inverter, some issues, important issues I will discuss and I will go. Four level inverter. Four level, number of switch. At any given time N minus 1 switches are on. Now, see the connections here. Capacitor required are N minus 1. Are you with me? N minus 1. So, there are three capacitors and here number of devices are 2N minus 1. 2N minus 1. N is 4. So, N minus 1 is 3. 2 into 3 is 6. 6 devices and each device has a free wheeling diode across it. I have not shown for other diodes. See the connections. It is tricky. What you have to do is top one, this is the output point. This is the output point. Top one and immediately after one device from the output point, take two diodes, connect back to back. Sorry, connect in this fashion and it goes to top most capacitor. This is a rule to be followed as the number of level increases. Same rule. One here, immediately one diode connection. Common point goes to capacitor. Second diode goes to the second capacitor. This is a rule to be followed as the level increases. Three level it is obvious. Four level it looks bit tricky. As five level increases, it looks really confusing. But then you follow this rule. One, one, back, first capacitor, second, second, back to this point. Now we will see. I will just show you how it works. At any given term, three switches are on. S 1, S 2, S 3. What is the output voltage? S 1, S 2. V ix is V dc. You have much given here. S 1, S 2, S 3 on. Positive direction of current is through S 1, S 2, S 3 and negative direction is d1, d2, d3. d1, d2, d3. S 2, S 3, S 4 on. At any given time, three switches on. You turn off S 1 and turn on S 1 of S 4 close. Trace it. Simple. Start from here. Positive direction of current, negative direction of current. What is V ix? 2 by 3 V dc. 2 by 3 V dc. S 4, S 5 on. What is the output voltage? So, S 1, S 2 on. Sorry, S 1, S 2 off. S 3, S 4 on. So, voltage is positive direction of current, negative direction of current. V dc by 3. Now I will open S 3, close positive direction as well as negative direction. What are our observations? Four level inverter. Four level. In the sense, there are four levels. What are our observations? The number of levels is bit advance, but let us see here. What are the issues? That is required. What are the, see there are six devices. See here for maximum voltage. No, no, no. Maximum voltage S 1, S 2, S 3 on. That is if I in an envelope, see here, assume that this is the envelope. Maximum voltage that appears only for a short duration. I don't know. So, in this duration, maximum voltage, all the three upper switches are on. Are you with me? In this, only two are on. Which are two? Lower two. No, no, voltage average, see the duty cycle for the devices is not the same. Are you able to understand or no? Here all the upper switches, all three switches are on. What happens here? The topmost switch has gone off, but lower are continue to remain on. Here topmost two are gone on. Here, so if you see in this entire zone that this one, no, no, no, lower on this one. For one third, for one third, the device, which device is continuously on? S 3 is continuously on. In the second level, S 2 is on and S 1 is on for a very short duration, only in that peak. So, duty cycle is not the same, duty cycle is not the same, duty cycle is not the same. So, maybe you may have to really worry about, duty cycle is not the same, conduction period and all those things may be different. Is that okay? Voltage rating of each switch is the same. What about these diodes? Why did I put a question mark there? What is the voltage rating of dA? What voltage it has to block? dA has to block only one third of VDC. See when S 1 is closed, listen to me, when S 1 is closed, this point gets connected here. I know it is here, cathode is here. So, voltage rating of dA is one third. What is the voltage rating of dA dash? You close S 5, S 6, sometime in the, during the time S 5, S 6 are closed. Come on, sometimes S 5, S 6 are closed. When S 5, S 6 are closed, what is the voltage, what is, when S 5, S 6 are closed, this point gets connected to negative DC bus. This point gets connected to negative DC bus whereas, whereas anode is, sorry cathode is connected here. What is the voltage rating? Two third VDC, two third VDC. So, voltage rating, so you have a choice, you have a, now you need to make a choice. Use diodes of different voltage rating or diodes of same voltage rating, same voltage rating, the losses and what not, see inventory versus, inventory versus, the moment I use two diodes, what are the issues? See if all the diodes are of same voltage rating, I have to buy only one type of diodes and stock them. Ayana, now I have different diodes rating, I need to have different stock, inventory increases. Now to reduce that, you are using the diodes of same voltage, what price you are paying? Sir, what price are you paying? Forget, I am not talking about performance wise, no serious doubt, he is saying some issues. Now, can I mount them on the same heat sink? Heat sink body may be, anode may be the body. Ayana, you have to mount it on the heat sink. That may be either anode or a cathode. Potential of two anodes is not the same, so I do not think I can mount, same heat sink, you may have to separate them. Plus overall losses, you have to find out, high voltage diode versus two low voltage diodes. How the voltage rating has, how do you increase the voltage rating? I mean it not so much but then, how do you increase the voltage rating? It has to, doping level has to be increased or depth level has to come down. Doping level has to, they are lightly doped. The moment you lightly doped them, what happens to your T off, off time, that also has to turn off, no. It may be an uncontrolled device, it has to also turn off, whether T off will increase or decrease? Increase, okay, find out, fine. So, you have to, you have to pay a price for that and you have to decide that. So, this is a number of diodes is not, not here four. If you have four of course, they are, they are different current rating or voltage rating, sorry different voltage rating. If you want to use same diodes, the expression here is, number of diodes is N minus 1 into N minus 2, N minus 1 into N minus 2. Either you have to connect only one diode or a higher voltage rating or same diodes, voltage rating is not the same.