 output voltage of the solar cells is dc, again is a variable dc, not a fixed dc, is not it? Variable dc, variable dc, bulk of the one way is to use directly charge the battery may be fine, depending upon the solar size, sorry size of the voltage rating the panel and the requirement, I may have to process this dc, depending upon the power level, depending upon the power level, depending upon the voltage level, the type of a dc to dc converter that is required will change, are you with me, dc to dc converter. So, let us see what are the possible possible topologies, most of them you may know and what application, which one, how do we use the selection of a proper topology, depending upon the power level. So, broadly these are divided into two groups, one is with transformer, another one is without transformer and with transformer, dc to dc with transformer, we are till in the in the transformer class that we should not give a very popular question in the why why you ask, what happens if I give dc to dc to a transformer, now tell me, let me tell you one thing, if I do not use a transformer in a dc to dc converter, I am telling you the size of the equipment would not have come down to this level, only thing is of course, I can use a transformer in a dc to dc, but I should not allow it to, I should not allow it to saturate, I should not allow it to saturate, that is all, the frequency of operation is fairly high. So, as frequency increases, n comes down, core losses increases. So, use a, see the problem use a ferrite core, the moment I use a ferrite core, it is going to be brittle, brittle, brittle, there are other cores are also and again BR is very low, the flux density, operating flux density operating flux density of a ferrite core is very low, could be of the order of 0.2 to 0.25 tesla, 0.25 to 0.25 tesla, so B average is low, B average is low. So, MR first alloy available, B average, this could be of the order of 1 to 1.1 tesla. So, if you want to use a good transfer rate of the size, you have to go in for MR first alloy, boost converter, that is what in the beginning of my talk, I told that either I have a low voltage solar panel, my application requires 230 volts line phase or 240, 440 volts, 440 volts line to line. I have a question to ask, output voltage here is suppose, this is equivalent circuit of a, equivalent circuit of a, what this could be? Sir, say something when come on, half bridge, half bridge, of course, this may not be the ground exactly. One AC source, I have shown only one leg of an inverter, one leg of an inverter, AC source here, inductor is a grid, are you with me? The inverter feeding power to the grid, equivalent, some equivalent circuit. Assuming that here is, I require 230 volts phase voltage or 440 line to line, 440 line, what is the DC link that is required for a 3 phase inverter? What is the DC link voltage that is required for 3 phase inverter? What is the DC link voltage that is required for a 3 phase inverter feeding power to the grid? What is the DC link that is required? What is the DC link voltage that is required for a, for a, I get a 3 phase AC here, I get a 3 phase AC, I have to, through an inductor, I have to connect it to the grid. If the voltage is, grid voltage is 440, line to line, what is the DC link that is required? What is the DC link required? What is the order of DC link? Order, around may be, if it is 440, this link is good, the order of 750 volts, could be of the order of 750 volts, minimum, depending upon the wave shaping, it could be higher than 750 also. How would it be, how this figure is around? We will discuss it later, we will discuss, because grid connection, I will emphasize on this sometime later. 750 volts, assuming that it is 750, I cannot have a 750 volts panel there, are you with me? I may have a, I may have a low voltage solar panel. In other way, we say that whatever the solar panel, low voltage, you invert it, use a transformer and feed it. The moment I use a transformer, there is a problem, size, cost, bulky, noisy, everything. So, this voltage I have to increase it to, this voltage I have to increase it to 750 volts or so, 750 volts or so. Assuming that I have a low voltage DC panel, what do I do? What do I do? I have to use, somehow I have to boost the DC voltage. Now, the configurations and all will change, let me tell you one thing you might, might have heard about it, there are various other issues, a very important circuit one has to understand, boost converter circuit, apology, principle of operation is fairly simple. Close the switch, close the switch, what is the equivalent circuit? When I close the switch, what is the equivalent circuit? Inductor is connected to DC source, at the output stage, capacitor is connected to the load. So, you store the energy in the inductor, open the switch, open the switch, stored energy in the inductor is transferred to the capacitor, transferred to the capacitor. So, if I derive the transfer function, what is the condition? Average voltage across the inductor should be 0, are you with me? Average voltage is, when it is, when the switch is closed, it is VDC, which is positive current increases. When the switch is open, the switch is open, voltage across the inductor should be, when the switch is closed, voltage across the inductor is VDC and it is positive, current increases. Are you with me? When the switch is closed, current increases, when I open the switch, current should decrease. So, voltage across the inductor should be negative, voltage across the inductor should be negative. So, when this voltage will be negative? When this is at a higher potential than this, this is the equal circuit when the switch is opened. So, voltage across the inductor is negative when this potential is higher than this. So, the circuit to work satisfactorily V naught output voltage must be higher than V input, must be higher than the E input that is the condition. So, you equate it, I do not want to get down to this mathematics average voltage of the inductor is 0, you will get a transfer function which says that V dc is equal to 1 minus d, V dc divided by 1 minus d. So, if I plot it when d tends to 1, V naught tends to, V naught tends to infinity, is it okay? V naught tends to infinity. Now, see the equivalent circuit, sorry see the circuit operation, that is we have not committed any mistake, analysis is perfectly okay, V dc, V naught is equal to V dc divided by 1 minus d, okay, d tends to 1, V naught tends to infinity. What happens here? This is equal to d naught, d tends to 1, this is the equivalent circuit, most of the time, most of the time switch is closed, most of the time switch is closed, most of the time switch is closed. So, most of the time switch is closed. So, dc is permanently connected, sorry source is permanently connected to inductor, inductor is permanently across a inductor. So, what will happen at the input side? What will happen to the input side? dc is permanently connected or again popular question in the undergraduate, we are connecting a dc to the inductor, current will increase linearly, therefore flux also will increase linearly, beyond a point it will get saturated, the moment it gets saturated, what happens? The moment it gets saturated, current is limited by its resistance, what is the steady state current here? V by R, steady state current is V by R. At the output stage, what happens? Capacitor is permanently connected across the load, output voltage tends to 0, output voltage tends to 0, output voltage tends to 0, why there is such a significant difference? Ideal condition, we have not done any mistakes perfectly, only thing is we have not taken the resistance into account, because this equation says that V L is equal to V dc, V L is equal to V dc when the switch is closed, only thing we ignored is V dc is equal to L df by dt plus plus plus R into i, that is all we have done, that is the small mistake that we have committed. So, but then why there is such a such a huge difference? Output voltage tends to 0, output voltage tends to infinity, see efficiency of ideal Carnot engine and efficiency of practical Carnot engine, ideal may be 100, but non ideal is not 0. So, why it is so? Where is our mistake? Where have you gone wrong? Of course, internal resistance you have to take into account, fine. The basic assumption that we have made here, V0 and Vdc are constant and triple feet, they are not valid. Can I assume V0 will remain constant in this case? Can I assume? No, if I, if I permanently if d is closed, switch is closed for most of the time, here V0 tends to, V0 tends to 0, V0 tends to 0, V0 tends to 0, maximum boost that I can get here is of the order of 7 to 10, 10 is the maximum, we cannot go above that. Your currents are very high, currents are very high, see here what happens here is to boost it, I have to close most of the time, are you with me? So, the moment I close most of the time here current peak current increases, increases, increases. So, maximum ratio that I can get is of the order of, of the order of 7 to 10, 7 to 10. Now, assume that I want to, you have to, you want a 230 volt AC, 230 volts AC. So, for that input voltage, if I have to boost it, input voltage itself could be of the order of you require around 350 volts. If I want a 230 volt RMS AC, inverter input required input is of the order of 350 to 400 volts. Now, as of now you take it, I will tell you how did I arrive at this figure, are you with me? For 230 volt RMS, I may require around 350 to 400. Assume that I have a 48 volt panel, maximum ratio I can get is maybe 7 to 10, a thumb rule, what do I do? I said input DC link voltage, see now we are talking about a miniaturization, 50 or transommer I cannot use it. So, I have to use, I have to boost, I have to boost, no, I have to boost the DC voltage. Somehow my DC link voltage should be of the order of 350 to 400 volts. I have a 48 volt panel or a 24 volt panel. So, assuming that I have a 48 volt panel, the ratio is of the order of 10. Again, I am hitting the limit there, what do I do? We require the DC link voltage is of the order of 450, 350 for that figure, do not? Cascade, see the moment I use cascade, the overall efficiency is again, see component count increases, component count increases, stages N1, NETA 1 into NETA 2, thought of issues, component count, what do I do sir? Increase the voltage level, go for higher panel, no, no, come on, what do I do? We may single stage with, see these are, I said there are two classes, one is, one is, one is with transformer and other without transformer. Without transformer, we cannot have or this, this topologies various buck, buck boost or a boost topologies can be used only if, only if the ratio of input and output is not very high. I cannot use a buck converter if the output voltage, output voltage that I want is very low compared to the input. I cannot use it, I cannot, there are various issues, are you with me? I cannot use it. Similarly, if I want to have a very high voltage here, I cannot use this boost, cannot use this boost, cannot use this boost. Now, can I use a transformer? Now, use a transformer then there is a various topology buck boost, quick converter, see what I am saying is not a very popular topology. If I have to reduce the capacitor size, can I use a quick converter? Quick converter, yeah, I have developed a simple theory, I have an inductor, I close this, close the switch, current in the inductor increases linearly. Can I open the switch? Sir, can I open the switch? I cannot open the switch, I have to provide a path for the inductor current. So, what do I do? Freewheeling diode, the moment I fit a freewheeling diode, what happens? Stored energy in the inductor is dissipated as heat with the internal resistance, are you with me? So, stored energy here is dissipated as heat. My question is, can I recover the energy? Can I recover the energy? Can I recover the energy? The question is, can I recover the energy? Yeah, no, no, no, capacitor 9. I will go back to a transformer, 50 hertz transformer. I have a transformer here, how do we place these dots? How do we place these dots? What is the dots convey? What are they? What do we, the moment I am showing a mutually coupled circuit 2 dots, what does it imply? Polarity, these two terminals are of the same polarity. If this is positive, this is positive. So, assume for me, in the sense when, so either to transform or when primary and secondary can carry the current at the same time, remember, in a conventional transformer what happens? Primary is connected to the source, secondary is connected to the load. If it is open circuit, of course, there is no current only the magnetizing current. Now, let me not discuss that. Assuming, so both the coils are carrying current simultaneously, are you with me? So, at that time when the current enters the dot, current enters the dot, what happens in the secondary? Current leaves the dot, is that okay? Current enters the dot, current leaves the dot, provided both the coils are carrying current simultaneously. Aayana, now what I will do is consider a case here, consider a case here. I have modified the circuit, the transformer, high frequency transformer, primary is connected to DC source. I will close the switch, current enters the dot. Aayana, current enters the dot. The natural direction should be current, current leaves the dot. Can current leave the dot? Current cannot leave the dot because of the diode. Are you with me? Current cannot leave the dot because of the diode. So, therefore, when the primary is carrying current, there is no current in the secondary. So, is the DC current, current in the flux in the core increases linearly. The core increases linearly. Can I open the switch now? Can I open the switch? Can I open the switch? If I have to open the switch, what should happen? Flux in the core should be continuous. Flux in the core should be continuous. So, what should be, if both the coils are carrying current, if one coil is carrying current at a time, what should be the direction of current? I am telling you, both the coils are carrying current simultaneously. Current enters the dot, current leaves the dot. So, the flux produced by I2 opposes the flux produced by I1 because both of them are carrying current simultaneously. Are you with me? Now, only one coil is carrying current. Only one coil is carrying current. I am not allowing the current. When the current is entering the dot, no current enters here. So, what happens when or what should be or what should be the direction of current in the secondary when the switch is opened? What should be the direction of flux and why and why? Why it is here they are opposing? See here, I1 flux produced by I1 flux produced by I2 opposing. Here, if only one coil is carrying current at a time, what should be the direction of flux? When only one coil is carrying current at a time, if current enters the dot, current should enter the dot here. If current leaves the dot, current should leave the dot, current should leave the dot. So, that flux produced by I1 should be the same or direction of flux produced by I1 should be same as that of direction of flux produced by I2. So, what happens is when I close the switch, current in the coil increases, the primary increases linearly. When I open the switch, sorry, when I close the switch, current enter the dot. So, when I open the switch in the secondary, in the secondary, current should enter the dot, current should enter the dot. So, diode connections are very important. What could be the transfer function? What philosophy can I use? What principle will I use? What should be the direction of, what is the transfer function? What is the transfer function? In boost, we said the average voltage across the inductor is 0. Come on, average voltage across the inductor is 0. What principle will I use it here? What is the transfer function? May I use the transformer now? What could be the transfer, what could be the transfer function? When I close the switch, flux in the core increases. When I open the switch, flux in the core decreases. What will I use? What principle will I use? In boost, average voltage across the inductor is 0. What will I use it here? Average flux linkage is 0. What is that? Defy rate of change of flux in the primary, volt turn, volt second per turn. What does that mean? Volt second per turn. What is it? Volt second per turn. Sir, what is volt second per turn? V is equal to N d phi by dt. So, this is volt V into dt divided by N is equal to d phi d phi. So, if there is a volt second per turn balance in the primary and secondary, that should be the condition, volt second per turn balance. So, flux rate of change of flux is equal to rate of, rate of change d phi d phi d phi. So, you equate it, you equate it in the primary and secondary. Now, you will get the transfer function as you can derive. I do not want to transfer function as V dc into N 2 by N 1 d divided by 1 minus d. This is the transfer function. V dc into N 2 divided by N 1 into d divided by 1 minus d, 1 minus d, 1 minus d. So, d divided by 1 minus d. What is sort of a transfer function is this? See, same thing. I have derived increase in flux, decrease in flux, various theory that you can go. V naught is equal to V dc divided by N 2 divided by N 1 divided by 1 minus d. This is what, what is this? Bug boost. For d less than 5, for d less than 5, V naught is less than, come on. d less than 5, if N 2 by N 1 is 1, if N 2 by N 1 is 1, N 2 by N 1 is 1. If N 2 by N 1 is 1, for d less than 5, 0.5, output voltage is less than input, for d greater than 0.5, output voltage is higher than input. So, is it the isolated bug boost, isolated bug boost, isolated bug boost. See, same problem you have, d tending to 1. What is the output voltage? d tending to 1. What is the output voltage? Infinity. See, we have a problem. Whatever the problem that we had in boost converter, same problem we have here also. That problem exists in bug boost. That problem exists in isolated also. You cannot have high values of d here. The moment you have high values of d, it will get saturated. Transformer is permanently connected to DC. It may get saturated. It may get saturated. It may get saturated. So, now I can choose suitable ratio N 2 by N 1. So, input voltage is low. You require a high output voltage. You can choose accordingly. So, basically it is a flyback converter. If your inverter rating is of the order of 75 watts, say not more than 100, 150 watts, you can use this. This is not suitable for, not suitable for, say, not more than 200 watts or so. Suitable for low power inverter, low power inverter, sorry, low power application, low power application, low power application. If I want to increase the power further, what do I do? If I want to increase the power further, what do I do? See, you cannot use this. See, topologies change as the power rating. This is suitable for, I told you here, written here, very popular up to 200 watts. About 200 watts, it is not popular. This, are you with me? I want a low voltage, 200 watts, I may be able to use. If I have a slightly, say, around 500 watt, 500 watt solar panel, what topologies to use? Why, what are we doing here? What are we doing here? We might have used a transformer, but the moment I use a transformer, one thing that comes to my mind is, both the coils should carry current simultaneously. See here, I am using the transformer only. I am using it as an inductor, storing the energy, dumping it here. Both the coils are not carrying current simultaneously. Trans transformer means it should carry simultaneously. Moreover, it is only DC excitation. So, in the BH loop, my operation is in the first quadrant. If I use AC excitation, it is a full cycle, full cycle, full cycle. Here, I am using it as when primary is carrying current, secondary is not carrying current. So, that is the reason, it is suitable only for low power. So, if I want to increase the power further, which one should I use? Slowly, go back, forward converter. Do not worry about this connection. We will see. Look at this circuit. Look at this circuit. Do not look for the third connection, N 1 and 2. See that, see, have you seen the circuit before? What is this? Buck converter, inductor, capacitor, load, diode. Here, there is a switch. Are you with me? So, basically, this is a buck converter, I guess. Now, what is this? There is a transformer, high frequency transformer. One like, no, no, no, no. So, fly back. Close the switch. Current enters the dot. Are you with me? Current enters the dot. Current can? Leave the dot. Current can leave the dot. Isn't it? So, now, both the coils are carrying current simultaneously. So, what is the current that is flowing in? What is I 1 now? What is I 1? In the previous case, only one coil was carrying current. I 1 was, is, is the magnetizing current alone? I have a transformer, fly back, low power, when the only one coil was carrying current, only one coil was carrying current. Secondary is open. So, primary current is the no load current itself. It is the magnetizing current. Hi and ah. Here, both the coils are carrying current. So, I 1 is, I 1 is magnetizing current plus, plus the secondary current, plus the secondary current, plus the secondary current, plus the secondary current. So, therefore, I cannot, I do not think I can use the same transformer here. So, my cross-sectional area of the conductor should be, see the, all the design issues you need to take. You should not say device, my equipment is not working. See, all of you are asking that you need to set up a lab there. Arrive with me. Lab, there will be converters, inverters. Now, things are going to be bit more difficult than managing the only street lights or the geyser. Arrive with me. Inverters will be there. DC to DC converter will be there. Storage oscilloscopes will be there. One of the problems that you will face is use of storage oscilloscope. Arrive with me. Use of oscilloscope in power electric circuit is extremely important. Very, a small mistake here and there, it will damage your control circuit as well as the, as well as the, as well as your oscilloscope, which is not the case in electronics, digital and analog electronics. There is only one ground. Here, there is grounds or reference point itself is changing, floating grounds. I will tell you sometime later. So, these are basic principles you need to understand. If you want to set up a lab there, you need to set up a lab there. What precaution do you need to take care? Yes, you will be able to do only when you understand these concepts. So, cross-sectional area of the conductor is high here. Now, I open the switch. See, but the primary current is magnetizing current plus, plus secondary current. Now, secondary, when I open the switch, what should happen? Secondary current, whatever that are flowing here, will continue to flow through freewheeling diode. Flux in the core should be continuous. There was some flux, magnetizing current was increasing linearly. Therefore, flux increased linearly. It has reached some value. Now, you want to open the switch. What should happen? Flux, you have to provide a path. Provide a path. You follow what I am saying. You have to provide a path for the flux in the core. Flux must be continuous. Flux must be continuous. What will you do? Flux must be continuous. I do not know why you are saying provide a one-guide looking at it or... What did you do here? Only one coil is carrying current at a time. So, logically, you will be, there is a reason I told you, you think logically, take a paper and pencil, you will hit the jackpot, you will get an answer. What did we do here? Only one coil is carrying current. We provided a path for the magnetizing current. When I close the switch, current here is only the magnetizing current, because secondary is zero. When I opened the switch, I allowed the current to flow in a second winding, second winding. What should be the direction of current there? If current enters the dot, current enters the dot. If current enters the dot, current enters the dot. When two coils are not carrying currents at the same time, if they are carrying currents at the same time, current enters the dot, current should leave the dot. If one coil is carrying current at a time, if current enters the dot, current should enter the dot when I open the switch. So, this we know. Now, I have to combine it. That is all. How to combine it? Did you follow or no? Is that clear? No. Sir, third one. So, this is worked. So, current enters the dot, current should enter the dot. This is nothing but the magnetizing and all that. So, here when I close the switch, current enters the dot, current enters, current can leave the dot. So, now, I2 is finite. This is nothing but our conventional transformer. Whatever the current that was flowing here, I2, it will continue. I will provide a path here. But I have not done anything for the magnetizing energy. But I know how to take care. So, I will have one more winding. I will have one more winding. I will have one more winding. But in that winding, current should flow only when, only when switch is opened, only when the switch is opened, only when the switch is opened. Here, I have connected to the same source, connected to the same source. Else, what I can do is, what else I can do is, see I had, what I can do is, in the third winding, what should happen? Current enters the dot, current should enter the dot. No, that is why I am storing energy in the capacitor, I can have some load there. That is not a problem. Do not worry. Where capacitor getting short circuited? No, I will not. What is this? This winding is, when current enters the dot, if this is a transformer, current enters the dot, current should leave the dot, not possible, not possible. Aayana, current enters the dot, current? Leave the dot. Leave the dot, yes. So, this current is now magnetizing current plus, I2 prime, I2 prime. Now, I2 prime when I secondary is open, so load load whatever the current is flowing in it circulate here, but magnetizing current has to be taken care. So, this is nothing but, when current, if current enters the dot, current can enter the dot, current can enter the dot. So, this is basically, what is this? The previous circuit that you have studied. I do not need to do all this. Instead, see, instead of charging the capacitor, what I can do? I can feed it back to the source, make sense or not? If I can store, if I can dump this magnetizing energy in a capacitor, cannot I feed it back to the source? Do not ask me how? Is it possible or no? Possible. That is what we can do. Instead of having a separate capacitor, feed it to the source. Current enters the dot, current can enter the dot. So, this you can use it till, say 500 watts or so. Suppose, if I want to increase it further, 1 kilowatt, what do I do? Again here, operation is still in the first quadrant, BH loop. Are you with me? So, what do I do? If I have to increase the power further, 1 kilowatt solar panel I have, what do I do? Sorry, we did not derive the transfer. What could be the transfer function here? What could be the transfer function? When the switch is opened, what is the voltage here? What is the voltage here? When the switch is, sorry, when the switch is closed, what is the voltage here? Turn's ratio into the input voltage. Are you with me? When the switch is off, when the switch is off, what is the voltage is? 0. So, this is nothing but a buck or naught of the forcing function of input voltage into turns ratio. So, N2 by N1 into VDC, this is okay for 500 watts or so. Suppose, if I have a 1 kilowatt panel, I have to use, I have to use the both the entire BH loop, AC excitation of the similar. What I will do is, what I will do is, see here, please try to understand, what am I doing here? This, what is this? You have seen this circuit. Gentlemen, please do not copy, do not try to understand. What is this? Centretrapped? Centretrapped transformer. What are you doing? Centretrapped transformer. It is a full wave rectifier. It is a full wave rectifier, full wave rectifier. So, what am I doing? This DC, see you people, I do not, sorry, should not be saying that, whether you have given thought over it or not. What am I doing here is, I have a DC, DC supply with a Centretrapped transformer. I will convert it into AC, high frequency AC. I will convert a high frequency AC, then rectify it, then filter it and it use it. I do not want to discuss the principle of operation. If you want, I can, I do not mind. Choice is yours. You tell me after 2 minutes, before that I will tell. I am doing, what I am doing is, only one switch is closed at a time. Same principle of operation, dots are there, you can analyze. Close the switch, close the switches, convert it into AC2, AC2, high frequency, sorry, this DC to high frequency AC. We had a choice, I will make this DC to AC, 50 hertz AC, then I will use, step it up. You follow what I am saying? Step it up, use a transformer, 50 hertz transformer, 50 hertz transformer there, 50 hertz transformer there. Instead what I am doing here is, doing here is high frequency, size the transformer is now very small, I have used rectify it till 1 kilowatt, it can use this. There are few kilowatts. If I want to increase the power further, what do we do? The concept is, somehow we have to, all of us are now agreed that I have to do, how to do? I have to use a DC to DC conversion, bring the voltage to a suitable value, invert it and do whatever in, feed it to the grid. Do not use 50 hertz transformer, assuming that there is no isolation required. Now you tell me, I want a constant DC at the end of the day, full wave rectification, is not it? It is a full wave rectification. Now I have a question to ask, in which size, in which case 1, 2, 3, the capacitors, size of the capacitor is small. I want a constant DC, I want a constant DC, I wanted a constant DC in that transformer. What do we do? In which case I need a smaller capacitor? Assuming that, of course, I do not want this, there is a load here, there is a load here, there is a load here. So, no, capacitor charge will get by game, chip chop. I want a load disconnected across the capacitor in all the three cases, in which case size of the capacitor is very low. It is quick, I have to wind up another 5 minutes time. I will just give the crux of the problem and I will leave you. Here more number of pulses. Here what are the capacitor is charged only for? I do not know, it is a very short duration, very short duration. In very short duration in one cycle, here twice in positive half also, negative half also, very for short duration. So, size of the capacitor is lowest here, it goes on gradually, gradually reducing. Now you tell me, I will ask another question. I said, what is the DC to AC is an inverter, full wave rectify, full wave rectify, full wave rectify, single phase full wave rectification. And I want a constant DC, I have filtered it. I want to reduce the size of the capacitor still further, because miniaturization is the theme. What do I do? I have to do high frequency conversion to DC. DC to AC, high frequency inversion has to be there. I want to reduce the size further. Is that again? No, no, you cannot increase the switching frequency beyond a certain value. It depends on what? See every time you switch, there are losses, fine, one switching losses. So, yes MOSFET may be a fast device, but then it all depends on the input voltage and the power rating. As the power rating increases switching frequency, it comes down. You have to reduce the switching frequency. You have to reduce the switching frequency. You cannot go on increasing the switching frequency. It depends on the power level, voltage level. What do I do? What could be the possible? I gave an example also. That was one, the full wave rectification. Now I want to reduce it further, because miniaturization is the theme. Somehow I want to reduce the size. What do we do? See, tell me. Here what are you doing? What am I doing here? No, no. I have this sort of a pulse. I have rectified it and I am getting something like that. Now instead what I will do is, number level should be the same. What I will do is, as this gentleman said, multiphase, multiphase and multilevel, multilevel, multilevel. You might have heard multilevel inverter, multilevel inverter. There are certain lot of advantages of multilevel inverter over conventional to level inverter. Maybe I do not have something, but not very clear. I will discuss this sometime later. But somehow I have to reduce the, because let me cover this topic then I will come back. Somehow I have to reduce the filter size requirement. I gave an example for half wave, full wave, three phase. You told me that minimum requirement, filter requirement is three phase. How to reach there? What are the possible ways? I told you this is suitable for 1 kilowatt, 1 kilowatt or so, 1 to 2, few kilowatt. But high power, our thought process has to change. What is the possible DC to DC converter that is useful? I will tell you maybe when I do DC to AC, then I have to come back. You follow what I am saying? Why am I saying that? Because there is a, there is a, though it is a DC to DC converter, I am saying we are, we are converting this DC to AC, AC. So, I need to have a rough idea about what are the possible ways of or what are the efficient ways of converting DC to AC, then can I combine? See that is all the research is all about. I know what is the best way of inverting DC to AC. Can I use it in DC to DC? Any way, any way, I have to use a transformer and therefore, thereby reducing the size of the transformer. The thought process will come back, revisit this topic sometime, later when I finish, when I cover DC to AC. If you have any questions, feel free, just 2 minutes. So, depending upon the power level, voltage level, your input stage has to change, it has to change. The tutorial process we will see how to select the proper size of transformer turns ratio, vector size we will see. But then, no, no, no, the name, let us not get stuck. Now, T, why the first one known as a fly back? This is known as fly back. What are my various, like stored energy here is being transferred there, stored energy here transferred to the secondary. Does it work as a fly? So, there are names have been generated for various reasons. There are, I will tell you, I will show you the textbook and I will give you, maybe you read yourself and get convinced. I do not want to say. So, forward here may be because primary and secondary may be, may be carrying current simultaneously. I had a question here, why fly back? See gentlemen, you have, we have no choice, but to step up DC for reducing the size. Do you agree? Do you agree? You have to step up DC. There is no way you can do, one way, one choice you have to connect boost in cascade.