 We were discussing about the heart switching, since this what I said whenever I feel like or device is turned on or off depending upon the output of the controller, we are what is the current rating, sorry what is the current that is flowing this prior to switching or what will the current that will flow immediately after switching, it is those issues are not taken into account at all. So, what this is known as the heart switching, heart switching. So, d i by d t and d v by d t are very high. Now, what are the limitations? Device stress increases, stress on the device increases, switching losses increases and electromagnetic interference and see the locus of voltage and current, this how when you turn the device when the device is turned on, when the device is turned on heart switching, it takes a path like this and when the device is turned off current takes the path something like this it comes and takes the path. This additional increase in height could be because of because of the reverse recovery current that it has to carry or various other issues or this could be the voltage drop voltage drop across the parasitic inductances. Now, we can we can try to reduce the losses that are taking place in the device and transfer it to an external circuit by using a snubber. Let me tell you one thing, we can try to reduce the losses that are taking place in the device and transfer them to an external circuit by using a snubber. What do you mean by snubber? It snubs, it does not allow a very fast rise of current or it will not allow a fast rise of voltage across the device. So, if I use a passive components like L and R sorry L and C, it is possible to reduce the losses that are taking place in the in the device. So, but then these are these losses are transferred to the other circuit. So, overall losses may remain the same, there are there are various snubbers one is known as the lossless snubber, lossy snubber. I am talking about the lossy snubber wherein in addition to L and C there could be a diode, there could be a resistor as well. So, if I use a snubber you can control the rate of change of voltage as well as the current. Now, you just see this how the locus is, this is during turn on and during turn off. So, definitely there is a significant reduction in the losses taking place in the device, but then these losses have been transferred to the external circuit if it is a lossy snubber. A lossless snubber you might be able to fit them back, but then there are area under the curve is finite and therefore, the losses do take place. Now, instead can I use what is known as a soft switching, what soft switching implies that I will ensure that voltage across the device is 0 or it is very low only then I will apply a gate drive, I will turn it on or current flowing through the device will gradually starts from 0. What is these are known as the soft switch converter see the low kind of how voltage and current change, voltage has fallen to a very low value and there and current is increasing from there. So, losses are reduced significantly, if the losses are reduced significantly device stress comes down EMI also comes down. Now, if the device stress comes down switching losses have come down my cooling requirements have come down. In addition, another important result that is obtained is I can now increase the switching frequency to a higher value. Now, it is possible to increase the switching frequency in soft switch converter. So, in a soft switch converter of you in a fly back soft switch converter switching frequency could be of the order of 500 kilo hertz 500 kilo hertz it could be that high. So, this is precise the reason I referring to throw some figures when some when the audience ask me sir switching frequency I do not want to answer that question it all depends on how you switch. If you do the soft switching it may be possible to go as high as 500 kilo hertz, if you do hard switching you may not be able to switch at 50 kilo hertz. So, stress on the device comes down to reduce overall. Now, switching frequency I can increase. So, what do you invest see there are soft switching means there is there are two types one is a 0 current switching and a 0 voltage switching simple they can be represent something like this see I have connected a small inductor. Now, if I turn on see prior to closing the switch there was no current flowing here. Now, when I close this switch because of this inductor current has to gradually increase. So, this could be a 0 current switch I have just shown a small part of the circuit there are other parts of the circuit which are not drawn they are bit complicated I do not want to scare you now there is a reason I just put a inductor here a switch. So, this is 0 current similarly here I have connected a small capacitor. So, when the capacitor is completely discharged I will turn it on. So, we know the voltage across the capacitor cannot change instantaneously or when I will turn off the device when the capacitor is complete turn off means it was conducting to a 0 anyway now voltage across the capacitor cannot change instantaneously. So, it will gradually rise. So, this is known as the 0 voltage switched I am reducing the rate of rise of voltage here I am controlling the rate of rise of current through the device. Hence, the name soft switched losses are reduced significantly I can increase the switching frequency my size comes down EMI issues are addressed at what cost circuit complexity has increased circuit complexity has increased switch on of the device from the voltage across it and or or the current through the current through the current is 0. So, I said you switch on of the device when the voltage across it and or or current through it is 0 if it is only is when you switch on or off when the voltage across it is 0 it is known as a 0 voltage switched and if the current is 0 0 current switched and both of them are 0 it is 0 voltage 0 current switched 0 voltage 0 current or 0 voltage soft switched. So, I am how to use a resonant somehow now it is not going to be a steady current current has to gradually rise and fall there are L C circuit there will be a the moment I use L C there is going to be a resonance and that resonance frequency is very high I said in 5 if it is a switching frequency is 500 kilohertz the resonance frequency could be of that order as well. So, I have to you but then since the resonant frequency is very high the size of L C also is very small. So, power electron equipment which utilizes a resonant L C circuit as a part of the power conversion process L C is very high depending upon the power level it could be of the order of 300 to 500 kilohertz size of L C is very small. So, this is just the philosophy those who are interested there is enough literature available, but then in the present day where the efficiency is the criteria size again it should be as small as possible there was a question that what is the power density power density it should be as high as possible for a given size or the ratio of power to the size should be as high as possible one may have to use soft switching, but then it is slightly complex, but then it is not it is not impossible there are converters are available market if not in. So, I will just show you the waveforms here see the ringing I am applying the gate signal. So, this is the device current see the ringing here it is going this is because the circuit parasitic effect because of the diode recovery as well diode recovery current because that current has to diode recovery current current now flows through in the opposite direction and it has to flow through some device it may happen that other device some the when the store none the main switch may have to carry the diode recovery current see the voltage across the device and see the spike here. Now, the product is product is finite and see the rate of rise now see the soft switching current is gradually increasing almost like a sinusoidal voltage is also slowly falling the product is most 0 in the first case this is the in this region there are losses taking place and in the second case this is the losses are switching turn on and turn off losses are turn on and turn off losses are eliminated conduction losses are there. So, these are the advantages of short switching. So, advantages are EMI is addressed size is size yeah the footprint make the output will come down, but then there are additional L and C required complexities increase you may have to use an additional switch as well there are various issues device stress low EMI switching losses of 0 separate LSE required. Now, parasitic elements can be enhanced see all this time we said this is due to the parasitic element now that parasitic inductance I can use it for resonance try to understand these are all these are this is spike due to the parasitic element L or C now that I can use it to our advantage. So, parasitic enhance rather than it is possible to improve diode recovery as well I can complete it turn off only then so diode recovery can be handled very efficiently. Now, this all depends on the circuit topology and type of depends on the circuit topology and the type of control these advantages everything comes to the with the price circuit become complicated you might have slight peak currents might see this is what it is safe operating area without snubber see this is without snubber 1 and 2 without snubber during turn on say during turn off with snubber I am controlling. So, thereby losses in the devices are reduced they are transferred to some other part this is the soft switch that is enough.