 Hello everyone, welcome to this session. I am Dr. Asha Thalange and today we are going to learn turn off methods of SCR. At the end of this session you will be able to differentiate natural commutation and force commutation method and also explain class A and class B type commutation method. These are the contents we will be covering in this session. In the previous session we studied turn on and turn off mechanism of SCR. Turn off process of an SCR means bringing back the SCR from forward conducting state to forward blocking state. This turn on process of an SCR is called as commutation of SCR. Commutation is nothing but transferring the currents from one part to another. We know that in order to turn off the SCR following conditions must be satisfied. The anode or forward current of SCR must be reduced to zero or below the holding current and a sufficient reverse voltage must be applied across the SCR to turn in off. This is done so because even after the SCR is turned off by reducing forward current to zero, still there exist some charge carriers in different layers which keeps SCR turned on. To avoid this, these excess carriers must be recombined and this is done by applying a reverse voltage across the SCR. This reverse voltage which commutates the SCR is called commutation voltage. Based on the source of this commutation voltage, commutation methods also known as turn off methods are classified into two major types natural commutation and forced commutation. Let us see natural commutation. Figure shows the circuit and waveform for natural commutation. In natural commutation, the supply voltage itself is the source of commutation voltage. Natural commutation is used in AC applications. During the positive half cycle of the supply, before the gate pulse is applied, SCR is in forward blocking state and acts as a open switch. Thus, all the input voltage appears across the SCR and load voltage is zero. As soon as the gate pulse is applied to the SCR, SCR turns on and all the supply voltage now appears across the load. The moment supply voltage becomes negative during the negative half cycle, this negative voltage reverse biases the SCR and turns it off. Thus, all the supply voltage now appears across the SCR and load voltage becomes zero. As here, SCR turns off automatically due to the negative supply voltage and without using any additional circuit it is known as natural commutation. This method is also known as line commutation or source commutation. This type of commutation takes place in phase controlled rectifiers, AC voltage controllers and cyclo converters. Let us now see forced commutation. In DC applications, natural commutation is not possible. Thus, to turn off the SCR in such circuits, additional circuitry is used known as commutating circuit. This circuit forces the reverse voltage across the SCR and brings the anode current to zero. The commutating circuit consists of inductor and capacitor called as commutating components. Based on how the commutating components are connected in the circuit and anode current is made zero, forced commutations are classified into different types such as class A, B, C, D and E. Force commutation is generally used in chopper and inverter circuits. With this discussion, pause the video and think what is the major difference between natural commutation and forced commutation. Well, the major difference is that natural commutation occurs in AC circuits with no additional circuitry requirement for commutation, whereas forced commutation is used in DC circuits along with additional circuitry needed for commutation. Let us see class A commutation. It is also known as self commutation or resonant commutation or load commutation. In this commutation, the source of commutation voltage is in the load. The commutating components L and C are connected either parallel or in series with the load resistance R. Load resistance and commutating components form an under damped resonant circuit to produce natural zero. Figure shows the circuit and waveform for class A commutation. When the SCR is triggered, the forward current starts flowing through it and the capacitor charges in the direction as shown. Once the capacitor is fully charged, more than the supply voltage, the SCR becomes reverse biased. And after the turn of time, SCR turns off and acts as an open switch and all the supply voltage now appears across the SCR. The capacitor now discharges through the load resistance and makes the circuit ready for the next operation. The switchoff time of the SCR depends on the resonant frequency which in turn depends on L and C components. This method is simple and reliable. Let us now see class B commutation which is a self commutation by LC circuit. Figure shows the circuit and waveform for class B commutation. In this L and C LC resonant circuit is connected across the SCR and the L and C components do not carry load current. When the DC supply is applied to the circuit before the SCR is triggered, the capacitor charges to the supply voltage EDC with the upper plate positive and lower plate negative as shown. When the gate pulse is applied, SCR triggers and the current flows in two directions. One is through plus EDC, through SCR, through RL, two minus EDC and another is through L and C. Once the SCR turns on, capacitor starts discharging through C plus, L, SCR and C minus. When the capacitor is fully discharged, it charges in the opposite polarity as shown. This voltage across the capacitor makes the SCR reverse biased and causes the commutation current IC to flow through the SCR in opposite direction of the load current IL. When the commutating current IC becomes greater than the load current IL, reverse current flows through the SCR making the anode current to flow below zero and after the turnoff time, T of SCR turns off. The moment SCR turns off, capacitor again starts charging with the original polarity as shown. In this process, the SCR is turned on for some time and then automatically turned off after some time. If a periodic gate pulse is applied, then this process is continuous and on-off frequency depends on the values of L and C. Thus, in this session, we discussed natural commutation class A and class B type force commutation methods. These are the references you can refer. Thank you.