 Hello everyone, welcome to this session. I am Dr. Asha Tharangi and today we are going to learn turn off methods of SCR. At the end of this session, you will be able to explain the class C and class D type commutation method of SCR. These are the contents we will be covering in this session. In the previous session, we discussed natural commutation, forced commutation, class A and class B type commutation methods. Now, before moving ahead, pause the video and recall whether the forced commutation is possible in AC circuits only, AC and DC circuits, DC circuits only or none of this. The answer is DC circuits only. Let us now discuss class C type commutation method. It is also known as complementary commutation method. Figure shows the circuit and waveform for class C type commutation method. Here, SCR T1 is the main thyristor and is connected in series with the load RL. SCR T2 is the complementary thyristor connected in parallel with the main thyristor T1 and is used to commutate T1. The whole commutation process is explained in different modes of operation. Let us start with mode 0, which is the initial state. In this state, none of the SCRs are triggered. So, both the SCRs are in off state and acts as open switch. Due to this, there is no path available for the capacitor to charge and hence voltage across the capacitor Vc is 0. Thus, at the end of mode 0, T1 is off, T2 is off and Vc is equal to 0. Next is mode 1 state. In this state, T1 is triggered. As soon as a gate pulse is applied to SCR T1, it turns on and voltage across it falls to its on state voltage level. Also, current starts flowing through T1. Two current flows through the circuit namely the load current IL, which flows through the path Vdc plus RL T1 to Vdc minus and the charging current Ic, which flows through the path Vdc plus Rc plus C minus T1 to Vdc minus. Due to this charging current, capacitor gets fully charged to supply voltage Vdc with the polarity as shown. Thus, at the end of this mode, T1 is on, T2 is off and Vc is equals to Vdc. In mode 2, T2 is triggered. When a triggering pulse is applied to T2, T2 turns on and acts as a close switch. Voltage across T2 becomes on stage voltage. The negative polarity of the capacitor C is applied to the anode of T1 and positive polarity of C gets applied to the cathode of T1. This makes T1 reverse biased and the current through T1 now becomes less than the holding current. After the turn of time, T1 turns off. The capacitor now starts charging again through the load and its polarity gets reversed. The path of this charging current is Vdc plus RL C plus C minus T2 to Vdc minus. Hence, at the end of mode 2, T1 is off, T2 is on and Vc is equals to minus Vdc. In mode 3, T1 is triggered again. When a gate pulse is applied to T1, T1 turns on and voltage across T1 drops to on state voltage. Due to this, the voltage across the capacitor now gets applied across the T2 which makes it reverse biased and the current through T2 becomes less than the holding current and after the turn of time, T2 turns off. Thus, the current through T1 is again due to the load current IL and the charging current Ic. This charging current charges the capacitor in the polarity as shown. Therefore, at the end of mode 3, T1 is in on state, T2 is in off state and Vc is equal to Vdc. This is same as the end of mode 1 operation. Class C commutation is very useful at frequencies below 1000 hertz and is very reliable commutation method. It is used in McMurray-Bettford inverter. Let us now see class D type commutation method. It is also known as auxiliary commutation method. Figure shows the circuit and waveform for class D commutation method. In this, the auxiliary thyristor T2 is used to commutate the main thyristor T1. Thyristor T1 and load resistance RL form the power circuit which is the main circuit. Whereas, inductor, diode and SCR T2 form the commutation circuit. Let us see how it works. In the initial state that is mode 0, battery Vdc is connected but no current flows through the circuit as both the thyristors are off and diode is reverse biased. Thus, at the end of mode 0, T1 is in off state, T2 is in off state and Vc is equals to 0. In mode 1, T2 is triggered. Initially, before triggering the main thyristor T1, T2 is triggered here. This makes T2 to act as close switch. As soon as the T2 turns on, capacitor starts charging with the polarity as shown. This charging path is through Vdc plus, C plus, C minus, T2 RL to Vdc minus. As soon as the capacitor is fully charged to Vdc, the voltage across the capacitor decreases the current through T2 and also the equivalent potential difference across the T2 makes it reverse biased. Due to this, T2 turns off. Thus, at the end of mode 1, T1 is off, T2 is off and Vc is equals to Vdc. In mode 2, T1 is triggered. When a gate pulse is applied to T1, T1 turns on and voltage across it drops to on state voltage. Here, the current flows in two parts. Load current IL flows through Vdc plus T1 RL to Vdc minus and the commutation current flows due to the discharging of capacitor through C plus, T1, L, D to C minus. This makes the capacitor voltage to decrease and charge the capacitor with the opposite polarity. Once the capacitor is fully discharged with the upper plate negative and the lower plate positive, this voltage reverse biases the diode and hence, reverse discharge of capacitor is not possible. The current through T1 is now just the load current. Hence, at the end of mode 2, T1 is on, T2 is off and Vc is equals to minus Vdc. In mode 3, T2 is triggered again. When a gate pulse is applied to T2, T2 turns on and voltage across it drops to on state voltage. This makes the negative voltage of the capacitor to appear across the T1. The capacitor now immediately discharges through T2 and T1. This discharging current passes through T2, the capacitor now charges again in the opposite direction with the upper plate positive and lower plate negative. This makes the load current IL to reduce. When the discharging current or commutation current Ic becomes more than the load current IL, current through T1 becomes less than 0. Thus, after its turn off time, T1 turns off. Therefore, at the end of mode 3, T1 is off and T2 is on. Now, the capacitor keeps charging through T2 and when the capacitor is fully charged to Vdc, SCR T2 turns off. This is same as in mode 1 operation. Thus, now T1 is off, T2 is off and Vc is equals to Vdc. This type of commutation method is used in Jones DC chopper circuit. Now, with this discussion on class C and class D type commutation method, pause the video, think and answer the following. Which of the commutation method is used in DC chopper circuits? Your options are class A, class B, class C or class D. The answer is class D. These are the references you can refer. Thank you.