 Hello everyone, welcome to this session. I am Dr. Asha Tharange and today we are going to learn switching characteristics of SCR. At the end of this session you will be able to describe turn on and turn off switching characteristics of SCR. These are the contents we will be covering in this session. In the previous session we discussed VI characteristics of SCR. This characteristics shows relationship between the voltage across the device and the current flowing through it during off-state and on-state. Now looking at the figure can you tell whether this characteristics gives information about the switching speed of SCR, that is from off-state to on-state and vice versa. Pause the video and think about it. The answer is no. This VI characteristics is static characteristics and it gives no information about the speed at which the SCR switches from forward blocking state to conducting state and vice versa. However, the transition from one state to another does not take place immediately. It takes a finite period of time to turn on and off. During this turn on and turn off process different voltage appears across the SCR and different current flows through it. The time variations of voltage across it with the current through SCR during turn on and turn off constitute the switching characteristics of SCR. Figure shows the switching characteristics of SCR. Let us see turn on and turn off characteristics in detail. When a positive gate signal is applied to forward biased SCR it changes its state from forward blocking state to conducting state and this process is known as turn on mechanism. This time taken by the SCR during turn on mechanism is known as turn on time. The complete turn on process of SCR with respect to time is represented in its dynamic turn on switching characteristics. Total turn on time is made up of three periods that is delay time, rise time and spread time. Figure shows the turn on characteristics of SCR. Delay time, rise time and spread time are defined in terms of the waveforms of the anode voltage and current obtained in a circuit in which the anode load consists of a pure resistance. Figure shows the gate voltage applied to the SCR. This figure shows the anode voltage across the SCR and the gate current through SCR with respect to time. The initial anode voltage across the SCR is VA. This figure shows the flow of anode current through the SCR with respect to time. Before applying the gate pulse SCR is in off state or forward blocking state and the current flowing through it is a very small forward leakage current as shown. Also IAE is the maximum anode current or load current which flows through the SCR when it is in conducting state and is controlled by the external load resistance connected in the circuit. This figure shows the power dissipation occurring in the SCR during the turn on mechanism. Let us see delay time TD. When the gate current reaches 90% of its maximum current IG, the anode current starts increasing and reaches to 10% of maximum anode current. This time taken for the anode current to reach 0.1 IAE is delay time given by TD. This can also be defined as the time during which the anode voltage falls from VA to 0.9 VA. The gate current has non-uniform distribution of current density over the cathode surface due to the P layer. Gate current is much higher near the gate terminal but it decreases rapidly as the distance from gate increases. Due to this during TD small amount of anode current flows in a narrow region near the gate where gate current density is the highest. Let us now see rise time TR. This is the time required for forward blocking of state voltage to fall from 0.9 VA to 0.1 VA that is 90% to 10% of its initial value. It can also be defined as the time required for anode current to rise from 10% to 90% of its final value IAE. This time is inversely proportional to the magnitude of gate current and its build up rate. Thus TR can be minimized if high and steep current pulses are applied at gate. For series RL circuit TR is more and for series RC circuit TR is less. Also during rise time turn on losses are the highest due to high anode voltage and large anode current occurring together in the SCR. Let us now see spread time TP. Spread time is the time required for the forward blocking voltage to fall from 10% of its initial value VA to on state voltage drop. It can also be defined as the time required for the anode current to rise from 90% to 100% of its final value IAE. After the spread time anode current attains steady state value and the voltage drop across the SCR is equal to the on state voltage drop of the order of 1 to 1.5 volts depending on the SCR's rating. Thus the total turn on time T on is the sum of delay time, rise time and spread time. This is typically of the order of 1 to 4 microseconds and depends upon the anode circuit parameters and the gate signal waveform. The width of the firing pulse should therefore be more than 10 microsecond preferably in the range of 2200 microseconds. The amplitude of the gate pulse should be 3 to 5 times the minimum gate current required to trigger the SCR. The high instantaneous power dissipation during the rise time due to SCR's large forward voltage and current may create local internal hotspots which can destroy the device. This can be avoided by inserting a small inductor called DI by DT inductor in the anode circuit to limit the rate of rise of current. Also as this power loss takes place during switching time this loss may be more in high frequency applications. Once the SCR turns on completely it then remains in a steady state condition. Figure shows the complete dynamic switching characteristics of SCR. We studied turn on mechanism. Let us now see turn off mechanism of SCR. Turn off of an SCR means bringing SCR from conducting state to blocking state. In order to turn off SCR two things must be done. First reduce the anode current below its holding current level and second apply reverse voltage. Figure shows the waveform of anode voltage across the SCR and the anode current through the SCR. Process of turning off is called as commutation. Various methods are used for turning off SCR. Once the SCR starts conducting sufficient forward current gate has no control on it and the device can be brought back to the blocking state only by reducing the forward current to a level below that of the holding current. But if a forward voltage is applied immediately after reducing the anode current to 0 it again starts conducting without even providing the gate pulse. It is therefore necessary to keep the device reverse biased for a finite period before a forward anode voltage can be reapplied. Thus the turn off time is defined as the minimum time interval between the instant at which the anode current becomes 0 and the instant at which the device is capable of blocking the forward voltage. The total turn off time is divided into two time intervals the reverse recovery time TRR and the gate recovery time TGR. At instant T1 the anode forward current becomes 0. During the reverse recovery time T1 to T3 the anode current flows in the reverse direction. At the instant T2 a reverse anode voltage is developed and the reverse recovery current continues to decrease. At T3 junction J1 and J3 are able to block a reverse voltage. But the thyristor is not able to block a forward voltage because carriers called trapped charges are still present at the junction J2. During the interval T3 to T4 these carriers recombine and SCR completely turns off. At T4 the recombination is complete and therefore even if a forward voltage is applied at this instant SCR remains in the off state. Thus the SCR turn off time is interval between T1 and T4. This time varies in the range 10 to 100 microsecond. Thus the total turn off time T of required for the device is the sum of duration for which reverse recovery current flows after application of reverse voltage and the time required for the recombination of all the excess carriers in the inner two layers of the device. In practical applications the turn off time required to the SCR by the circuit called the circuit turn off time TQ must be greater than the device turn off time T of by a safe margin to avoid undesired turn on known as commutation failure. With this we have seen complete turn on and turn off mechanism of SCR. You can refer these references. Thank you.