 Hello everyone, welcome to this session. I am Dr. Asha Thalange and today we are going to learn Silicon Control Rectifiers principle of operation and VI characteristics. At the end of this session, you will be able to explain principle of operation of Silicon Control Rectifier, describe static VI characteristics of SCR state the effect of gate current on triggering of SCR. These are the contents we will be covering in this session. We all have studied and used different semiconductor devices. Before moving ahead, pause this video for few seconds and write down what is a semiconductor device. Yes, the semiconductor devices are electronic components made up of semiconductor materials such as silicon, germanium etc. Semiconductor diodes, transistors are example of semiconductor devices. Let us now discuss what power semiconductor devices are. Power semiconductor devices are those semiconductor devices which are used as switches or rectifiers in power electronic applications for controlling purpose and also for the electric power conversion. These devices are able to carry large amount of current during their on state and can withstand large reverse bias voltage in off state. Some of the examples are power diode, thyristor, power MOSFET and IGBT. Thyristor is a general name given to a family of power semiconductor switching devices. On controlled rectifiers, popularly known as SCR is an important and most widely used member of thyristor family. Let us discuss SCR's construction and its principle of working. Figure shows the structure and symbol of SCR. As shown, the SCR is a four layered PN-PN switching device having three junctions J1, J2 and J3. It has three external terminals anode, cathode and gate as shown. The gate terminal is taken out from the P layer near to the cathode and hence it is also known as cathode gate. The anode and cathode are always connected to the main power circuit and gate terminal is used to turn on the SCR by passing a gate current from gate to cathode. Let us see the operation of SCR. Consider the state when anode is made positive with respect to cathode. Two outer junctions J1 and J3 are forward biased, middle junction J2 is reverse biased and thus forms a depletion layer which blocks the current from flowing through the device. Some small amount of leakage current flows through the device due to drift of mobile charges but this current is insufficient to make the device conduct. Thus, even though SCR is in forward biased condition, it does not conduct. This is known as forward blocking state or off state of the device. But when this positive anode to cathode voltage is increased, the width of the depletion layer at the junction J2 decreases. If this voltage is kept on increasing, then after a certain point, depletion layer at junction J2 vanishes as the reverse biased junction J2 breaks down due to the large potential difference across its depletion layer. This is called Avalanche breakdown. At this stage as J1 and J3 are already forward biased, there will be a large free carrier movement resulting in a large amount of current flowing from anode to cathode of the device. Due to this, the device starts conducting and is in a conducting state or on state. Anode is made negative with respect to the cathode. Junction J1 and J3 becomes reverse biased and J2 becomes forward biased. Reverse biased junctions J1 and J3 do not allow any current to flow through the device but then to a small leakage current flows because of the drift of charges. This current is again insufficient to turn on the device. This is known as reverse blocking state or off state of the device. Reverse working can also be represented in the form of a VI characteristics of SCR. Figure shows the circuit diagram used to obtain the VI characteristics of a SCR. As shown, anode and cathode are connected to the main source through a load. And another source ES is connected between gate and cathode through variable resistor and a switch to provide gate current pulse. Figure shows VI characteristics of SCR. VA is anode to cathode voltage across SCR and IA is anode current of SCR. This VI characteristics can be divided into three regions of operations. Let us see the forward blocking region. In this, anode is made positive with respect to cathode. As explained before, during this state J1 and J3 are forward biased and J2 is reverse biased. Even though the anode voltage is increased, very small forward leakage current flows through the device as shown in the figure. Keeping it in the off state only. This region represented by the region OM of the VI characteristics is known as the forward blocking region. Next region is forward conduction region. As explained before, as anode to cathode voltage is still increased keeping gate circuit open, a stage comes when Avalanche breakdown occurs at junction J2 at a particular anode to cathode voltage known as forward breakover voltage VBO, say at point M and SCR latches. That is, immediately SCR switches to a low impedance condition or high conduction state denoted at point N shown by the dotted lines in figure. This makes a large current to flow through the SCR. The region MN of the characteristics shows that as soon as the device latches or turns on the voltage across the device drops to a very lower value, say about 1 to 2 volts depending on its rating and a large amount of current starts flowing through the device. This part is represented by the region NK in the figure and is known as forward conduction state. During this region the anode current is determined by the external load impedance. Thus, as the drop across the SCR is very less during this region with large current flow the SCR is said to be as a closed switch. The third region is reverse blocking region. When the anode is made negative with respect to cathode with gate circuit open, the thyristor is reverse biased. As explained before, J1 and J3 are reverse biased and J2 is forward biased. Thus, only a small reverse leakage current flows through the device as shown in the figure. Region O to P is thus known as reverse blocking mode. But if this reverse voltage is further increased then at a particular reverse voltage called reverse breakdown voltage VBR an Avalanche breakdown will occur at junction J1 and J3 due to which reverse current increases sharply in the device. If this current crosses the safer limit then due to the high power dissipation the device gets destroyed. Region PQ represents reverse Avalanche region. During region OP when the reverse voltage is less than the reverse breakdown voltage the device just behaves as a high impedance device or as open switch. Also, the inner two layers of the SCR are lightly doped compared to the outer layer. Thus, the thickness of the depletion layer at junction J2 during forward bias condition is greater than the total thickness of the two depletion layers at J1 and J3 when the reverse is reverse biased. Due to this the forward breakover voltage VBO is generally higher than the reverse breakover voltage VBR. Now, we discussed VI characteristics without applying the gate signal. When no gate signal is applied gate current IG is equal to zero and the SCR turns on at forward breakover voltage VBO as shown. The moment switch S is closed a current pulse is passed from gate to cathode say IG equal to IG1. This makes the SCR to turn on early before the anode voltage reaches the VBO as shown. If still higher gate current is passed say IG equals to IG2 such that IG2 is greater than IG1 greater than zero the SCR turns on more early. With this can you state the relationship between the gate current and triggering of the SCR? Pause the video for few seconds and think about it. Yes, the forward voltage at which the device turns on depends upon the magnitude of the gate current that is higher the gate current lower is the forward anode voltage. Thus using gate drive device can be turned on early. The typical gate current magnitude are of the order of 20 to 20 milliampere. Once the SCR turns on it starts conducting forward current that is greater than the minimum value called latching current shown at point N. The gate signal is no longer required to maintain the device in on state. Even though gate signal is removed SCR still remains in on state. The SCR will return to its original forward blocking state if the anode current falls below a lower level known as holding current as shown in the figure. Thus holding current is less than the latching current. Thus we can say latching current is related to turn on process and holding current is related to turn off process. Lastly, to summarize in this session we discuss construction principle of operation and VI characteristics of SCR. These are the references you can refer. Thank you.