siliconcontrolledrectifierscr-210624134015.pptx

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1. IDEAL INSTITUTE OF ENGINEERING NAME :- DHIRAJ CHANDRA DAS
DEPT. :- ELECTRICAL ENGINEERING
ROLL NO. :- 27901623011
REG. NO. :- 232790120118
YEAR :- 3 RD (5 TH SEM.)
SUBJECT :- POWER ELECTRONICS
SUBJECT CODE :- PC-EE-504
TOPIC NAME :- V-I CHARACTERISTIC OF SCR

2. CONTENTS Introduction What is SCR SCR Construction Working of SCR Characteristic curve Importance of SCR Applications of SCR Disadvantages of SCR Conclusion

3. What is SCR ? A SCR is a three terminal , four layer solid state device which controls current flow. It’s terminals are: Gate Cathode Anode (Image source: daenotes.com)

4. SCR Construction SCRs are constructed from silicon and can be used as controlled switch That’s why it is called “Silicon Controlled Rectifier”. It is a four-layered semiconductor device that forms NPNP or PNPN structure, which eventually forms three junctions J1, J2, and J3. Among the three terminals of the SCR, the Anode is a positive electrode, it will be on the P-layer and Cathode is a negative electrode, it will be on the N-layer of the SCR, the Gate acts as a control terminal of the SCR. The outer P and N layers where the two electrodes are placed will be heavily doped and the middle P and N layers will be lightly doped, the gate terminal will be connected to the P-layer in the middle. (Image Source: learnaboutelectronics.com)

5. Working of SCR The anode terminal of SCR is always kept at positive potential w.r.t. cathode. In a silicon controlled rectifier, load is connected in series with anode. Depending on the biasing given to the SCR, the operation of SCR is divided into three modes. They are Forward blocking Mode Forward Conduction Mode and Reverse Blocking Mode

6. Forward Blocking Mode In this mode of operation, the Silicon Controlled Rectifier is connected such that the anode terminal is made positive with respect to cathode while the gate terminal kept open. In this state junctions J1 and J3 are forward biased and the junction J2 reverse biased. Due to this, a small leakage current flows through the SCR. Until the voltage applied across the SCR is more than the break over voltage of it, SCR offers a very high resistance to the current flow. Therefore, the SCR acts as a open switch in this mode by blocking forward current flowing through the SCR as shown in the VI characteristics curve of the SCR.

7. 2. Forward Conduction Mode In this mode, SCR comes into the conduction mode from blocking mode. It can be done in two ways as either by applying positive pulse to gate terminal or by increasing the forward voltage (or voltage across the anode and cathode) beyond the break over voltage of the SCR. Once any one of these methods is applied, the avalanche breakdown occurs at junction J2. Therefore the SCR turns into conduction mode and acts as a closed switch thereby current starts flowing through it. In this mode, maximum current flows through the SCR and its value depends on the load resistance or impedance.

8. 3. Reverse Blocking Mode In the reverse blocking mode, the positive voltage is applied to the Cathode (-) and the Negative voltage is given to the Anode (+), There will not be any pulse given to the gate, it will be kept as an open circuit. During this mode of operation the Junctions J1 and J3 will be reverse biased and the junction J2 will be forward biased. Since the junctions J1 and J3 are reverse biased there will not be any current flowing through the SCR. Although there will be a small leakage current flowing due to the drift charge carriers in the forward-biased Junction J2, it is not enough to turn on the SCR.

9. VI Characteristics of SCR The VI characteristics of the SCR are obtained by operating the SCR in three different regions, namely forward blocking region, forward conduction region and reverse blocking region. ( Image Source : Google )

10. Importance of SCR The SCR (silicon controlled rectifier) can handle large voltage, current and power. The Triggering circuit for silicon controlled rectifier (SCR) is simple. Easy to turn ON. It can be protected with the help of fuse. It is able to control AC power. Simple to control. Cost is low. The switching speed is very high as it can perform 1 nano operations per second.

11. Applications of SCR Silicon Controlled Rectifiers are used as Switch in electronic circuits. There is a huge application of Silicon Controlled Rectifier in Inverter Circuits. In controlled Rectifier Circuits SCRs are used. SCRs are used in AC Voltage Stabilizer circuits. SCRs are also used in Battery Charger circuits. SCRs are used to control electrical power. SCRs are used for High voltage protection purpose. SCRs are used in power regulator devices.

12. The SCR (silicon controlled rectifier) is unidirectional devices, so it can control power only in DC power during positive half cycle of AC supply, thus only DC power is controlled with the help of SCR. The gate current cannot be negative. In AC circuit, it needs to be turned on each cycle. It cannot be used at higher frequencies. Disadvantages

13. Conclusion An SCR has two states i.e. either it does not conduct or it conducts heavily. There is no state inbetween . Therefore, SCR behaves like a switch. There are two ways to turn on the SCR. The first method is to keep the gate open and make the supply voltage equal to the breakover voltage. The second method is to operate SCR with supply voltage less than breakover voltage and then turn it on by means of a small voltage applied to the gate. Applying small positive voltage to the gate is the normal way to close an SCR because the breakover voltage is usually much greater than supply voltage. To open the SCR e. to make it non-conducting, reduce the supply voltage to zero.

14. Thank You!