Silicon controlled rectifier (SCR).ppt

Lessons about silicon controlled rectifier

1. Silicon Controlled Rectifier (SCR) The Silicon Controlled Rectifier (SCR), also known as a Thyristor , is a fundamental component in power electronics. SCR is a  three-terminal  and four-layer unidirectional current-controlling  semiconductor  device. It is made up of silicon materials and is mainly used for controlling  high power  and conversion of high AC current into DC current. Hence the named silicon-controlled rectifier. INTRODUCTION

2. Silicon Controlled Rectifier (SCR) Symbol: A  Silicon Controlled Rectifier (SCR)  consists of three terminals namely  Anode (A) ,  Cathode (K) , and  Gate (G)  terminal. The device can be turned  ON  or  OFF  by controlling the gate input or  biasing condition  of SCR. The schematic symbol of SCR is shown in the figure below. The arrow of the  diode  represents the  direction  of conventional current flow.

3. The Silicon Controlled Rectifier (SCR) is a powerful switch for controlling high voltage and current. Its construction involves a unique arrangement of semiconductor materials to achieve its functionality. Here's a breakdown of its key aspects: Structure : Four Layers: An SCR is made of four alternating layers of P-type and N-type semiconductor material, forming a PNPN structure. Construction of Silicon Controlled Rectifier (SCR)

4. Three Junctions: These four layers create three junctions (J1, J2, and J3) between the different regions. Three Terminals: The SCR has three terminals: Anode (A): Connected to the outer P-type layer. Cathode (K): Connected to the outer N-type layer. Gate (G): Connected to a thin layer of P-type material within the structure, providing control over conduction.

5. STRUCTURE OF SCR

6. The three terminals  Anode (A) ,  Cathode (K) , and  Gate (G)  are arranged in such a way that Gate (G) terminal is attached to the p-type layer nearer to the Cathode (K) terminal in the  PNPN structure . Anode (A)  is a  positively charged electrode  and the conventional current enters into the device through this terminal.  Cathode (K)  is a  negatively charged electrode  and conventional current leaves the device through this terminal.  Gate (G)  is a control terminal that controls the flow of current between  Anode (A)  and  Cathode (K) .

7. From the  structure of SCR , a single SCR is a combination of one  PNP   transistor (TR1)  and one  NPN transistor (TR2) . The emitter of TR1 acts as the Anode terminal and the emitter of TR2 acts as the cathode terminal of SCR. Further, the base of TR1 is connected to the collector of TR2 and the collector of TR1 is connected to the base of TR2. The Gate terminal of SCR is connected to the base of TR2. This analogy of SCR as a combination of  two transistors  is called  two   transistor model . The structure of SCR as two transistor model

8. The structure of SCR as two transistor model

9. Depending on the biasing given to SCR, there are three modes of operation. They are Forward Blocking Mode (OFF State) Forward Conducting Mode (ON State) Reverse Blocking Mode (OFF State) Modes of operation in Silicon Controlled Rectifier (SCR):

10. Forward Blocking Mode (OFF State)

11. Under this condition, junctions J 1  and J3 are  forward biase d whereas junction J 2  is in  reverse biased  condition. The  depletion region  at junction  J 2  blocks the flow of current from junction J1 to junction  J3 as it acts obstacle or wall between them. However, a small amount of  leakage current  flows between these junctions  J2 and J3. Forward Blocking Mode (OFF State)

12. When the applied voltage across the SCR reaches a  breakdown voltage , the  avalanche breakdown  occurs due to high energy minority carriers. The current starts flowing through the SCR at this breakdown voltage and there is no current flow below the breakdown voltage because SCR offers very  high resistance  to the current below the breakdown voltage and acts as an  open switch  by blocking the forward current. Hence it will be in an  OFF state . Forward Blocking Mode (OFF State)

13. 2. Forward Conducting Mode (ON State): In this mode of operation, the SCR comes into the  conduction mode  from blocking mode. It can be done in two ways, i.e. either by  increasing the forward bias voltage  (voltage across Anode and Cathode) beyond the  breakdown voltage  or by  applying positive pulse  or voltage at the  Gate  terminal. The biasing of SCR in this mode is shown in the figure below.

14. 2. Forward Conducting Mode (ON State):

15. In the first case, the forward bias voltage applied between Anode and Cathode is increased beyond the breakdown voltage, the  depletion region breakdown  occurs at J 2,  and the current starts flowing through SCR. In this condition, the SCR will be in an  ON state . In the second case, a small  positive pulse  or voltage VG is applied to the Gate terminal of SCR as shown in the figure above. When the gate voltage is applied to the gate terminal, the  reverse biased junction  J 2  in forward blocking mode will become forward biased, and the  depletion region  width becomes very narrow. In this condition, a small forward bias voltage between  Anode  and  Cathode  can easily penetrate this narrow depletion region. Therefore on applying a small  forward bias voltage , an electric current starts flowing through the SCR and it will be in an  ON state . 2. Forward Conducting Mode (ON State):

16. 3. Reverse Blocking Mode (OFF State): In this mode of operation, a  positive voltage (+)  is given to Cathode (K) terminal, and a  negative voltage (-)  is given to Anode (A), Gate (G) terminal is an  open circuit  as shown in the figure below.

17. Under this condition, junctions J 1  and J3  are  reverse biased  whereas junction  J 2  is in  forward biased  condition. As junctions J 1  and J3  are reverse-biased, there is  no current flow  through the SCR. But due to the drift of the charge carrier in a forward-biased junction J 2 , there is small  leakage current  flow in SCR which is not sufficient to turn  ON  the device. Hence the SCR will be in an  OFF  state and acts as an  open switch . 3. Reverse Blocking Mode (OFF State):

18. V-I Characteristics of Silicon Controlled Rectifier (SCR): In this V-I characteristic, the horizontal line represents the amount of voltage applied  V A  across the SCR and the vertical line represents the amount of current flow  I A  in the SCR. The  V-I characteristics  of SCR are shown in the figure below.

19. V-I Characteristics of Silicon Controlled Rectifier (SCR):

20. T he  V-I characteristics  of SCR are divided into  three regions . Forward Blocking Region Forward Conduction Region Reverse Blocking Region V-I Characteristics of Silicon Controlled Rectifier (SCR):

21. Forward Blocking Region The region OA in V-I characteristics is called the forward blocking region. This region represents the forward-blocking mode of SCR operation. In this region, the forward bias voltage is given to SCR where positive voltage is given to Anode, the negative is given to SCR and Gate is open-circuited. In this condition, the junctions  J 1  and J3 become forward biased whereas junction J 2  becomes reverse biased. A small leakage current flows from the Anode terminal to the Cathode terminal of SCR which is known as a forward leakage current.  The SCR does not conduct electric current and the device is in an OFF state in this region.

22. 2. Forward Conduction Region The region BC in V-I characteristics is called the forward conduction region. This region represents the forward conduction mode of SCR operation. In this region, the current flowing from Anode to Cathode increases rapidly. When the forward bias voltage applied between Anode and Cathode is increased beyond the breakdown voltage, the depletion region breakdown occurs at junction J 2  and the current starts flowing through the SCR and it will be in the ON state.

23. 2. Forward Conduction Region The current flow in this region increases rapidly after junction J 2  breakdown occurs. The voltage at which the junction  breakdown occurs when the Gate is open is known as forward breakdown voltage (V BF ) The region AB in V-I characteristics indicates that as soon as the SCR becomes ON, the voltage across the SCR drops to some volts.

24. 3. Reverse Blocking Region The region OE in the V-I characteristics is called the reverse blocking region. This region represents the reverse blocking mode of SCR operation. In this region, the reverse bias voltage is applied to SCR where a positive voltage is given to Cathode, a  negative voltage  is given to Anode, and the Gate terminal is open-circuited. In this condition, junctions J 1  and J3 are reverse biased whereas the junction is forward biased. As junction J 1  and J3  are in reverse biased condition, there is no current flow through SCR.

25. But due to the drift of the charge carrier in forward-biased junction J 2 , there is small leakage current flow in SCR which is not enough to turn ON the device. Hence the SCR will be in an OFF state in this region. When the reverse bias voltage between Anode and Cathode is increased beyond the reverse breakdown voltage V BR , an avalanche breakdown occurs, and the current increases rapidly. The region EF in V-I characteristics is known as the reverse avalanche region. 3. Reverse Blocking Region

26. Advantages of SCR: The followings are some  advantages  of SCR. It can handle large voltage, current, and power. The voltage drop across conducting SCR is small which will reduce the power dissipation. Triggering circuits are simple. Easy to turn ON. It has a higher switching speed. It can be protected with the help of a fuse. It is simple to control.

27. Disadvantages of SCR: The followings are the  disadvantages  of SCR. It can conduct only in one direction. So power control can be done only during half cycle of ac. The gate current cannot be negative It cannot be used at high frequency as it can be operated at a maximum frequency of 400 HZ. It needs to be turned on each cycle in ac circuits.

28. Applications of SCR: The main  application of SCR  is  switching  and  power control . The followings are some applications that use switching and power control properties of SCR. It is used as a switch It is used b in AC voltage stabilizers It is used in choppers (DC to Dc converters) It is used for inverters (DC to AC converters) It is used in battery charger

29. It is used for power control circuits It is used in DC circuit breaker It is used for AC power control with a solid relay It is used to control motors speed It is used to adjust the light dimmer It is used in fan speed regulators. Applications of SCR: