Thyristor family

The complete list of thyristor family members include diac (bidirectional diode thyristor), triac (bidirectional triode thyristor), SCR (silicon controlled rectifier), Shockley diode, SCS (silicon controlled switch), SBS (silicon bilateral switch), SUS (silicon unilateral switch) also known as com­plementary SCR or CSCR, LASCR (light activated SCR), LAS (light activated switch) and LASCS (light activated SCS).

1. UNIT 3 INTRODUCTION TO POWER ELECTRONICS DEE 402 Thyristor Family by VINOD SRIVASTAVA Department of Electrical Engineering University Polytechnic IFTM University, Moradabad

2. CONTENTS Programmable UJT (PUT) Silicon Unilateral Switch (SUS) Silicon Control switch (SCS) Light Activated Thyristor Static Induction Thyristor (SITH) DIAC TRIAC Important Questions MCQs 2

3. Programmable UJT (PUT) Programmable unijunction transistor or PUT is a close relative of the thyristor family. It has a four layered construction just like the thyristors and have three terminals named anode(A), cathode(K) and gate(G) again like the thyristors . Yet some authors call it a programmable UJT just because its characteristics and parameters have much similarity to that of the unijunction transistor. 3

4. Programmable UJT (PUT) It is called programmable because the parameters like intrinsic standoff ratio (η), peak voltage( Vp ) etc can be programmed with the help of two external resistors. In a UJT, the parameters like Vp , η etc are fixed and we cannot change it. The main application of programmable UJT are relaxation oscillators , thyristor firing, pulse circuits and timing circuits. 4

5. SUS Silicon Unilateral Switch 5 Because of the presence of diode, SUS turns on for a fixed anode to cathode voltage. SCR’s trigger voltage and/or current vary widely with changes in ambient temperature.

6. SUS Silicon Unilateral Switch 6 SUS is mainly used in Timing, logic and trigger circuits. Its largest ratings are about 20 v and 0.5 A.

7. An SCS is a tetrode . It is a four electrode thyristor . It has two gates, one anode gate (AG) like a PUT and another cathode gate (KG) like an SCR. An SCS is a four layer, four terminal pnpn device with anode A, cathode K, anode gate AG and cathode gate KG. An SCS can be turned on by either gate. 7 SCS( Silicon Controlled Switch)

8. SCS( Silicon Controlled Switch) When a negative pulse is applied to gate AG, junction J1 is forward biased and SCS is turned on. A positive pulse at AG will reverse bias junction J1 and turns off the SCS. When a positive pulse is applied to gate KG, junction J2 is forward biased and SCS is turned on. (Just like an SCR) A negative pulse at KG will reverse bias junction J2 and turns off the SCS. (Just like a G.T.O.) Its ratings are about 100 v and 200 mA . It can be operated like an OR gate. 8

9. Light Activated Thyristor  LASCR or light activated SCR is a semiconductor device which turns ON when it is exposed to light. The constituent element of SCR is silicon, and it works like a rectifier, and thus, it is termed as Silicon Controlled Rectifier.  The LASCR is a type of thyristor which is triggered by photons present in the light rays. 9

10. Light Activated Thyristor It is a three terminal device, consists of cathode, anode and gate terminal. The gate terminal is used when the electrical triggering is supplied to the LASCR. The advantage of using triggering of the thyristor by light is prevention from electrical noise disturbances. Thus, LASCR is considered to be one of the best devices. 10

11. Light Activated Thyristor 11 Applications of the Light Activated SCR Low Power Applications Motor Control Computer Applications Optical light Controls Solid State Relay

12.  STATIC INDUCTION THYRISTOR(SITH) 12 The static induction thyristor ( SITh ) or field-controlled diode (FCD) was first introduced by Teszner in the 1960’s. This device is capable of conducting large currents with a low forward voltage and turn-off quickly. It is a self-controlled GTO-like on-off device that was commercially introduced by Toyo Electric Co. (Toyo Denki) of Japan in 1988. It belongs to a family of Static Induction Device and is a high power high frequency power semiconductor device. It is essentially a p+nn + diode with a buried p+ grid like gate structure.The device structure is analogous to SIT except that a p+ layer has been added to the anode side.

13. 13 DIAC The DIAC is a bi-directional semiconductor switch that can be switched on in both polarities. The full form of the name DIAC is diode alternating current. Diac is connected back to back using two zener diodes and the main application of this DIAC is, it is widely used to help even activating of a TRIAC when used in AC switches, dimmer applications and starter circuits for florescent lamps.

14. DIAC Construction and Operation of DIAC Basically, the DIAC is a two terminal device; it is a combination of parallel semiconductor layers that allows activating in one direction. This device is used to activating device for the triac . The basic construction of diac consist of two terminals namely MT1 and MT2. When the MT1 terminal is designed + Ve with respect to the terminal MT2, the transmission will take place to the p-n-p-n structure that is another four layer diaode . The diac can be performing for both the direction.   14

15. DIAC Characteristics 15

16. TRIAC A Triac device comprises of two thyristors  that are connected in opposite direction but in parallel but, it is controlled by the same gate. Triac is a 2-dimensional thyristor which is activated on both halves of the i /p AC cycle using + Ve or - Ve gate pulses. The three terminals of the Triac are MT1; MT2 & gate terminal (G). Generating pulses are applied between MT1 and gate terminals. The ‘G’ current to switch 100A from triac is not more than 50mA or so. 16

17. TRIAC Construction and Operation of DIAC Traic is a three terminal device and the terminals of the triac are MT1, MT2 and Gate. Here the gate terminal is the control terminal. The flow of current in the triac is bi directional that means current can flow in both the directions. In the above structure, the MT1 and gate terminals are near to each other. When the gate terminal is open, the triac will obstruct the both the polarities of the voltage across the MT1 & MT2. 17

18. TRIAC Characteristics 18

19. 19 IMPORTANT QUESTIONS Q.1. Describe briefly the following members of thyristor family ( i ) DIAC (ii) TRIAC Q.2. Explain PUT. Q.3 Explain SUS and SCS in detail. Q.4. Explain the following members of Thyristor family- (a) Static induction Thyristor (b) TRIAC Q.5.Explain Light activated Thyristor .

20. MULTIPLE CHOICE QUESTIONS Q1. A triac is a 2 terminal switch 2. 2 terminal bilateral switch 3. 3 terminal bilateral switch 4. 3 terminal bidirectional switch Q.2. In PUT, Gate terminal is connected to Inner n-layer ii) Inner p- layer iii) Upper p- layer iv) Lower n- layer Q3. A Triac has three terminals viz ……………… Drain, source, gate 2.Two main terminal and a gate terminal 3. Cathode, anode, gate 4. None of the above 20

21. MULTIPLE CHOICE QUESTIONS Q4. A triac is equivalent to two SCRs ………….. In parallel 2. In series 3. In inverse-parallel 4 . None of the above Q5. A triac is a …………. switch Bidirectional 2. Unidirectional 3. Mechanical 4. None of the above Q6. A triac can pass a portion of …………… half-cycle through the load Only positive 2. Only negative 3. Both positive and negative 4. None of the above Q7. A diac has ………….. terminals Two 2. Three 3. Four 4.None of the above 21

22. MULTIPLE CHOICE QUESTIONS Q8. A triac has …………….. semiconductor layers Two Three Four Five Q9. A diac has …………… pn junctions Four Two Three None of the above Q10. The device that does not have the gate terminal is ………………. 1. Triac 2. FET 3. SCR 4. Diac 22

23. THANK YOU 23