Lecture-5 : Semiconductor Power Switching Devices-2

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Semiconductor Power Switching Devices-2 (Lecture-5) R S Ananda Murthy Associate Professor and Head Department of Electrical & Electronics Engineering, Sri Jayachamarajendra College of Engineering, Mysore 570 006 R S Ananda Murthy Semiconductor Power Switching Devices-2
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This is the fifth lecture on Power Electronics. In this lecture we discuss Silicon Controlled Rectifiers (SCRs).

Transcript of Lecture-5 : Semiconductor Power Switching Devices-2

  • 1. Semiconductor Power Switching Devices-2 (Lecture-5) R S Ananda Murthy Associate Professor and Head Department of Electrical & Electronics Engineering, Sri Jayachamarajendra College of Engineering, Mysore 570 006 R S Ananda Murthy Semiconductor Power Switching Devices-2

2. Silicon Controlled Rectier (SCR) p+ p Gate (G)Cathode (K) Anode (A) n- J1 J2 n+ J3 Structure of SCR p+ indicates heavily doped p-type region n- indicates lightly doped n-type region n+ indicates heavily doped n-type region A K G Circuit Symbol of SCR ON OFF = Latching Current = Holding Current Reverse Leakage Current Forward Leakage Current Thicker n- layer gives higher voltage blocking capability to the device. The forward voltage at which the device turns on decreases with increase in gate current. R S Ananda Murthy Semiconductor Power Switching Devices-2 3. Control Characteristics of SCR vo iG vo + iG V t R + V t SCR is semicontrolled and pulse triggered. R S Ananda Murthy Semiconductor Power Switching Devices-2 4. Two Transistor Model of SCR p1 p2 n1 n2 p1 n1 2p n2 p2 1n T1 T2 + Gate Control Circuit IG + IK IA 1 IC1 IC2 2 T2 IB 1 IB 2 T1 K A K J1 J2 J3 G R V A K G G V R A 1.0 0.8 0.6 0.4 0.2 0 0 (mA)1 IE (a) (b) (c) (d) R S Ananda Murthy Semiconductor Power Switching Devices-2 5. Conditions for Turn ON and Turn OFF of SCR In Figure-(c) of previous slide, by applying KCL to the device we get IK = IA +IG and IA = IC1 +IC2 (1) Considering each transistor we have IC1 = 1IA +ICBO1 and IC2 = 2IK +ICBO2 (2) Substituting Eq.(2) in (1) and by rearranging terms we get IA = 2IG +(ICBO1 +ICBO2) [1(1 +2)] (3) When (1 +2) VBO. This should be avoided since it may permanently damage the device. Rise in device temperature can cause unwanted turn ON and hence should be avoided by cooling the device. By injecting positive gate current IG until IA = IL where IL is the latching value. This is the preferred method of turning ON the device. Forward dvAK /dt > rated value causes undesirable turn ON and should prevented by connecting a snubber circuit across the SCR. Light radiation of specic wavelength incident on junctions of SCR turns ON the device. LASCRs are turned ON by this method. R S Ananda Murthy Semiconductor Power Switching Devices-2 7. How to Turn Off SCR? Gate current has no control over the SCR after it turns ON. IA should be reduced below the holding value IH in order to make (1 +2) 0 to stop the internal regenerative action to turn OFF the device. After IA drops to zero, the device should be reverse biased for a duration tq > tOFF where tOFF is known as the device turn OFF time and tq is known as the circuit turn-off time. R S Ananda Murthy Semiconductor Power Switching Devices-2 8. Gate Characteristics of SCR Load Line Equation for load line A K G A B Trigger Source SCR The VG-IG plot of SCR is similar to that of a diode. The VG-IG characteristics of individual SCRs belonging to a family will have a spread between A and B as shown. R S Ananda Murthy Semiconductor Power Switching Devices-2 9. Reverse Voltage Protection for Gate A K G Trigger Source SCR Connecting resitance between gate and cathode --- Increases dv/dt capability, lowers the turn off time of SCR, minimizes reverse voltage across the device, and increases holding and latching current. (a) (b) (c) A K G Trigger Source SCR A K G Trigger Source SCR R S Ananda Murthy Semiconductor Power Switching Devices-2 10. SCR Turn ON Characteristics (a) (b) (c) tON increases with increase in the inductance of the load. R S Ananda Murthy Semiconductor Power Switching Devices-2 11. SCR Turn OFF Characteristics (a) (b) (c) tq should be > tOFF and dvAK /dt should be less than rated value for proper turn off of SCR. R S Ananda Murthy Semiconductor Power Switching Devices-2 12. Snubber Circuit for SCR When SCR is turned ON di/dt through SCR is limited to safe value by connecting this inductor. When SCR is turned ON this resistor limits the capacitor discharging current through the SCR to safe value. When SCR is turned OFF and forward voltage appears across the SCR D1 bypasses R2 and dv/dt across SCR is limited to safe value by this R1C1 network. A K SCR G R S Ananda Murthy Semiconductor Power Switching Devices-2 13. Letter Symbols used to Specify Ratings of SCR A Anode, ambient J Junction (AV) Average K Cathode (BO) Breakover M, m Maximum (BR) Breakdown Q, q Turn off D Off state or non-trigger R, r Reverse, repetitive d Delay time (RMS) RMS value F, f Forward, falltime rr Reverse recovery H Holding S surge, nonrepetitive G, g Gate terminal T, t On-state, trigger Examples: VRRM Reverse repetitive maximum voltage. ITSM On-state maximum surge current. VGT Gate voltage required for triggering. IT(RMS) On-state RMS current. R S Ananda Murthy Semiconductor Power Switching Devices-2 14. Specications of BT145 Thyristor Series R S Ananda Murthy Semiconductor Power Switching Devices-2 15. Specications of BT145 Thyristor Series R S Ananda Murthy Semiconductor Power Switching Devices-2 16. Specications of BT145 Thyristor Series R S Ananda Murthy Semiconductor Power Switching Devices-2 17. Specications of BT145 Thyristor Series R S Ananda Murthy Semiconductor Power Switching Devices-2 18. Disc Type of SCRs (Source: www.china-rectifer.com) R S Ananda Murthy Semiconductor Power Switching Devices-2 19. Disc Type of SCR (Source: www.abb.com) R S Ananda Murthy Semiconductor Power Switching Devices-2 20. Stud Type of SCRs (Source: www.china-rectifer.com) R S Ananda Murthy Semiconductor Power Switching Devices-2 21. SCR Power Module (Source: saishe.en.made-in-china.com) R S Ananda Murthy Semiconductor Power Switching Devices-2 22. SCR Power Module (Source: http://theelectrostore.com) R S Ananda Murthy Semiconductor Power Switching Devices-2 23. Specications of SKKT 210/20E Module R S Ananda Murthy Semiconductor Power Switching Devices-2 24. Specications of SKKT 210/20E Module R S Ananda Murthy Semiconductor Power Switching Devices-2 25. Next Lecture... In the next lecture we will discuss some more power semiconductor switching devices used in power electronics. Thank You. R S Ananda Murthy Semiconductor Power Switching Devices-2