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L7 gto

Part of Lecture series on EE321N, Power Electronics-I delivered by me during Fifth Semester of B.Tech. Electrical Engg., 2012
Z H College of Engg. & Technology, Aligarh Muslim University, Aligarh
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L7 gto

  1. 1. EE‐321 N Lecture‐7 Gate Turn OFF (GTO) Thyristor
  2. 2. Introduction • The conventional SCR is the ruling device for the power electronics industry due to very high ratings • However, it is a semi‐controlled device because it can’t be turned OFF by the gate terminal • GTO achieves this objective of being turned OFF by –ve gate current and thus is a fully controlled switch 27‐Jun‐15 EE‐321N, Lec‐7 2
  3. 3. Contd... • Two configurations available: asymmetric and symmetric • Available ratings up to : 6 kV, 6 kA (Mitsubishi) 27‐Jun‐15 EE‐321N, Lec‐7 3 Popular
  4. 4. Device Symbol
  5. 5. Structure 27‐Jun‐15 EE‐321N, Lec‐7 5
  6. 6. Contd... • Turn OFF capability by supplying –ve IG can be guaranteed if the access resistance from the gate to all parts of the device cathode is very low • This is done by using an “interdigitated” structure in which the gate‐cathode area is distributed over the whole pellet and interleaving their respective regions 27‐Jun‐15 EE‐321N, Lec‐7 6
  7. 7. Interdigitized Structure (Front View) 27‐Jun‐15 EE‐321N, Lec‐7 7
  8. 8. Interdigitized Structure (Top View) 27‐Jun‐15 EE‐321N, Lec‐7 8 Cathode Gate
  9. 9. Appearance 27‐Jun‐15 EE‐321N, Lec‐7 9
  10. 10. Characteristics 27‐Jun‐15 EE‐321N, Lec‐7 10 Real Ideal
  11. 11. Turn OFF Mechanism  27‐Jun‐15 EE‐321N, Lec‐7 11
  12. 12. Remember ? 27‐Jun‐15 EE‐321N, Lec‐7 12
  13. 13. Analysis • Above circuit was used to explain the turn ON mechanism of SCR • Now we use this to understand how turn‐OFF is achieved • Both transistors are in saturation • A –ve gate current can bring T2 out of saturation and eliminate this “regenerative” effect thereby turning OFF the device 27‐Jun‐15 EE‐321N, Lec‐7 13
  14. 14. Contd... • The anode current is given by: • Suppose, we neglect ICBO’s then IA = 0 means  that  27‐Jun‐15 EE‐321N, Lec‐7 14 )(1 21 212 αα α +− ++ = CBOCBOG A III I ( )22 1 2 1 21 ( ) 1 ( ) GG A II I αα α α α α −− = = − + − +
  15. 15. Contd... • Turn‐OFF gain βOFF for the turn‐OFF process of a GTO is defined as: • βOFF lies in the range 4‐5 (low) but this calls for a very high –IG. E. g. if IA = 2000 A then –IG = 500 A (around) • A high value of βOFF can be obtained by making α2 high and α1 low 27‐Jun‐15 EE‐321N, Lec‐7 15 ( ) 2 OFF 1 2 1 A G I I α β α α = = + −
  16. 16. Comparison with Conventional SCR 27‐Jun‐15 EE‐321N, Lec‐7 16
  17. 17. Advantages Disadvantages Elimination of commutating components in forced commutation resulting in reduction in cost, weight and volume. ON state voltage drop and associated losses are higher in GTO than thyristor Reduction in acoustic and  electromagnetic noise due to the elimination of commutation chokes Triggering gate current required for GTOs is more than those of thyristor Faster turn OFF permitting high switching frequency Latching and holding current is more in GTO than those of thyristor Improved converters efficiency Gate drive circuit loss is more than those of thyristor, and, It has more di/dt rating at turn ON Its reverse voltage block capability is less than its forward blocking capability. (But this is no disadvantage for inverters/choppers) 1727‐Jun‐15 EE‐321N, Lec‐7
  18. 18. HOME ASSIGNMENT Q1. Discuss the switching characteristics of SCR. Emphasize on the following terms: Delay time, rise time, spread time, turn‐OFF time, circuit turn OFF time. Q2. Discuss briefly other devices of the thyristor family. Highlight their circuit symbol, characteristics, ideal characteristics, main applications 27‐Jun‐15 EE‐321N, Lec‐7 18
  19. 19. Reminder • Internet connection is now available • Please complete the Today’s Task assigned in  lecture‐2 • Will be checked randomly 27‐Jun‐15 19EE‐321N, Lec‐7
  20. 20. End of Unit‐1 27‐Jun‐15 EE‐321N, Lec‐7 20 ☺