Characteristics: Triac - nptel

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A A A I A G p n - p n + n + K (a) p J 1 n - J 2 p n - G J 2 J 3 p n + K (b) J 3 i C2 (α 2 ) Q 2 I K K (c) Q 1 (α 1 ) i C1 I G G Fig. 4.2: Two transistor analogy of a thyristor construction. (a) Schematic Construction, (b) Schematic division in component transistor (c) Equivalent circuit in terms of two transistors. a) Schematic construction, b) Schematic division in component transistor c) Equivalent circuit in terms of two transistors. Let us consider the behavior of this p n p n device with forward voltage applied, i.e anode positive with respect to the cathode and the gate terminal open. With this voltage polarity J 1 & J 3 are forward biased while J 2 reverse biased. Under this condition. 2 2 K co2 ( ) ( ) ic 1= ∝1I A+ I co1 4.1 ic = ∝ I + I 4.2 Where ∝ 1 & ∝ 2 are current gains of Q 1 & Q 2 respectively while I co1 & I co2 are reverse saturation currents of the CB junctions of Q 1 & Q 2 respectively. Now from Fig 4.2 (c). i c1 + i c2 = I A ( 4.3) & I A = I K ( 4.4 ) ( ∵ I G = 0) Combining Eq 4.1 & 4.4 I +I I ( ) co1 co2 co I A = = 4.5 1- ( ∝1+ ∝2) 1- ( ∝1+ ∝2) Version 2 EE IIT, Kharagpur 6
Where I I +I is the total reverse leakage current of J co co1 co2 2 Now as long as V AK is small I co is very low and both ∝ 1 & ∝ 2 are much lower than unity. Therefore, total anode current I A is only slightly greater than I co . However, as V AK is increased up to the avalanche break down voltage of J 2, I co starts increasing rapidly due to avalanche multiplication process. As I co increases both ∝ 1 & ∝ 2 increase and ∝ 1 + ∝ 2 approaches unity. Under this condition large anode current starts flowing, restricted only by the external load resistance. However, voltage drop in the external resistance causes a collapse of voltage across the thyristor. The CB junctions of both Q 1 & Q 2 become forward biased and the total voltage drop across the device settles down to approximately equivalent to a diode drop. The thyristor is said to be in “ON” state. Just after turn ON if I a is larger than a specified current called the Latching Current I L , ∝ 1 and ∝ 2 remain high enough to keep the thyristor in ON state. The only way the thyristor can be turned OFF is by bringing I A below a specified current called the holding current (I H ) where upon ∝ 1 & ∝ 2 starts reducing. The thyristor can regain forward blocking capacity once excess stored charge at J 2 is removed by application of a reverse voltage across A & K (ie, K positive with respect A). It is possible to turn ON a thyristor by application of a positive gate current (flowing from gate to cathode) without increasing the forward voltage across the device up to the forward break-over level. With a positive gate current equation 4.4 can be written as K A G ( ) I = I + I 4.6 Combining with Eqns. 4.1 to 4.3 ∝ I +I ( ) 2 G co I A = 4.7 1- ( ∝1+ ∝2) Obviously with sufficiently large I G the thyristor can be turned on for any value of I co (and hence V AK ). This is called gate assisted turn on of a Thyristor. This is the usual method by which a thyristor is turned ON. When a reverse voltage is applied across a thyristor (i.e, cathode positive with respect to anose.) junctions J 1 and J 3 are reverse biased while J 2 is forward biased. Of these, the junction J 3 has a very low reverse break down voltage since both the n + and p regions on either side of this junction are heavily doped. Therefore, the applied reverse voltage is almost entirely supported by junction J 1 . The maximum value of the reverse voltage is restricted by a) The maximum field strength at junction J 1 (avalanche break down) b) Punch through of the lightly doped n - layer. Since the p layers on either side of the n - region have almost equal doping levels the avalanche break down voltage of J 1 & J 2 are almost same. Therefore, the forward and the reverse break down voltage of a thyristor are almost equal.Up to the break down voltage of J 1 the reverse current of the thyristor remains practically constant and increases sharply after this voltage. Thus, the reverse characteristics of a thyristor is similar to that of a single diode. Version 2 EE IIT, Kharagpur 7
Page 1 and 2: Module 1 Power Semiconductor Device
Page 3 and 4: Instructional objects On completion
Page 5: A A p n - G K p n + n + (a) G (b) K
Page 9 and 10: 4.4 Steady State Characteristics of
Page 11 and 12: V g V g max A E c d R g • S 2 P g
Page 13 and 14: Answer: (i) gate; (ii) break down,
Page 15 and 16: I av Amps 120 100 φ = 180° φ = 1
Page 17 and 18: Average Gate Power dissipation (P G
Page 19 and 20: Delay time (t d ): After switching
Page 21 and 22: The reverse recovery charge Q rr is
Page 23 and 24: 1 2 V 2 V V ( ∝) πω π i i H E
Page 25 and 26: G I G MT1 ( - ) I G N 2 I G I G N 3
Page 27 and 28: A 200 Bidirectional ON state curren
Page 29 and 30: vii. viii. To avoid unwanted turn o
Page 31 and 32: Lesson Summary • Thyristor is a f
Page 33 and 34: 1. Explain the effect of increasing
Page 35 and 36: Answers to Practice Problems Versio
Page 37 and 38: Since V g = 10 - i g R is tangent t
Page 39: 5. As soon as THA is turned on the

Characteristics: Triac - nptel

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