DE71/DE110 POWER ELECTRONICS DEC 2015

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1 Q.2 a. What is power loss in an ideal switch? Explain the conduction losses in a bipolar junction transistor with the help of circuit diagram. (8) Answer: IETE 1

2 b. Explain, how the power diode must be protected against the following:-(8) (i) Overvoltage (ii) Overcurrent (iii) Transients Answer: IETE 2

3 Q.3 a. How does a Power Bipolar Junction Transistor used as a switch? Draw its V- I characteristics and explain the significance of saturation, active and cut-off regions. (8) Answer: IETE 3

4 IETE 4

5 IETE 5

6 IETE 6

7 b. A Power MOSFET has I DSS = 2 ma, R DS(ON) = 0.3 Ω, duty cycle d = 50%, I D = 6 A, V DS = 100 V, t r = 100 ns and t f = 200 ns. If the frequency of switching is 40 KHz, then find (8) (i) on-state loss (ii) off-state loss (iii) turn-on switching loss (iv) turn-off switching loss Answer: Given Data I DSS = 2 ma, R DS(ON) = 0.3 Ω, duty cycle (d) = 50%, I D = 6 A, VDS = 100 V, t r = 100 ns, t f = 200 ns and frequency of switching(f) = 40 khz 1 1 The Time Period (T) = = = 25 µ S f 3 40(10 ) T Therefore, t ON = t OFF = = 12.5 µ S 2 IETE 7

t (i) On-State Loss (P ON ) = I 2 D R ON DS(ON) = T 6 X 0.3X12.5(10 6 25(10 ) 2 6 ) = 5.4 W. 3) 6 t (ii) Off-State Loss (P OFF ) =V DS(max) I OFF 100X 2(10 X12.

8 t (i) On-State Loss (P ON ) = I 2 D R ON DS(ON) = T 6 X 0.3X12.5( (10 ) 2 6 ) = 5.4 W. 3) 6 t (ii) Off-State Loss (P OFF ) =V DS(max) I OFF 100X 2(10 X12.5(10 ) DSS = 6 T 25(10 ) (iii) Turn-on Switching Loss (P SW(ON) ) = 9 VDS (max) I Dtr 100X 6X100(10 ) 3 X f = X 40(10 ) = 0.4 W. 6 6 (iv) Turn-off Switching Loss (P SW(OFF) ) = 9 VDS (max) I Dt f 100X 6X 200(10 ) 3 X f = X 40(10 ) = 0.8 W. 6 6 = 0.1W Q.4 a. What is the necessity of connecting two SCRs in parallel? Draw the circuit of connecting two SCRs in parallel and explain its operation with the help of onstate characteristics. (9) Answer: IETE 8

9 b. What are the most common methods of achieving commutation? Explain the commutation method by external source and explain its operation with the help of waveforms. (7) Answer: IETE 9

10 1. Capacitor Commutation 2. Commutation by Resonance 3. AC Line Commutation IETE 10

11 Q.5 a. Draw a neat diagram for Single Phase Full Wave Controlled Bridge Rectifier with a resistive load and explain its operation with the help of waveforms. (10) Answer: IETE 11

12 IETE 12

13 IETE 13

14 b. A single phase half wave controlled rectifier connected to a 150 V, 60 Hz source to supplying a resistive load of 10 Ω. If the delay angle α is30, then find: (i) the maximum load current (ii) the average load current (6) Answer: Given Data: VS = 150 V, f = 60 Hz, Resistive Load (R) = 10 Ω and Delay Angle (α ) = 30 Peak Load Voltage = V m = 2 V S = X 150 = 212 V. V (i) Maximum Load Current (I m ) = m 212 = = A R 10 ( I )(1 + cosα) (ii) Average Load Current = m (21.2)(1 + cos30 ) = = 6.3A 2π 2π Q.6 a. Draw a neat diagram for Three Phase Half Wave Controlled Rectifier circuit with a resistive load and explain its operation with waveforms. (10) Answer: IETE 14

15 IETE 15

16 IETE 16

17 b. A six pulse half controlled bridge rectifier is connected to a three phase 220 V AC source. Calculate the firing angle if the terminal voltage of the rectifier is 240 V. What is the maximum value of the DC output voltage? (6) Answer: Given that the terminal voltage of the rectifier (V o(avg.) ) = 240 V and the source voltage V L(S) = 220 V 220 V = 311 V. Therefore, the maximum voltage (V L(m) ) = 2 ( ) L(S ) (i) The formula for finding of Firing Angle ( ) VO( 3. V 2π 240VX 2π 3X 311V Hence ( + ) = = = cosα avg.) L( m) So that ( cos α ) = Therefore, the Firing Angle ( α ) = 52 V = 2 ( ) 3 2π α is ( V ) V ( 1 cosα ) O( avg.) = L( m) + (ii) Maximum value of DC output voltage is obtained with ( α = 0 ) is given by IETE 17

18 3 ( V )( 1+ cosα ) = ( 311)( 1+ 1) V 3 VO(max) = L( m) = 297 2π 2π Q.7 a. What is a DC Chopper? Explain its principle with the help of suitable diagram and waveforms. What are its various industrial applications? (8) Answer: IETE 18

19 IETE 19

20 IETE 20

21 b. What is a Buck-Boost Chopper? Draw its circuit configuration and explain its working with the help of voltage and current waveforms. (8) Answer: IETE 21

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24 IETE 24

25 Q.8 a. Explain the working of a full bridge VSI with a neat circuit diagram and waveforms. (8) Answer: IETE 25

26 IETE 26

27 IETE 27

28 b. What are the most commonly used methods of pulse width modulation? Explain multiple pulse width modulation with the help of waveforms when the number of pulses (m) = 2 and 3. (8) Answer: IETE 28

29 IETE 29

30 Q.9 a. What do you mean by AC power control? Discuss the differences between integral cycle control and AC phase control. (8) Answer: IETE 30

31 IETE 31

32 IETE 32

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34 IETE 34

35 b. Compare the advantages and disadvantages of semiconductor switches over mechanical switches. (8) Answer: IETE 35

36 IETE 36

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40 TEXT BOOK I. Power Electronics for Technology, First Impression (2006), Ashfaq Ahmed, Purdue University - Calumet, Pearson Education IETE 40

(a) average output voltage (b) average output current (c) average and rms values of SCR current and (d) input power factor. [16]

(a) average output voltage (b) average output current (c) average and rms values of SCR current and (d) input power factor. [16] Code No: 07A50204 R07 Set No. 2 1. A single phase fully controlled bridge converter is operated from 230 v, 50 Hz source. The load consists of 10Ω and a large inductance so as to reach the load current