Hamdard University – Islamabad Campus
Hamdard Institute of Engineering & Technology
Department of Electrical Engineering
Course Title: Power Electronics (Theory)
Exam: Mid-Term
Maximum Marks: 20
Course Instructor: Engr. Muhammad Abdullah Umar
Semester: Fall, 2020
Date: 04 January, 2021
Time: 48 hours
Note: Attempt all questions
(CLO_1): (Cognitive Level C2, i.e., Understanding) (PLO_1, i.e., Engineering Knowledge)
Question No. 1: (02 Marks)
Describe the two protection methods for gate-pulse generating circuits against induced voltage
transients in thyristor based circuits.
(CLO_1): (Cognitive Level C2, i.e., Understanding) (PLO_1, i.e., Engineering Knowledge)
Question No. 2: (02 Marks)
Explain the current-time waveforms of soft-recovery and abrupt-recovery power diodes. Plot the
neat and clean current-time waveforms by-hand on graph paper with proper labeling and your
name on top-right corner of graph, for following AC diode parameters in waveforms.
i- Reverse recovery time
ii- Reverse recovery current
(CLO_2): (Cognitive Level C4, i.e., Analyzing) (PLO_2, i.e., Problem Analysis)
Question No. 3: (02 Marks)
Draw the waveforms for load voltage, load current and voltage across the SCR for half-wave
controlled rectifier for following cases. Plot the neat and clean waveforms by-hand on graph
paper with proper labeling and your name on top-right corner of graph.
i-A resistive load with firing angle of 45o
ii-An inductive load with firing angle of 90o
(CLO_2): (Cognitive Level C4, i.e., Analyzing) (PLO_2, i.e., Problem Analysis)
Question No. 4: (02 Marks)
Draw the waveforms for load voltage, load current and voltage across the SCR for full-wave
controlled bridge rectifier for following cases. . Plot the neat and clean waveforms by-hand on
graph paper with proper labeling and your name on top-right corner of graph.
i-A resistive load with firing angle of 60o
ii-An inductive load with firing angle of 120o
(CLO_2): (Cognitive Level C4, i.e., Analyzing) (PLO_2, i.e., Problem Analysis)
Question No. 5: (02 Marks)
Draw the waveforms for load voltage, load current and voltage across the two SCR’s for fullwave center-tap rectifier for following cases. . Plot the neat and clean waveforms by-hand on
graph paper with proper labeling and your name on top-right corner of graph.
i-A resistive load with firing angle of 45o
ii-An inductive load with firing angle of 110o
(CLO_3): (Cognitive Level C3, i.e., Applying) (PLO_2, i.e., Problem Analysis)
Question No. 6: (02 Marks)
Consider the following diode-based circuit where load current of 20 A is flowing through the
freewheeling diode Dm and switch S1 is closed at t=0, calculate the expression for current
through the switch, when R =20 ohms, L= 4mH and Vs = 200V. (02 Marks)
(CLO_3): (Cognitive Level C3, i.e., Applying) (PLO_2, i.e., Problem Analysis)
Question No. 7: (02 Marks)
An IGBT switch controls the flow of power to a 30 Ohms resistive load. The DC source Voltage
Vs = 400V,VCE(sat) = 1.5 V, the switching frequency = 2 kHz, tON = 10ns and duty cycle = 0.5.
Calculate the following:
i-The IGBT minimum current rating
ii-The on-state power loss
iii-The turn-on power loss
(CLO_3): (Cognitive Level C3, i.e., Applying) (PLO_2, i.e., Problem Analysis)
Question No. 8: (02 Marks)
A single Phase bridge rectifier has a pure resistive load , R = 50 ohms, the peak value of supply
voltage is 150V, the supply frequency is 60 Hz, Find the average output voltage of rectifier if
source inductance is negligible.
(CLO_3): (Cognitive Level C3, i.e., Applying) (PLO_2, i.e., Problem Analysis)
Question No. 9: (02 Marks)
If the peak value of input voltage for a single phase rectifier with RL load in the figure below is
Vm = 150V, the supply frequency, f= 60 Hz, and load resistance R = 30 Ohms, determine the
values of load inductance to limit load current to 5% of the average value Idc.
(CLO_3): (Cognitive Level C3, i.e., Applying) (PLO_2, i.e., Problem Analysis)
Question No. 10: (02 Marks)
A MOSFET has the following parameters:
IDSS=2mA, RDS(on)=0.3Ohms , duty cycle d = 50%, ID = 5 A, VDS = 120 V, tr = 120 ns, and tf=
250 ns. If the frequency of switching is 100 kHz, find the total power loss.