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Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE) PDF Download

Q1: If the following switching devices have similar power ratings, which one of them is the fastest?      (2024)
(a) SCR
(b) GTO
(c) IGBT
(d) Power MOSFET
Ans: 
(d)

Q2: A semiconductor switch needs to block voltage V of only one polarity (V > 0) during OFF state as shown in figure (i) and carry current in both directions during ON state as shown in figure (ii). Which of the following switch combination(s) will realize the same?         (2023)
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)(a) Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)(b) Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)(c) Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)(d) Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)Ans:
(a, d)
Sol: I-V characteristic of given switch :
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)this characteristic shows by options (A) and (D) switch.

Q3: A charger supplies 100 W at 20 V for charging the battery of a laptop. The power devices, used in the converter inside the charger, operate at a switching frequency of 200 kHz. Which power device is best suited for this purpose?       (2022)
(a) IGBT
(b) Thyristor
(c) MOSFET
(d) BJT
Ans: 
(c)
Sol: Ratings of different power devices:
BJT : 1200 V, 800 A, (10 - 20)kHz
SCR : 10000 V, 3000 A
IGBT : 1200 V, 500 A, 50 kHz
MOSFET : 500 V, 140 A, 1 MHz

Q4: A resistor and a capacitor are connected in series to a 10 V dc supply through a switch. The switch is closed at t = 0, and the capacitor voltage is found to cross 0 V at  t = 0.4τ, where τ is the circuit time constant. The absolute value of percentage change required in the initial capacitor voltage if the zero crossing has to happen at t = 0.2τ is _______ (rounded off to 2 decimal places).      (2020)
(a) 24.24
(b) 78.83
(c) 12.45
(d) 54.99
Ans: 
(d)
Sol: If initial charge polarities on the capacitor is opposite to the supply voltage then only the capacitor voltage crosses the zero line.
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)
Q5: A non-ideal diode is biased with a voltage of -0.03 V, and a diode current of I1 is measured. The thermal voltage is 26 mV and the ideality factor for the diode is 15/13. The voltage, in V, at which the measured current increases to 1.5 I1 is closest to:       (2020)
(a) -0.02
(b) -0.09
(c) -1.5
(d) -4.5
Ans: 
(b)
Sol: Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)
Q6: A single-phase inverter is fed from a 100 V dc source and is controlled using a quasisquare wave modulation scheme to produce an output waveform, v(t). as shown. The angle σ is adjusted to entirely eliminate the 3rd harmonic component from the output voltage. Under this condition, for v(t), the magnitude of the 5th harmonic component as a percentage of the magnitude of the fundamental component is _______(rounded off to 2 decimal places).        (2020)
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)

(a) 15
(b) 10
(c) 20
(d) 25
Ans:
(c)
Sol: Using result,
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)
Q7: Four power semiconductor devices are shown in the figure along with their relevant terminals. The device(s) that can carry dc current continuously in the direction shown when gated appropriately is (are)      (2018)
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)(a) Triac only
(b) Triac and MOSFET
(c) Triac and GTO
(d) Thyristor and Triac
Ans:
(b)

Q8: For the power semiconductor devices IGBT, MOSFET, Diode and Thyristor, which one of the following statements is TRUE?      (SET-1 (2017))
(a) All of the four are majority carrier devices.
(b) All the four are minority carrier devices
(c) IGBT and MOSFET are majority carrier devices, whereas Diode and Thyristor are minority carrier devices.
(d) MOSFET is majority carrier device, whereas IGBT, Diode, Thyristor are minority carrier devices.
Ans:
(d)

Q9: The voltage (vs) across and the current (is) through a semiconductor switch during a turn-ON transition are shown in figure. The energy dissipated during the turn-ON transition, in mJ, is _______.       (SET-1 (2016))
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)(a) 25
(b) 50
(c) 75
(d) 100
Ans:
(c)
Sol: Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)
Q10: A steady dc current of 100 A is flowing through a power module (S,D) as shown in Figure (a). The V-I characteristics of the IGBT (S) and the diode (D) are shown in Figures (b) and (c), respectively. The conduction power loss in the power module (S,D), in watts, is ________.      (SET-1  (2016))
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)(a) 100
(b) 135
(c) 240
(d) 170
Ans:
(d)
Sol: No current flows through IGBT. So current flows only in diode. Equivalent circuit of diode is as shown below,
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)Power loss during conduction,
P = V x I
= 1.7 x 100 = 170 W

Q11: The circuit shown is meant to supply a resistive load RL from two separate DC voltage sources. The switches S1 and S2 are controlled so that only one of them is ON at any instant. S1 is turned on for 0.2 ms and S2 is turned on for 0.3 ms in a 0.5 ms switching cycle time period. Assuming continuous conduction of the inductor current and negligible ripple on the capacitor voltage, the output voltage V0 (in Volt) across RL is ___________.       (SET-1 (2015))
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)(a) 3
(b) 7
(c) 9
(d) 11
Ans:
(b)
Sol: Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)S1 is turn ON for 0.2 ms
S2 is turn ON for 0.3 ms.
Switching cycle time period is 0.5 ms.
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)the output voltage V0 across RL is  
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)
Q12: Figure shows four electronic switches (i), (ii), (iii) and (iv). Which of the switches can block voltages of either polarity (applied between terminals 'a' and 'b') when the active device is in the OFF state?       (SET-1  (2014))
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)(a) (i), (ii) and (iii)
(b) (ii), (iii) and (iv)
(c) (ii) and (iii)
(d) (i) and (iv)
Ans:
(c)
Sol: Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)As given that the active device is in OFF state it means the device can block the voltage when 'a' is positive with respect to 'b' and when 'b' is positive with respect to 'a' then the diode is reverse baised and voltage is blocked.
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)When 'a' is positive with respect to 'b', as given the device is OFF so it will block the voltage.
When 'b' is positive with respect to 'a' it will block the voltage.
Where as the other two devices will conduct when 'b' is positive with respect to 'a'.

Q13: The typical ratio of latching current to holding current in a 20 A thyristor is       (2012)
(a) 5
(b) 2
(c) 1
(d) 0.5
Ans:
(b)
Sol: For medium power thyristor of rating 6 A to 60 A the ratio of the latching current to holding current is 1.5 to 2.

Q14: A voltage commutated chopper circuit, operated at 500 Hz, is shown below.
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)If the maximum value of load current is 10 A, then the maximum current through the main (M) and auxiliary (A) thyristors will be       (2011)
(a) iMmax=12AiMmax = 12A and iAmax =10A
(b) iMmax = 12A and iAmax = 2A
(c) iMmax=10AiMmax = 10A and iAmax = 12A
(d) iMmax=10AandiAmax=8AiMmax = 10A and iAmax = 8A
Ans:
(a)
Sol: When main thyristor (M) is turned on, an oscillatory current in the circuit C, M, L and diode is set up and it is given by Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)
Peak value of current through capacitor
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)Current through main thyristor
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)So maximum value of Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)
When auxiliary thyristor (A) is turned on, capacitor voltage applies a reverse voltage across main thyristor and main thyristor is turned off. The load current is now carried by C nad auxiliary thyristor . Current through auxiliary thyristor,
iA = I0
maximum value of i= maximum value of I= 10A.

Q15: Circuit turn-off time of an SCR is defined as the time      (2011)
(a) taken by the SCR turn to be off
(b) required for the SCR current to become zero
(c) for which the SCR is reverse biased by the commutation circuit
(d) for which the SCR is reverse biased to reduce its current below the holding current
Ans:
(c)
Sol: It is defined as the time between the instant anode current becomes zero and the instant reverse voltage is applied across SCR.

Q16: Figure shows a composite switch consisting of a power transistor (BJT) in series with a diode. Assuming that the transistor switch and the diode are ideal, the I-V characteristic of the composite switch is    (2010)
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)(a)Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)(b) Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)(c)Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)(d) Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)Ans: 
(c)
Sol: BJT blocks positive voltage and diode blocks negative voltage and both devices aloows positive current.

Q17: Match the switch arrangements on the top row to the steady-state V - I characteristics on the lower row. The steady state operating points are shown by large black dots.     (2009)
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)(a) A-I, B-II, C-III, D-IV
(b) A-II, B-IV, C-I, D-III
(c) A-IV, B-III, C-I, D-II
(d) A-IV, B-III, C-II, D-I
Ans:
(c)
Sol: Device-A:
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)When diode is forward baised (ON state),  Vs = 0, is > 0
diode is reversed baised (OFF state) Vs < 0, is = 0:
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)
when thyristor is in reverse blocking mode,  Vs < 0, is = 0
thyristor is in forward blocking, Vs > 0, is = 0
thyristor is in forward conduction mode, Vs = 0, is > 0  
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)When device is ON, Vs = 0, is > 0
The device is OFF, Vs > 0, is = 0
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)Reverse current can flow through the diode so  is < 0 and Vs = 0
During ON state of the device Vs = 0, is > 0
During OFF state of the device,  Vs = 0, is = 0  
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)
Q18: The circuit shows an ideal diode connected to a pure inductor and is connected to a purely sinusoidal 50 Hz voltage source. Under ideal conditions the current waveform through the inductor will look like.      (2009)
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)(a) Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)(b) Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)(c) Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)(d) Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)Ans:
(c)
Sol: Frequency of the voltage source, f = 50Hz
Time period, Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)
During positive half cycle of the source voltage,  0 < t < T/2, energy is stored in the inductor and current increases.
During negative half cycle of the source voltage,  T/2 ≤ t ≤ T, current decreases and energy stored in the inductor is delivered to source.

Q19: An SCR is considered to be a semi-controlled device because      (2009)
(a) It can be turned OFF but not ON with a gate pulse.
(b) It conducts only during one half-cycle of an alternating current wave
(c) It can be turned ON but not OFF with a gate pulse.
(d) It can be turned ON only during one half-cycle of an alternating voltage
Ans: 
(c)
Sol: During one half cycle, SCR can be in forward blocking mode and by applying gate pulse, the SCR operates in forward conduction mode (ON state).
But SCR can not be turned OFF by applying gate pulse.

Q20: A 1:1 Pulse Transformer (PT) is used to trigger the SCR in the below figure. The SCR is rated at 1.5 kV, 250 A with I= 250 mA,  I= 150 mA, and IGmax = 150 mA, IGmin = 100 mA. The SCR is connected to an inductive load, where L = 150 mH in series with a small resistance and the supply voltage is 200 V dc. The forward drops of all transistors/diodes and gate-cathode junction during ON state are 1.0 V
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)The minimum approximate volt-second rating of pulse transformer suitable for triggering the SCR should be : (volt-second rating is the maximum of product of the voltage and the width of the pulse that may applied)         (2007)
(a) 2000 μV-s
(b) 200 μV-s
(c) 20 μV-s
(d) 2 μV-s
Ans:
(a)
Sol: Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)Forward voltage drop of SCR during ON-state
VT = 1V
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)Gate pulse width required = time taken by i_a to rise up to
IL = T
Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE)

The document Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 | Power Electronics - Electrical Engineering (EE) is a part of the Electrical Engineering (EE) Course Power Electronics.
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FAQs on Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 1 - Power Electronics - Electrical Engineering (EE)

1. What are the main types of power semiconductor devices used in electrical engineering?
Ans. The main types of power semiconductor devices include diodes, transistors (such as BJTs, MOSFETs, and IGBTs), thyristors, and SCRs (Silicon Controlled Rectifiers). Each of these devices has unique characteristics that make them suitable for different applications, such as switching, rectification, and voltage regulation.
2. How do IGBT devices differ from MOSFETs in power electronics applications?
Ans. IGBTs (Insulated Gate Bipolar Transistors) combine the high input impedance of MOSFETs with the high current and voltage handling capabilities of BJTs. They are typically used in high power applications due to their better efficiency and lower conduction losses at high voltages, while MOSFETs are preferred for low to medium power applications due to their faster switching speeds.
3. What is commutation in power electronics, and why is it important?
Ans. Commutation is the process of turning off a conducting power semiconductor device and ensuring that it can switch to a non-conducting state. It is important because it affects the performance of converters and inverters, influencing efficiency, switching losses, and the overall stability of power electronic systems.
4. What are the different commutation techniques used for thyristors?
Ans. The main commutation techniques for thyristors include natural commutation, forced commutation, and resonant commutation. Natural commutation relies on the AC waveform to turn off the device, while forced commutation uses external circuits to provide the necessary voltage or current to turn off the thyristor. Resonant commutation involves the use of resonant circuits to achieve turn-off.
5. What are the key parameters to consider when selecting a power semiconductor device for a circuit?
Ans. Key parameters include maximum voltage rating (Vmax), maximum current rating (Imax), on-state voltage drop (Von), switching speed, thermal resistance, and power dissipation capability. Additionally, factors like gate drive requirements, cost, and reliability should also be considered based on the specific application.
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