Which of the following devices is a unidirectional switch in power electronics?
Answer: A
A diode conducts only in one direction (forward bias), making it a unidirectional switch
Q.42Medium
The switching frequency of a PWM inverter is 10 kHz. What is the fundamental frequency of the output if the modulation index is 0.8?
Answer: D
The output fundamental frequency is typically half the switching frequency in standard PWM, or determined by reference signal. With modulation, output frequency follows the reference signal, typically much lower than switching frequency.
Q.43Easy
In a half-bridge inverter, what is the minimum number of switches required to generate AC output from DC source?
Answer: A
A half-bridge inverter requires 2 switches (typically with 2 diodes) to generate AC output from a DC source
Q.44Medium
Which modulation technique provides the best harmonic performance in high-power applications?
Answer: B
SVPWM provides superior harmonic performance, higher DC link utilization (~15% more), and better efficiency in high-power applications compared to SPWM
Q.45Easy
What is the primary purpose of a freewheeling diode in a buck converter?
Answer: B
The freewheeling diode provides a continuous path for the inductor current during the switch OFF period, preventing voltage spikes
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Q.46Easy
In a three-phase full-bridge inverter, the total number of thyristors required is:
Answer: C
A three-phase full-bridge inverter requires 6 thyristors (2 per phase) to generate three-phase AC output
Q.47Medium
What is the conduction angle of each thyristor in a three-phase full converter operating at 60° firing angle?
Answer: B
In a three-phase full converter, each thyristor conducts for 120° (π/3 radians) regardless of the firing angle within the commutation period
Q.48Medium
The turn-off time of an IGBT at 25°C is typically in the range of:
Answer: A
Modern IGBTs have turn-off times in the range of 100-500 nanoseconds depending on the device rating and switching conditions
Q.49Hard
In a forward converter, the energy stored in the magnetizing inductance during the ON time is transferred to:
Answer: C
In a forward converter, when the switch turns OFF, the magnetizing inductance energy is transferred back to the input source through the demagnetization winding
Q.50Medium
What is the typical efficiency range of modern power MOSFETs in switching applications?
Answer: C
Modern power MOSFETs in well-designed switching converters achieve efficiency in the range of 90-98%, depending on operating point and circuit design
Q.51Easy
In a buck-boost converter, the polarity of output voltage compared to input is:
Answer: B
A buck-boost converter produces an output voltage of opposite polarity to the input voltage, with magnitude |Vo| = Vin × D/(1-D)
Q.52Medium
The dv/dt rating of a thyristor is critical because:
Answer: B
If dv/dt exceeds the rated value, capacitive current through the junction can turn on the thyristor prematurely, causing malfunction
Q.53Medium
In a resonant DC-DC converter, what is the primary advantage over PWM converters?
Answer: C
Resonant converters achieve zero-voltage or zero-current switching, significantly reducing switching losses and electromagnetic interference compared to PWM converters
Q.54Hard
The average output voltage of a three-phase half-wave uncontrolled rectifier is:
Answer: A
For a three-phase half-wave uncontrolled rectifier, Vdc = (3√23π) × Vm ≈ 0.827 × Vm
Q.55Medium
Which of the following is NOT a source of harmonic distortion in power converters?
Answer: C
Ambient temperature affects thermal behavior but does not directly cause harmonic distortion. Harmonics arise from non-linear switching, circuit inductances, and device mismatches.
Q.56Hard
In a synchronous rectifier design, the gate drive signal for the MOSFET switch should be synchronized with:
Answer: D
Synchronous rectifier gate drive must be timed with the primary switch to replace the body diode conduction, typically during the primary switch OFF period
Q.57Easy
What is the maximum theoretical efficiency of a step-down chopper with duty cycle 0.5?
Answer: C
In an ideal chopper, theoretical efficiency is 100% as it transfers energy without dissipation. Practical efficiency is reduced by component losses
Q.58Hard
The phenomenon of 'latch-up' in power semiconductor devices occurs due to:
Answer: B
Latch-up in CMOS and power devices occurs when parasitic p-n-p and n-p-n transistors form a regenerative feedback loop, causing high current and potential device destruction
Q.59Medium
In soft-switching topologies, Zero Current Switching (ZCS) means switching occurs when:
Answer: B
ZCS occurs when the device current reduces to zero before switching, eliminating switching losses due to current commutation
Q.60Medium
In a three-phase fully controlled bridge rectifier operating at 50 Hz with a firing angle of 60°, what is the average output voltage if the RMS line-to-line voltage is 415V?
