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.222Medium
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.223Medium
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.224Medium
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.225Medium
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
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Q.226Medium
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.227Medium
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.228Medium
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.229Medium
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.230Medium
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?
In a boost converter with duty cycle D = 0.4, input voltage Vi = 48V, and output voltage Vo = 80V, what is the voltage stress across the switch?
Answer: A
In a boost converter, voltage stress on switch = Vo = 80V. The diode experiences stress of (Vo - Vi) = 32V.
Q.232Medium
Which of the following topologies is most suitable for soft-switching operation in modern DC-DC converters?
Answer: B
Resonant converters inherently provide soft-switching conditions through LC resonance, reducing switching losses and EMI compared to hard-switching topologies.
Q.233Medium
In an IGBT, the latching current phenomenon occurs when:
Answer: B
IGBTs contain a parasitic thyristor structure. When collector current becomes too high, this thyristor can turn on, causing the device to latch and potentially causing permanent damage.
Q.234Medium
In a three-phase VSI (Voltage Source Inverter), if the modulation index (Ma) is 0.8 and DC link voltage is 400V, what is the peak phase voltage?
Answer: B
Peak phase voltage = (Ma × Vdc)/2 × √3 for three-phase. Vpeak = 0.8 × 2400 × √3/√3 ≈ 184.8V (considering line-to-neutral relationship)
Q.235Medium
Which PWM technique provides the lowest THD in the output voltage of an inverter?
Answer: C
Space Vector Modulation provides superior harmonic performance with THD approximately 30% lower than SPWM and enables higher DC utilization (15.5% improvement in voltage utilization).
Q.236Medium
In a step-down DC-DC converter (buck), the inductor current ripple is ΔI = 2A at 100 kHz switching frequency. If D = 0.5 and Vi = 24V, what is the approximate inductance value?
Answer: A
For buck converter: L = (Vi × D × (1-D))/(f × ΔI) = (24 × 0.5 × 0.5)/(100×10³ × 2) = 120 μH
Q.237Medium
Which cooling method is most effective for high-power semiconductor modules operating above 5 kW?
Answer: C
Liquid cooling provides superior heat transfer coefficient (500-5000 W/m²K) compared to forced air (50-500 W/m²K), essential for high-power applications with stringent thermal requirements.
Q.238Medium
The power factor of a thyristor-based rectifier with firing angle 60° is approximately:
Answer: C
Power factor = cos(α) = cos(60°) = 0.5 for fully controlled single-phase rectifier. For 3-phase, it's slightly higher at approximately 0.67 considering the waveform.
Q.239Medium
In a current-source inverter (CSI), the DC link component is typically:
Answer: B
CSI uses a large inductor in series with the DC source to maintain nearly constant current. This differs from VSI which uses a capacitor for constant voltage.
Q.240Medium
A thyristor has a forward breakover voltage of 800V and a holding current of 200mA. What is the minimum gate current required to trigger the device if the gate sensitivity is 40mA/V?
Answer: B
Gate current = Gate sensitivity × Gate voltage. With 800V forward breakover and 40mA/V sensitivity, minimum gate current ≈ 20mA for reliable triggering.
