Which power semiconductor device is best suited for high-frequency switching applications above 100 kHz?
Answer: B
MOSFETs have switching frequencies up to several MHz, making them ideal for high-frequency applications. Thyristors are limited to lower frequencies (few kHz).
Q.62Medium
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.63Easy
The holding current (Ih) specification of a thyristor is important to ensure:
Answer: B
Holding current is the minimum anode current required to keep the thyristor in ON state after the gate signal is removed. If anode current falls below Ih, the device turns OFF.
Q.64Hard
A single-phase half-wave rectifier with a firing angle of 45° is connected to a 230V, 50Hz AC source. Calculate the RMS output voltage.
Answer: D
For half-wave rectifier: Vrms = (Vm/2)√[(π - α + sin(2α))/(2π)] where α = 45° = π/4, Vm = 230√2 = 325.3V. Vrms ≈ 108.6V
Q.65Medium
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.
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Q.66Medium
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.67Easy
What is the purpose of a freewheeling diode in a converter circuit?
Answer: B
A freewheeling diode provides a discharge path for inductor current when the main switch opens, preventing voltage spikes and ensuring continuous current flow in inductive circuits.
Q.68Medium
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.69Medium
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.70Medium
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.71Easy
What does the snubber circuit in a power converter primarily protect against?
Answer: B
Snubber circuits limit the rate of change of voltage (dv/dt) across semiconductor switches during switching transients, preventing false triggering and overvoltage damage.
Q.72Hard
In a controlled rectifier circuit, the commutation overlap angle (μ) depends on:
Answer: B
Commutation overlap occurs due to finite source impedance. The overlap angle μ = sin⁻¹(ωLₛI/√2×Vline), depending on source inductance and current.
Q.73Medium
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.74Easy
In a full-bridge DC-DC converter, when both upper switches conduct simultaneously, what occurs?
Answer: B
Simultaneous conduction of upper switches creates a direct short across the DC link, causing massive current spike and potential device failure. This is prevented using dead-time insertion.
Q.75Medium
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.76Medium
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.77Hard
Which parameter determines the minimum OFF-time in a PWM converter?
Answer: B
Minimum OFF-time must account for semiconductor recovery characteristics: tOFF(min) = ts + tf, allowing complete device turn-off before the next cycle.
Q.78Hard
For an AC voltage controller with RMS output voltage of 230V from a 230V, 50Hz source at 30° firing angle, what is the power factor?
Answer: A
For AC voltage controller: PF = (1/π)√(π² - 4α²) × sin(2α)/(2α) where α = 30° = π/6. This yields PF ≈ 0.87 with leading reactive current.
Q.79Easy
In modern 2024-25 EV charging systems, what is the typical efficiency target for on-board DC-DC converters?
Answer: B
Modern automotive DC-DC converters target 90-95% efficiency using advanced topologies, SiC MOSFETs, and optimized gate drives to minimize losses in 12V auxiliary systems.
Q.80Medium
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.
