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.462Medium
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.463Medium
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.464Medium
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.465Easy
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.
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Q.466Hard
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.467Medium
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.468Easy
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.469Medium
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.470Medium
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.471Hard
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.472Hard
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.473Easy
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.474Medium
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.
Q.475Easy
In a three-phase bridge rectifier, if the input AC voltage is 400V (line), what is the approximate average DC output voltage?
Answer: A
For a three-phase bridge rectifier: Vdc = (3√2/π) × Vline = 1.35 × 400 = 540V approximately.
Q.476Easy
Which of the following devices has the highest switching frequency capability among modern power semiconductors?
Answer: A
MOSFETs have switching frequencies up to several MHz, while IGBTs typically reach 20-50kHz, thyristors <10kHz, and GTOs ~5kHz.
Q.477Medium
In a boost converter operating in continuous conduction mode (CCM) with duty cycle D=0.6, input voltage 24V, and output voltage 60V, what is the efficiency if losses are 5%?
Answer: A
Theoretical efficiency = 95% (ideal boost), minus 5% losses = 90% actual efficiency.
Q.478Medium
A PWM inverter produces an output frequency of 50Hz with a carrier frequency of 10kHz. What is the modulation index if the peak modulating signal is 0.8V and the peak carrier signal is 1V?
Answer: C
Modulation index m = Vm/Vc = 0.18 = 0.8, where Vm is peak modulating signal and Vc is peak carrier signal.
Q.479Medium
In a half-bridge inverter, what is the minimum dead time required to prevent shoot-through if the switching frequency is 20kHz?
Answer: C
Dead time should be 2-3 times the device switching time (~0.5-1μs for modern IGBTs), typically 1-3μs to ensure safe commutation.
Q.480Medium
Which protection method is MOST suitable for protecting a thyristor against voltage transients in industrial applications?
Answer: C
Combined MOV and RC snubber provides both transient suppression (MOV) and dV/dt protection (RC), offering comprehensive protection.
