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.442Medium
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.443Hard
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.444Medium
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.445Easy
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)
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Q.446Medium
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.447Medium
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.448Hard
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.449Medium
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.450Hard
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.451Easy
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.452Hard
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.453Medium
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.454Medium
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?
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.456Medium
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.457Easy
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.458Hard
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.459Medium
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.460Medium
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.
