A single-phase full-wave bridge rectifier with a 230V AC input produces an average output voltage of approximately:
Answer: C
For a full-wave rectifier: Vdc = (2√2/π) × Vrms = 0.9 × Vrms = 0.9 × 230 = 207 V approximately.
Q.2Medium
Which of the following thyristors can be turned off by a gate signal?
Answer: B
GTO can be turned on and off by gate signals. SCR can only be turned on by gate signal; turn-off requires anode current to become zero.
Q.3Medium
In a step-down DC-DC converter (buck converter), the output voltage is always:
Answer: B
A buck converter reduces voltage. Output voltage Vo = D × Vin, where D is duty cycle (0 < D < 1). For step-up, use boost converter.
Q.4Medium
The switching frequency of power electronic devices is increased to:
Answer: B
Higher switching frequency allows use of smaller inductors and capacitors, reducing converter size. Trade-off is increased switching losses.
Q.5Medium
A 3-phase diode rectifier (6-pulse) produces a ripple frequency of:
Answer: C
For a 3-phase 6-pulse rectifier: Ripple frequency = 6 × supply frequency = 6 × 50 = 300 Hz (or 6 × 60 = 360 Hz for 60 Hz supply).
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Q.6Medium
The holding current of an SCR is important because:
Answer: B
Holding current (Ih) is the minimum anode current below which SCR turns off. If Ia < Ih, SCR will not remain in conducting state.
Q.7Medium
A PWM (Pulse Width Modulation) controlled DC motor drive uses which power electronic device?
Answer: C
PWM control of DC motors uses switching devices like MOSFETs or IGBTs in chopper configuration to vary average voltage and control speed.
Q.8Medium
The commutation of a thyristor in a circuit means:
Answer: C
Commutation is the process of turning off a thyristor by reducing anode current to below holding current value (natural commutation in AC circuits or forced commutation in DC).
Q.9Medium
The reverse recovery time of a diode is critical in power electronics because it:
Answer: D
Reverse recovery time affects switching frequency limits, causes switching losses, and creates EMI. Fast recovery diodes are preferred in high-frequency applications.
Q.10Medium
Which of the following devices can block voltage in both forward and reverse directions?
Answer: C
GTO can be turned on and off by gate control and blocks voltage bidirectionally when off, making it suitable for AC circuit applications.
Q.11Medium
In a flyback converter, the energy transfer occurs during:
Answer: B
In flyback converters, energy is stored in the transformer core during ON time and transferred to the output during OFF time through the secondary winding.
Q.12Medium
In a three-phase bridge inverter with 120° conduction mode, each switch conducts for:
Answer: B
In 120° conduction mode, each IGBT or switch conducts for 120° of the fundamental frequency cycle, reducing harmonics compared to 180° mode.
Q.13Medium
In an induction motor fed by a variable frequency drive (VFD), the V/f ratio is maintained constant to:
Answer: B
Maintaining constant V/f ratio ensures the magnetic flux in the motor remains constant across the speed range, preventing core saturation and maintaining torque capability.
Q.14Medium
The parasitic capacitance in a power MOSFET causes:
Answer: B
The gate-source (Cgs), gate-drain (Cgd), and drain-source (Cds) capacitances require energy to charge/discharge, causing switching losses and limiting frequency.
Q.15Medium
A series RLC filter connected at the output of an inverter serves to:
Answer: B
Output LC filters attenuate high-frequency switching harmonics and reduce voltage rise time (dv/dt), protecting connected equipment like motors.
Q.16Medium
The snubber capacitor in a power converter acts to:
Answer: B
Snubber capacitors provide a path for displacement current during switching transitions, clamping the voltage and reducing dv/dt stress on semiconductor devices.
Q.17Medium
The phenomenon of commutation in a thyristor-based rectifier circuit refers to:
Answer: A
In line-commutated converters, commutation occurs naturally due to phase voltage transitions. Thyristor turns off when the anode voltage becomes negative relative to cathode.
Q.18Medium
In a multilevel inverter topology, the total harmonic distortion (THD) compared to a two-level inverter is:
Answer: B
Multilevel inverters generate more voltage levels, creating a staircase waveform closer to sinusoid, resulting in lower THD and fewer harmonics compared to two-level inverters.
Q.19Medium
The thermal resistance (Rtheta-JA) of a power semiconductor determines:
Answer: C
Thermal resistance is the temperature difference per unit power dissipation: ΔT = Rθ × P. Lower thermal resistance means better heat transfer and lower junction temperature.
Q.20Medium
In a current-source inverter (CSI), the DC link contains:
Answer: B
CSI uses an inductor in the DC link to maintain a relatively constant current source, in contrast to VSI (Voltage Source Inverter) which uses a capacitor for constant voltage.
