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.22Medium
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.23Medium
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.24Easy
The maximum frequency of a thyristor is typically limited by:
Answer: B
Turn-off time (tq) determines the minimum period required between successive gate pulses, thus limiting the maximum operating frequency of thyristors.
Q.25Medium
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.
Advertisement
Q.26Hard
The dv/dt rating of a power semiconductor is important because it:
Answer: B
High dv/dt can cause displacement currents through parasitic capacitances, potentially triggering devices unintentionally. dv/dt rating specifies the maximum safe rate of voltage change.
Q.27Easy
A PWM inverter with sinusoidal modulation produces output voltage with:
Answer: B
Sinusoidal PWM produces a voltage waveform containing the fundamental frequency plus high-frequency harmonics that can be filtered out.
Q.28Medium
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.29Medium
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.30Medium
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.31Hard
In a three-level diode-clamped inverter (Neutral Point Clamped), the advantage over two-level inverter is:
Answer: B
Three-level inverter generates three voltage levels, reducing the step size and dv/dt compared to two-level inverter, thus reducing EMI and stress on motor windings.
Q.32Medium
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.33Hard
Which control strategy for a DC-DC converter provides the tightest output voltage regulation under varying input voltage and load conditions?
Answer: B
Closed-loop feedback control with error amplifier and compensation networks provides superior regulation by continuously adjusting duty cycle based on output voltage error.
Q.34Hard
In a synchronous buck converter, the advantages of using a low-side MOSFET instead of a diode are:
Answer: D
Synchronous MOSFETs have lower on-state resistance than diode forward voltage drop, reducing losses. Active switching also allows higher frequency operation with better control.
Q.35Medium
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.36Easy
The presence of freewheeling diode in a DC motor drive is essential because it:
Answer: A
The motor inductance tends to maintain current flow. The freewheeling diode provides a path for this current when the main switch is off, preventing voltage spikes.
Q.37Medium
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.38Medium
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.39Medium
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.
Q.40Easy
In a boost converter operating in continuous conduction mode (CCM), if the duty cycle is 0.5 and input voltage is 12V, what is the output voltage?
Answer: A
For a boost converter, Vo/Vin = 1/(1-D). With D=0.5, Vo = 12/(1-0.5) = 24V
