Which of the following is a unidirectional device in power electronics?
Answer: D
Both diodes and thyristors are unidirectional devices that conduct current in only one direction. MOSFETs are bidirectional.
Q.2Easy
The forward voltage drop across a silicon diode at room temperature is approximately:
Answer: B
Silicon diodes have a typical forward voltage drop of 0.6-0.7 V at room temperature. Germanium diodes have ~0.3 V.
Q.3Easy
Which device is used to convert DC to AC power?
Answer: B
An inverter converts DC power to AC power. Rectifiers do the opposite (AC to DC). Choppers convert DC to DC.
Q.4Easy
The peak inverse voltage (PIV) rating of a diode is important to prevent:
Answer: B
PIV rating defines the maximum reverse voltage a diode can withstand without breaking down. Exceeding this causes reverse breakdown and device failure.
Q.5Easy
In MOSFET, the gate-source voltage at which current starts to flow is called:
Answer: B
Threshold voltage (Vth) is the gate-source voltage required to create an inversion layer and allow current flow in MOSFET.
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Q.6Easy
In a buck converter, the output voltage is always:
Answer: A
A buck converter steps down the input voltage. The output voltage Vo = D×Vin where D is the duty cycle (0 < D < 1), making output always less than input.
Q.7Easy
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.8Easy
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.9Easy
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.10Easy
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
Q.11Easy
Which of the following devices is a unidirectional switch in power electronics?
Answer: A
A diode conducts only in one direction (forward bias), making it a unidirectional switch
Q.12Easy
In a half-bridge inverter, what is the minimum number of switches required to generate AC output from DC source?
Answer: A
A half-bridge inverter requires 2 switches (typically with 2 diodes) to generate AC output from a DC source
Q.13Easy
What is the primary purpose of a freewheeling diode in a buck converter?
Answer: B
The freewheeling diode provides a continuous path for the inductor current during the switch OFF period, preventing voltage spikes
Q.14Easy
In a three-phase full-bridge inverter, the total number of thyristors required is:
Answer: C
A three-phase full-bridge inverter requires 6 thyristors (2 per phase) to generate three-phase AC output
Q.15Easy
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)
Q.16Easy
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.17Easy
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.18Easy
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.19Easy
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.20Easy
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.
