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.5Medium
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.
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Q.6Medium
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.7Medium
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.8Medium
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.9Medium
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).
Q.10Medium
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.11Easy
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.
Q.12Medium
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.13Medium
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.14Hard
In a three-phase inverter, 180° conduction mode means:
Answer: A
In 180° conduction, each switch conducts for half the fundamental cycle (180°), typically producing a 6-step output. 120° conduction has each switch on for 120° only.
Q.15Hard
Which of the following has the lowest on-state voltage drop among power semiconductor devices?
Answer: B
MOSFETs have the lowest on-state voltage drop (few hundred millivolts) at moderate currents. IGBTs: ~1-2V, SCRs: ~1-2V, Diodes: ~0.7-1V depending on rating.
Q.16Hard
The snubber circuit in power electronics is used to:
Answer: A
Snubber circuits (RC networks) protect switching devices from transient voltage spikes during switching transitions and reduce switching losses.
Q.17Hard
In a soft-start thyristor converter, the firing angle is gradually increased to:
Answer: A
Soft-start increases firing angle gradually from maximum value, reducing dv/dt and inrush current, protecting equipment and supply system from transients.
Q.18Hard
The turn-off time (toff) of a power transistor includes:
Answer: B
Turn-off time (toff) = Storage time (ts) + Fall time (tf). Storage time is delay before current starts decreasing; fall time is the time for current to reach zero.
Q.19Hard
In a boost converter operating in continuous conduction mode (CCM), the average output voltage relationship is:
Answer: B
Boost converter output voltage: Vo = Vin / (1 - D), where D is duty cycle. This always produces Vo > Vin for 0 < D < 1.
Q.20Easy
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.
