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.402Medium
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.403Medium
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.404Medium
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.405Easy
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.406Medium
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.407Medium
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.408Hard
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.409Hard
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.410Hard
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.411Hard
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.412Hard
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.413Hard
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.414Easy
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.415Medium
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.416Medium
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.417Medium
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.418Easy
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.419Medium
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.420Hard
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.
