Electrical Engineering questions for GATE, PSU recruitment and SSC JE draw from network theory, electrical machines, power systems, control systems, measurements and instrumentation, analog and digital electronics, and electromagnetic fields. Numerical answers include the formula used and the unit at each stage, which is where marks are commonly lost even when the approach is correct.
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.
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.
