Q.1 Which device is most commonly used as a switch in low‑power DC‑DC converters?
Silicon Controlled Rectifier (SCR)
Metal‑Oxide‑Semiconductor Field‑Effect Transistor (MOSFET)
Thyristor
Triac
Explanation - MOSFETs have low on‑resistance and fast switching speeds, making them ideal for low‑power DC‑DC converter applications.
Correct answer is: Metal‑Oxide‑Semiconductor Field‑Effect Transistor (MOSFET)
Q.2 In a buck converter, the output voltage is given by Vout = D × Vin. What does D represent?
Duty cycle of the switch
Diode forward voltage
Inductor ripple current
Capacitor ESR
Explanation - The duty cycle D is the fraction of the switching period that the switch is ON, directly determining the output voltage in a buck converter.
Correct answer is: Duty cycle of the switch
Q.3 Which of the following topologies can provide both step‑up and step‑down voltage conversion?
Buck converter
Boost converter
Cuk converter
Flyback converter
Explanation - The Cuk converter can produce an output voltage that is either higher or lower than the input voltage depending on the duty cycle.
Correct answer is: Cuk converter
Q.4 What is the primary function of the free‑wheeling diode in an inductive load driven by a PWM switch?
Limit the input voltage
Provide a path for inductor current when the switch is OFF
Regulate output voltage
Increase switching frequency
Explanation - When the switch turns OFF, the free‑wheeling diode conducts, allowing the stored energy in the inductor to continue flowing, preventing voltage spikes.
Correct answer is: Provide a path for inductor current when the switch is OFF
Q.5 In a three‑phase voltage‑source inverter (VSI), which PWM technique minimizes harmonic distortion?
Sinusoidal PWM (SPWM)
Square‑wave PWM
Random PWM
Phase‑shifted PWM
Explanation - SPWM shapes the inverter output to follow a sinusoidal reference, reducing total harmonic distortion compared with square‑wave methods.
Correct answer is: Sinusoidal PWM (SPWM)
Q.6 Which semiconductor device can handle high voltage and high current simultaneously, making it suitable for medium‑power inverters?
IGBT (Insulated Gate Bipolar Transistor)
MOSFET
Schottky diode
BJT
Explanation - IGBTs combine the high‑current capability of BJTs with the voltage rating and ease of gate drive of MOSFETs, ideal for medium‑power applications.
Correct answer is: IGBT (Insulated Gate Bipolar Transistor)
Q.7 What is the purpose of a snubber circuit across a switching device?
To increase switching frequency
To limit the voltage spike caused by parasitic inductance
To provide feedback control
To improve thermal performance
Explanation - Snubber circuits (RC or RCD) absorb the energy from parasitic inductance during turn‑off, protecting the switch from over‑voltage.
Correct answer is: To limit the voltage spike caused by parasitic inductance
Q.8 In a resonant converter, the switching frequency is typically set:
Well above the resonant frequency
Well below the resonant frequency
Exactly at the resonant frequency
At a random value
Explanation - Operating at resonance minimizes the voltage and current stress on the switching devices, resulting in higher efficiency.
Correct answer is: Exactly at the resonant frequency
Q.9 Which parameter most directly determines the switching losses of a power MOSFET?
Gate charge (Qg)
On‑resistance (RDS(on))
Breakdown voltage (VDSmax)
Thermal resistance (RθJC)
Explanation - Switching losses are proportional to the charge that must be moved on and off the gate during each transition.
Correct answer is: Gate charge (Qg)
Q.10 For a boost converter, which condition must be satisfied for continuous‑conduction mode (CCM) operation?
Inductor current never reaches zero during a switching period
Output voltage is equal to input voltage
Switch duty cycle is 50 %
Diode reverse recovery time is zero
Explanation - CCM occurs when the inductor current remains above zero throughout the entire switching cycle.
Correct answer is: Inductor current never reaches zero during a switching period
Q.11 Which of the following is a key advantage of a soft‑switching (zero‑voltage‑switching) power converter?
Higher peak current capability
Reduced electromagnetic interference (EMI)
Increased on‑state resistance
Simplified control algorithm
Explanation - Soft‑switching eliminates abrupt voltage or current transitions, thereby reducing EMI and switching losses.
Correct answer is: Reduced electromagnetic interference (EMI)
Q.12 In a three‑level NPC (Neutral Point Clamped) inverter, how many distinct voltage levels are present at the output?
Two
Three
Four
Five
Explanation - NPC inverters produce three output voltage levels: +Vdc, 0, and –Vdc.
Correct answer is: Three
Q.13 Which loss mechanism dominates in an IGBT during high‑frequency switching?
Conduction loss in the channel
Switching loss due to charge redistribution
Thermal runaway loss
Leakage current loss
Explanation - IGBTs have significant switching losses caused by the charging and discharging of the internal capacitances at high frequencies.
Correct answer is: Switching loss due to charge redistribution
Q.14 A DC‑link capacitor in a voltage‑source inverter primarily serves to:
Limit the input current ripple
Store energy for load transients and filter voltage ripple
Provide a path for inductor current
Regulate the output frequency
Explanation - The DC‑link capacitor smooths the DC bus voltage and supplies transient power demands of the inverter.
Correct answer is: Store energy for load transients and filter voltage ripple
Q.15 What is the main benefit of using SiC (Silicon Carbide) MOSFETs over traditional Si MOSFETs in high‑temperature applications?
Lower gate charge
Higher breakdown voltage and lower on‑resistance at high temperatures
Reduced cost
Simpler packaging
Explanation - SiC devices maintain low on‑resistance and can endure higher temperatures and voltages, improving efficiency in demanding environments.
Correct answer is: Higher breakdown voltage and lower on‑resistance at high temperatures
Q.16 In a half‑bridge inverter, the minimum dead‑time between turning OFF one switch and turning ON the opposite switch is required to:
Increase output voltage
Prevent shoot‑through (short circuit)
Reduce ripple current
Boost switching frequency
Explanation - Dead‑time ensures that both high‑side and low‑side switches are not ON simultaneously, avoiding a direct short across the DC supply.
Correct answer is: Prevent shoot‑through (short circuit)
Q.17 Which control technique is commonly used to regulate the output voltage of a buck converter while maintaining a constant switching frequency?
Voltage‑mode control
Current‑mode control
Hysteretic control
Resonant control
Explanation - Current‑mode control senses the inductor current, providing fast response and inherent over‑current protection while keeping frequency constant.
Correct answer is: Current‑mode control
Q.18 For a three‑phase induction motor drive, the most common power electronic interface is:
Single‑phase diode bridge
Six‑step inverter (six‑pulse)
Full‑bridge rectifier
PWM voltage‑source inverter
Explanation - A PWM VSI provides variable frequency and voltage, enabling precise speed control of three‑phase induction motors.
Correct answer is: PWM voltage‑source inverter
Q.19 In a flyback converter, the energy is stored in:
The output capacitor
The input inductance
The transformer magnetizing inductance
The diode junction
Explanation - During the switch‑ON period, energy is stored in the magnetizing inductance of the flyback transformer and transferred to the output during the OFF period.
Correct answer is: The transformer magnetizing inductance
Q.20 Which of the following is NOT a typical advantage of using a multilevel converter?
Reduced voltage stress on devices
Lower harmonic distortion
Higher switching frequency
Smaller filter size
Explanation - Multilevel converters reduce voltage stress and harmonics, allowing smaller filters, but they do not inherently increase the switching frequency.
Correct answer is: Higher switching frequency
Q.21 What is the main purpose of the gate driver in a power electronics system?
Provide isolation between control and power stages
Regulate output voltage
Measure current
Generate PWM carrier signal
Explanation - Gate drivers buffer and isolate low‑level control signals, delivering the required voltage/current to switch power devices quickly and safely.
Correct answer is: Provide isolation between control and power stages
Q.22 In a synchronous buck converter, the synchronous rectifier is implemented using:
Schottky diode
MOSFET
IGBT
Thyristor
Explanation - A MOSFET replaces the diode, offering lower conduction loss and improved efficiency, especially at higher currents.
Correct answer is: MOSFET
Q.23 Which loss component becomes dominant in a power diode at high forward current?
Reverse recovery loss
Conduction loss (I²R)
Switching loss
Leakage loss
Explanation - At high forward current, the I²R loss due to the diode's forward voltage drop outweighs other loss mechanisms.
Correct answer is: Conduction loss (I²R)
Q.24 In a dual‑active‑bridge (DAB) converter, power transfer is controlled primarily by:
Phase shift between the two bridges
Duty cycle of one bridge
Output voltage magnitude
Inductor value
Explanation - Adjusting the phase shift between the primary and secondary bridges regulates the average power transferred across the transformer.
Correct answer is: Phase shift between the two bridges
Q.25 The term ‘hard switching’ refers to:
Switching with zero current overlap
Switching with zero voltage overlap
Switching with significant voltage and current overlap, causing high losses
Switching at a fixed frequency
Explanation - Hard switching occurs when both voltage and current are high during transition, leading to high switching losses and EMI.
Correct answer is: Switching with significant voltage and current overlap, causing high losses
Q.26 In a Phase‑Shifted Full‑Bridge (PSFB) converter, the zero‑voltage‑switching (ZVS) condition is achieved by:
Increasing the switching frequency
Adding a large output capacitor
Choosing an appropriate phase shift to allow resonant current to flow before switching
Using a Schottky diode
Explanation - Proper phase shift creates resonant conditions that discharge the switch voltage before it turns on, achieving ZVS.
Correct answer is: Choosing an appropriate phase shift to allow resonant current to flow before switching
Q.27 Which characteristic of a transformer determines its ability to block high‑frequency noise?
Core material permeability
Leakage inductance
Winding resistance
Turns ratio
Explanation - Leakage inductance acts as a low‑pass filter, attenuating high‑frequency components and noise.
Correct answer is: Leakage inductance
Q.28 In a PWM inverter, the carrier frequency is usually:
Lower than the fundamental output frequency
Equal to the fundamental output frequency
Higher than the fundamental output frequency
Variable with load
Explanation - The carrier frequency (switching frequency) must be higher than the output fundamental to properly shape the waveform and allow filtering.
Correct answer is: Higher than the fundamental output frequency
Q.29 Which parameter of an IGBT defines its ability to block reverse voltage when the device is off?
Collector‑emitter on‑resistance (RCE(on))
Gate threshold voltage (VGE(th))
Collector‑emitter breakdown voltage (VCES)
Forward voltage drop (VF)
Explanation - VCES specifies the maximum reverse voltage an IGBT can withstand in the off state without breakdown.
Correct answer is: Collector‑emitter breakdown voltage (VCES)
Q.30 The efficiency of a power converter is most affected by:
Switching frequency only
Both conduction and switching losses
Ambient temperature only
Control algorithm complexity
Explanation - Efficiency is reduced by power dissipated as conduction (I²R) and switching losses; both must be minimized for high efficiency.
Correct answer is: Both conduction and switching losses
Q.31 In a three‑phase PWM inverter, the number of switching events per electrical cycle per phase is:
One
Two
Six
Twelve
Explanation - With a typical sinusoidal PWM, each phase experiences six switching events (three on, three off) per fundamental cycle.
Correct answer is: Six
Q.32 What is the main reason for using a synchronous rectifier in a high‑efficiency boost converter?
To increase the output voltage
To reduce the forward voltage drop compared with a diode
To simplify the control loop
To limit input current ripple
Explanation - A synchronous MOSFET has a much lower on‑resistance than a diode’s forward voltage, decreasing conduction losses.
Correct answer is: To reduce the forward voltage drop compared with a diode
Q.33 Which of the following converters is inherently a current‑source converter?
Buck converter
Boost converter
Cuk converter
Flyback converter
Explanation - The Cuk converter transfers energy through its inductor, behaving as a current source at the input.
Correct answer is: Cuk converter
Q.34 In a resonant half‑bridge converter, the resonant tank is composed of:
Inductor only
Capacitor only
Inductor and capacitor in series
Inductor and capacitor in parallel
Explanation - A series resonant tank (L and C) defines the resonant frequency, allowing zero‑voltage‑switching operation.
Correct answer is: Inductor and capacitor in series
Q.35 Which of the following is a typical symptom of device overheating in a power electronics module?
Increase in switching frequency
Decrease in on‑state resistance
Shift in threshold voltage and possible thermal runaway
Lower input voltage requirement
Explanation - Temperature rise can cause threshold voltage drift and increase conduction loss, potentially leading to thermal runaway.
Correct answer is: Shift in threshold voltage and possible thermal runaway
Q.36 The term ‘dead‑time’ in a half‑bridge inverter is introduced to:
Reduce electromagnetic interference
Prevent simultaneous conduction of both switches
Increase output voltage amplitude
Improve current ripple
Explanation - Dead‑time guarantees a brief interval where both high‑side and low‑side transistors are OFF, avoiding a short circuit.
Correct answer is: Prevent simultaneous conduction of both switches
Q.37 What is the effect of increasing the switching frequency on the size of the passive filter components in a PWM inverter?
Filter size increases
Filter size decreases
No effect on filter size
Filter becomes unnecessary
Explanation - Higher switching frequency pushes the harmonic spectrum higher, allowing smaller inductors and capacitors to attenuate the remaining low‑frequency harmonics.
Correct answer is: Filter size decreases
Q.38 In a three‑phase full‑bridge inverter, the maximum line‑to‑line voltage is:
Vdc
Vdc / √3
√3 × Vdc
2 × Vdc
Explanation - Each phase can swing between +Vdc/2 and –Vdc/2, giving a maximum line‑to‑line voltage of Vdc.
Correct answer is: Vdc
Q.39 Which device characteristic is most critical when selecting a MOSFET for high‑frequency operation?
Breakdown voltage (VDS)
Gate charge (Qg)
Package size
Thermal resistance (RθJA)
Explanation - Low gate charge reduces the energy required to turn the MOSFET on and off, minimizing switching losses at high frequencies.
Correct answer is: Gate charge (Qg)
Q.40 A boost converter operating in discontinuous‑conduction mode (DCM) has an output voltage that:
Is independent of the duty cycle
Depends on both duty cycle and load current
Is always equal to the input voltage
Is inversely proportional to the duty cycle
Explanation - In DCM, the output voltage is a function of duty cycle and the ratio of output current to the inductance value.
Correct answer is: Depends on both duty cycle and load current
Q.41 Which topology provides galvanic isolation and can also step up or step down the voltage?
Buck converter
Boost converter
Flyback converter
Buck‑boost converter
Explanation - The flyback transformer provides isolation, and the turns ratio determines whether the output voltage is higher or lower than the input.
Correct answer is: Flyback converter
Q.42 In a voltage‑source inverter, the modulation index (ma) is defined as:
Ratio of carrier frequency to fundamental frequency
Amplitude of the reference sinusoid divided by the amplitude of the carrier triangular wave
Switching frequency divided by load frequency
Output voltage divided by input voltage
Explanation - The modulation index controls the amplitude of the inverter output voltage in PWM schemes.
Correct answer is: Amplitude of the reference sinusoid divided by the amplitude of the carrier triangular wave
Q.43 Which of the following is the most common cause of voltage overshoot during the turn‑off of an IGBT?
Gate resistance too low
Parasitic inductance in the circuit
Excessive gate drive voltage
Low collector current
Explanation - When the IGBT switches off, the stored magnetic energy in parasitic inductances can generate voltage spikes (overshoot).
Correct answer is: Parasitic inductance in the circuit
Q.44 What is the main advantage of using a bidirectional DC‑DC converter in battery‑electric vehicles (BEVs)?
Simpler control algorithm
Ability to charge and discharge the battery through the same converter
Higher switching frequency
Lower cost
Explanation - Bidirectional converters allow power flow in both directions, enabling regenerative braking and vehicle charging.
Correct answer is: Ability to charge and discharge the battery through the same converter
Q.45 In a PWM inverter, increasing the carrier frequency generally results in:
Higher total harmonic distortion
Lower total harmonic distortion
Higher output voltage
Lower switching losses
Explanation - Higher carrier frequency pushes switching harmonics further away from the fundamental, making them easier to filter and reducing THD.
Correct answer is: Lower total harmonic distortion
Q.46 Which component in a power electronics system primarily determines the thermal time constant?
Switching frequency
Heat sink thermal resistance
Gate driver voltage
Inductance value
Explanation - The thermal time constant is the product of thermal resistance and thermal capacitance, dominated by the heat‑sink design.
Correct answer is: Heat sink thermal resistance
Q.47 In a dual‑inductor buck‑boost converter, the output voltage polarity is:
Always positive
Always negative
Depends on the duty cycle
Same as the input polarity
Explanation - The dual‑inductor topology provides a non‑inverting output, regardless of step‑up or step‑down operation.
Correct answer is: Always positive
Q.48 The term ‘soft‑switching’ usually refers to:
Zero‑current‑switching (ZCS) or zero‑voltage‑switching (ZVS)
Switching at a lower frequency
Using a larger gate driver
Operating at high temperature
Explanation - Soft‑switching techniques ensure that either current or voltage is near zero during transitions, reducing losses.
Correct answer is: Zero‑current‑switching (ZCS) or zero‑voltage‑switching (ZVS)
Q.49 In a three‑phase inverter with SPWM, the carrier frequency is typically set to:
10–20 % of the output fundamental frequency
3–5 times the output fundamental frequency
At least 10 times the output fundamental frequency
Equal to the output fundamental frequency
Explanation - A carrier frequency at least ten times higher ensures adequate resolution for the PWM waveform and low harmonic content.
Correct answer is: At least 10 times the output fundamental frequency
Q.50 Which of the following statements about the Cuk converter is true?
It provides a non‑inverting output
It uses a single inductor
It has continuous input current
It cannot operate in continuous‑conduction mode
Explanation - The Cuk converter maintains continuous input current, which is advantageous for reducing input ripple.
Correct answer is: It has continuous input current
Q.51 The primary loss mechanism in a Schottky diode is:
Reverse recovery loss
Conduction loss due to forward voltage drop
Switching loss
Leakage loss
Explanation - Schottky diodes have negligible reverse recovery, so the dominant loss is the I×VF conduction loss.
Correct answer is: Conduction loss due to forward voltage drop
Q.52 Which power electronic device is most suitable for very high‑frequency (above 500 kHz) applications?
IGBT
SiC MOSFET
Thyristor
SCR
Explanation - SiC MOSFETs have low capacitance and fast switching, allowing operation at frequencies exceeding 500 kHz.
Correct answer is: SiC MOSFET
Q.53 In a PWM inverter, the term ‘carrier‑signal amplitude’ primarily affects:
Switching frequency
Fundamental output voltage magnitude
Harmonic spectrum shape
Dead‑time
Explanation - The carrier amplitude determines the modulation index, which directly sets the magnitude of the fundamental output voltage.
Correct answer is: Fundamental output voltage magnitude
Q.54 Which topology inherently provides voltage regulation without an external feedback loop?
Buck converter
Flyback converter
Resonant converter
Cuk converter
Explanation - Resonant converters regulate voltage through the natural resonance of the LC tank, reducing the need for active feedback.
Correct answer is: Resonant converter
Q.55 The term ‘gate‑driver isolation’ is important because:
It reduces the cost of the driver
It protects low‑level control circuitry from high voltages and transients
It increases the switching speed
It eliminates the need for a heat sink
Explanation - Isolation prevents high‑voltage spikes from reaching the microcontroller or PLC that generates the gate signals.
Correct answer is: It protects low‑level control circuitry from high voltages and transients
Q.56 In a boost converter, the diode must be rated to withstand:
Input voltage only
Output voltage only
Both input and output voltage
Maximum switch voltage
Explanation - During off‑state, the diode experiences the sum of input and output voltages; thus it must be rated accordingly.
Correct answer is: Both input and output voltage
Q.57 What is the main advantage of using a ‘phase‑shifted full‑bridge’ topology in high‑power applications?
Lower component count
Zero‑voltage‑switching capability
Higher output voltage without transformer
Simpler control scheme
Explanation - Phase‑shifted operation creates resonant currents that enable ZVS, reducing switching losses in high‑power converters.
Correct answer is: Zero‑voltage‑switching capability
Q.58 In a dual‑converter (buck‑boost) arrangement for battery management, the two converters share:
A common output capacitor
A common inductance
A common switching frequency
A common control loop
Explanation - Sharing the same switching frequency simplifies synchronization and reduces component size.
Correct answer is: A common switching frequency
Q.59 The term ‘Total Harmonic Distortion (THD)’ in power electronics primarily measures:
Efficiency loss
Voltage ripple
Deviation of waveform from a pure sine wave
Thermal resistance
Explanation - THD quantifies the presence of harmonic components relative to the fundamental in a waveform.
Correct answer is: Deviation of waveform from a pure sine wave
Q.60 Which of the following is true about the operation of a ‘buck‑boost’ converter?
It can only step down voltage
It provides an inverted output voltage
It is non‑inverting and can step up or step down
It requires no inductors
Explanation - The classic buck‑boost topology yields an output voltage that is opposite in polarity to the input and can be either higher or lower in magnitude.
Correct answer is: It provides an inverted output voltage
Q.61 What is the primary purpose of adding a snubber resistor in series with a snubber capacitor across a MOSFET?
To increase the switching frequency
To damp oscillations and limit peak voltage
To improve gate drive strength
To reduce on‑state resistance
Explanation - The resistor dissipates energy from the resonant LC circuit, preventing voltage spikes and ringing.
Correct answer is: To damp oscillations and limit peak voltage
Q.62 In a three‑phase induction motor drive, the term ‘field‑oriented control (FOC)’ refers to:
Controlling the stator voltage magnitude only
Aligning the stator current vector with the rotor flux for decoupled torque and flux control
Using a fixed frequency inverter
Directly varying the supply frequency without feedback
Explanation - FOC transforms the three‑phase currents to a rotating reference frame, enabling independent control of torque-producing and flux‑producing components.
Correct answer is: Aligning the stator current vector with the rotor flux for decoupled torque and flux control
Q.63 Which of the following statements is correct regarding the use of a ‘soft‑starter’ for induction motors?
It provides voltage regulation for the motor at full speed
It reduces inrush current by gradually ramping up voltage
It increases the motor starting torque
It eliminates the need for a motor protection relay
Explanation - A soft‑starter slowly increases voltage to limit the high starting current of an induction motor.
Correct answer is: It reduces inrush current by gradually ramping up voltage
Q.64 In a resonant converter, the quality factor (Q) of the resonant tank influences:
Switching frequency only
Bandwidth and the shape of the voltage/current waveforms
Gate driver voltage
Thermal resistance of the devices
Explanation - Higher Q leads to narrow bandwidth and sharper resonance, affecting the converter’s voltage regulation and transient response.
Correct answer is: Bandwidth and the shape of the voltage/current waveforms
Q.65 What is the main disadvantage of using a diode bridge rectifier in high‑power applications compared to a synchronous rectifier?
Higher reverse recovery loss
Increased conduction loss due to diode forward voltage drop
More complex control algorithm
Larger physical size
Explanation - Diodes have a fixed forward voltage drop, causing higher I²R losses at high currents relative to low‑RDS(on) MOSFETs used in synchronous rectifiers.
Correct answer is: Increased conduction loss due to diode forward voltage drop
Q.66 In a high‑frequency transformer used in a power inverter, the core material is typically:
Silicon steel
Ferrite
Aluminum
Copper
Explanation - Ferrite cores have low eddy‑current loss at high frequencies, making them suitable for high‑frequency power transformers.
Correct answer is: Ferrite
Q.67 The main advantage of a ‘two‑level’ inverter over a ‘three‑level’ inverter is:
Lower harmonic distortion
Reduced number of semiconductor devices
Higher voltage capability
Smaller filter size
Explanation - Two‑level inverters require fewer devices, simplifying the hardware at the cost of higher harmonic content.
Correct answer is: Reduced number of semiconductor devices
Q.68 In a buck‑boost converter, the voltage gain is given by Vout/Vin = D/(1‑D). For a gain of 2, the required duty cycle is:
33 %
40 %
50 %
66 %
Explanation - Solving D/(1‑D) = 2 gives D = 2/3 ≈ 66 %.
Correct answer is: 66 %
Q.69 Which of the following is a typical characteristic of an IGBT compared to a MOSFET?
Higher switching speed
Lower on‑state voltage drop at high current
Higher input capacitance
Lower breakdown voltage
Explanation - IGBTs combine low on‑state voltage drop for high currents with the gate drive simplicity of MOSFETs, though they switch slower.
Correct answer is: Lower on‑state voltage drop at high current
Q.70 In a PWM inverter, the term ‘carrier‑to‑reference frequency ratio’ influences:
The amplitude of the output voltage
The number of pulses per fundamental cycle
The dead‑time
The gate driver voltage
Explanation - A higher carrier‑to‑reference ratio produces more pulses per cycle, improving waveform resolution.
Correct answer is: The number of pulses per fundamental cycle
Q.71 What is the purpose of a ‘pre‑charge resistor’ in a DC‑link circuit of an inverter?
To limit the inrush current when the DC‑bus capacitor charges
To increase the switching frequency
To provide voltage regulation
To improve thermal performance
Explanation - The pre‑charge resistor slows the capacitor charging, preventing high surge currents that could damage components.
Correct answer is: To limit the inrush current when the DC‑bus capacitor charges
Q.72 In a three‑phase inverter, the line‑to‑neutral voltage is:
√3 times the line‑to‑line voltage
Equal to the line‑to‑line voltage
1/√3 times the line‑to‑line voltage
Zero
Explanation - For a balanced three‑phase system, V_line‑to‑neutral = V_line‑to‑line / √3.
Correct answer is: 1/√3 times the line‑to‑line voltage
Q.73 Which control method is commonly used for grid‑connected inverters to synchronize with the utility frequency?
Voltage‑mode control
Current‑mode control
Phase‑locked loop (PLL)
Hysteresis control
Explanation - A PLL tracks the grid voltage phase and frequency, enabling synchronization of the inverter output.
Correct answer is: Phase‑locked loop (PLL)
Q.74 In a boost converter, the diode conducts during:
Switch ON time
Switch OFF time
Both ON and OFF times
Never; it is a synchronous design
Explanation - When the switch turns OFF, the inductor current flows through the diode to the output.
Correct answer is: Switch OFF time
Q.75 The main advantage of a ‘Zero‑Voltage‑Switching’ (ZVS) converter over a conventional hard‑switching converter is:
Higher output voltage
Reduced switching losses and EMI
Simpler circuit topology
Lower cost components
Explanation - ZVS ensures voltage across the switch is near zero during turn‑on, dramatically cutting losses and radiated EMI.
Correct answer is: Reduced switching losses and EMI
Q.76 In a three‑phase PWM inverter, the ‘space vector modulation’ technique offers which benefit compared to sinusoidal PWM?
Lower switching frequency requirement
Higher voltage utilization and lower harmonic distortion
Simpler implementation
Reduced need for dead‑time
Explanation - Space vector modulation (SVM) maximizes the linear modulation range and reduces total harmonic distortion.
Correct answer is: Higher voltage utilization and lower harmonic distortion
Q.77 Which of the following statements about an isolated DC‑DC converter is correct?
It cannot step down voltage
It always uses a transformer
It provides galvanic isolation between input and output
It requires a synchronous rectifier on both sides
Explanation - Isolation is achieved using a transformer or coupled inductor, separating input and output grounds.
Correct answer is: It provides galvanic isolation between input and output
Q.78 When a MOSFET is driven with insufficient gate voltage, the device operates in:
Linear region, causing high dissipation
Saturation region, improving efficiency
Cut‑off region, no conduction
ZVS region
Explanation - Partial turn‑on leads to a high on‑resistance, causing large I²R loss and heating.
Correct answer is: Linear region, causing high dissipation
Q.79 The primary function of a ‘current sensor’ in a closed‑loop power converter is to:
Measure input voltage
Provide feedback for over‑current protection and regulation
Increase switching frequency
Generate the PWM signal
Explanation - Current sensing enables the controller to limit current peaks and maintain stable operation.
Correct answer is: Provide feedback for over‑current protection and regulation
Q.80 In a three‑phase bridge inverter, how many switches are required in total?
3
4
6
12
Explanation - Each phase requires a high‑side and low‑side switch, totaling 6 switches; a full‑bridge per phase doubles that to 12.
Correct answer is: 12
Q.81 Which loss component becomes dominant in a SiC MOSFET when operating at very high frequencies (≥ 1 MHz)?
Conduction loss
Switching loss due to gate charge
Reverse recovery loss
Thermal runaway loss
Explanation - At high frequencies, the energy needed to charge and discharge the gate capacitance each cycle dominates the loss budget.
Correct answer is: Switching loss due to gate charge
Q.82 What is the main purpose of a ‘clamp circuit’ in a resonant converter?
To limit the peak voltage across the switching device
To increase the output voltage
To provide a reference voltage
To improve thermal conduction
Explanation - Clamp circuits protect switches from voltage spikes caused by resonance overshoots.
Correct answer is: To limit the peak voltage across the switching device
Q.83 In a high‑power inverter, the term ‘carrier‑based PWM’ refers to:
Using a sinusoidal carrier wave
Comparing a high‑frequency triangular carrier with a reference sinusoid
Modulating the carrier frequency directly
Using pulse width modulation without a carrier
Explanation - Carrier‑based PWM generates switching pulses by comparing a high‑frequency triangular carrier with the desired reference waveform.
Correct answer is: Comparing a high‑frequency triangular carrier with a reference sinusoid
Q.84 Which parameter is most directly responsible for the ripple current in the output capacitor of a buck converter?
Inductor value
Switching frequency
Capacitor ESR
Load resistance
Explanation - Higher switching frequency reduces the magnitude of the ripple current seen by the output capacitor.
Correct answer is: Switching frequency
Q.85 The main advantage of using a ‘synchronous buck converter’ over a traditional buck with a diode is:
Lower switching frequency required
Higher efficiency especially at low output voltage
Simpler control loop
No need for an inductor
Explanation - Replacing the diode with a low‑RDS(on) MOSFET eliminates the diode's forward voltage drop, improving efficiency.
Correct answer is: Higher efficiency especially at low output voltage
Q.86 In a three‑phase induction motor supplied by a VFD, the slip (s) is defined as:
s = (Ns – Nr) / Ns, where Ns is synchronous speed and Nr is rotor speed
s = (Nr – Ns) / Ns
s = Ns / Nr
s = Nr / Ns
Explanation - Slip indicates how much slower the rotor rotates compared to the rotating magnetic field.
Correct answer is: s = (Ns – Nr) / Ns, where Ns is synchronous speed and Nr is rotor speed
Q.87 Which of the following is a common technique to improve the thermal performance of power semiconductor packages?
Using a higher gate drive voltage
Mounting the device on a heat sink with thermal interface material
Increasing the switching frequency
Reducing the input voltage
Explanation - Heat sinks, often with thermal paste or pads, conduct heat away from the semiconductor to ambient.
Correct answer is: Mounting the device on a heat sink with thermal interface material
Q.88 In a phase‑shifted full‑bridge converter, the output voltage is controlled by:
Changing the duty cycle of the switches
Varying the phase shift between the two bridge legs
Altering the transformer turns ratio
Modifying the input voltage
Explanation - The average power transferred is proportional to the phase shift angle between the primary and secondary bridge voltage waveforms.
Correct answer is: Varying the phase shift between the two bridge legs
Q.89 What is the typical purpose of a ‘flyback diode’ in a buck converter?
To provide a path for inductor current during switch OFF time
To step up the voltage
To limit the input current
To increase switching frequency
Explanation - The flyback diode (free‑wheel diode) conducts the inductor current when the switch opens, preventing voltage spikes.
Correct answer is: To provide a path for inductor current during switch OFF time
Q.90 Which of the following converters is best suited for a solar PV system requiring a wide input voltage range and high efficiency?
Buck converter
Boost converter
Buck‑boost (Cuk) converter
Multilevel inverter
Explanation - Cuk converters can handle input voltages both above and below the desired output, offering high efficiency across a wide range.
Correct answer is: Buck‑boost (Cuk) converter
Q.91 In a PWM inverter, the term ‘carrier frequency’ is also known as:
Fundamental frequency
Switching frequency
Modulation index
Reference frequency
Explanation - The carrier frequency determines how fast the PWM switches, and is the switching frequency of the inverter.
Correct answer is: Switching frequency
Q.92 Which of the following is an advantage of using a ‘multiphase’ interleaved buck converter in a high‑current application?
Reduced input ripple current
Higher output voltage
Simpler control loop
Lower component count
Explanation - Interleaving phases spreads the current over multiple inductors, decreasing input and output current ripple.
Correct answer is: Reduced input ripple current
Q.93 The term ‘reverse recovery time’ is associated with which type of device?
MOSFET
IGBT
Schottky diode
Capacitor
Explanation - IGBTs contain a bipolar junction, which exhibits reverse recovery when the device transitions from forward to reverse bias.
Correct answer is: IGBT
Q.94 In a three‑phase bridge inverter, the line‑to‑line voltage waveform can be obtained by:
Subtracting two phase‑to‑neutral voltages
Adding two phase‑to‑neutral voltages
Multiplying phase voltage by √3
Dividing phase voltage by √3
Explanation - Line‑to‑line voltage equals the difference between the instantaneous phase‑to‑neutral voltages of the two lines.
Correct answer is: Subtracting two phase‑to‑neutral voltages
Q.95 Which of the following is a common method to achieve soft‑switching in a resonant converter?
Increasing the gate resistance
Adding a snubber resistor
Designing the resonant tank to achieve zero‑voltage‑switching (ZVS)
Operating at a lower switching frequency
Explanation - A properly designed resonant tank forces the voltage across the switch to be zero before turn‑on, achieving soft‑switching.
Correct answer is: Designing the resonant tank to achieve zero‑voltage‑switching (ZVS)
Q.96 In a buck converter, increasing the inductance value primarily affects:
Switching frequency
Output voltage ripple
Gate charge
Diode reverse recovery time
Explanation - A larger inductor reduces the ripple current, which translates to lower voltage ripple at the output.
Correct answer is: Output voltage ripple
Q.97 Which converter topology is intrinsically capable of providing isolation and also a gain less than unity?
Buck converter
Flyback converter
Boost converter
Cuk converter
Explanation - The flyback transformer can be wound for a turns ratio that yields a step‑down (gain < 1) while providing isolation.
Correct answer is: Flyback converter
Q.98 The primary reason for using a ‘half‑bridge’ configuration in high‑voltage converters is:
To halve the input voltage seen by each switch
To eliminate the need for a gate driver
To increase the number of switching devices
To provide three‑phase output directly
Explanation - In a half‑bridge, each switch only sees half the DC‑bus voltage, allowing the use of lower‑voltage devices.
Correct answer is: To halve the input voltage seen by each switch
Q.99 In a power electronic system, the term ‘EMI’ stands for:
Electro‑Mechanical Interference
Electromagnetic Interference
Energy Management Interface
Electronic Modulation Index
Explanation - EMI refers to unwanted electromagnetic emissions that can disturb nearby electronic equipment.
Correct answer is: Electromagnetic Interference
Q.100 Which of the following is a key design consideration when selecting a transformer core for a high‑frequency resonant converter?
Core loss at the operating frequency
Core color
Number of windings only
Physical weight only
Explanation - At high frequencies, core loss (eddy current + hysteresis) dominates; a material with low loss (e.g., ferrite) is essential.
Correct answer is: Core loss at the operating frequency
Q.101 Which of the following control schemes is most commonly used to regulate the output voltage of a flyback converter?
Peak current mode control
Voltage‑mode control
Hysteretic control
Phase‑shift control
Explanation - Peak current mode control monitors the primary winding current, providing good line regulation and inherent over‑current protection.
Correct answer is: Peak current mode control
Q.102 In a three‑phase inverter, the term ‘six‑step operation’ refers to:
Six PWM pulses per cycle per phase
Six distinct voltage levels generated per cycle
Six switching events per electrical cycle per phase
Six-phase system
Explanation - Six‑step operation (also called six‑pulse) uses six transitions per cycle per phase, producing a quasi‑square waveform.
Correct answer is: Six switching events per electrical cycle per phase
Q.103 The term ‘dead‑time’ is essential in which type of power converter topology?
Isolated DC‑DC converters
Half‑bridge and full‑bridge inverters
Linear regulators
Resonant converters
Explanation - Dead‑time prevents simultaneous conduction of complementary switches, avoiding shoot‑through faults.
Correct answer is: Half‑bridge and full‑bridge inverters
Q.104 In a bidirectional DC‑DC converter used for regenerative braking, the power flow direction is reversed during:
Acceleration
Coasting
Braking
Idle
Explanation - During regenerative braking, mechanical energy is converted back to electrical energy and returned to the battery.
Correct answer is: Braking
Q.105 Which of the following is the most common method to limit the voltage stress on a MOSFET in a high‑frequency inverter?
Adding a series resistor
Using a snubber circuit
Increasing the gate drive voltage
Operating at lower temperature
Explanation - Snubber circuits clamp voltage spikes caused by parasitic inductance, protecting the MOSFET from over‑voltage.
Correct answer is: Using a snubber circuit
Q.106 In a three‑phase PWM inverter, the fundamental component of the line‑to‑line voltage is:
√3 times the phase voltage fundamental
Equal to the phase voltage fundamental
Half the phase voltage fundamental
Zero
Explanation - Line‑to‑line fundamental voltage equals √3 times the phase‑to‑neutral fundamental voltage for a balanced system.
Correct answer is: √3 times the phase voltage fundamental
Q.107 Which converter topology uses two inductors and a capacitor to achieve a non‑inverting output?
Buck‑boost converter
Cuk converter
SEPIC converter
Flyback converter
Explanation - The Single‑Ended Primary‑Inductor Converter (SEPIC) provides a non‑inverting output with the ability to step up or step down.
Correct answer is: SEPIC converter
Q.108 In power electronics, the term ‘soft‑turn‑on’ generally means:
Turning on the switch at a higher voltage
Ensuring the voltage across the switch is near zero at turn‑on
Increasing the gate drive resistance
Operating the switch at a lower temperature
Explanation - Soft‑turn‑on (ZVS) reduces switching losses and EMI by having minimal voltage across the device during turn‑on.
Correct answer is: Ensuring the voltage across the switch is near zero at turn‑on
Q.109 The main function of a ‘gate‑charge driver’ is to:
Measure the output voltage
Supply the required gate charge quickly to turn the device on/off
Regulate the input current
Provide thermal management
Explanation - Gate‑charge drivers deliver high peak currents to charge and discharge the gate capacitance rapidly.
Correct answer is: Supply the required gate charge quickly to turn the device on/off
Q.110 In a resonant converter, the resonant frequency fr is given by fr = 1/(2π√LC). If L is doubled while C remains the same, the new resonant frequency will be:
Unchanged
Reduced by √2
Increased by √2
Halved
Explanation - Since fr ∝ 1/√L, doubling L reduces fr by a factor of √2.
Correct answer is: Reduced by √2
Q.111 Which of the following devices is inherently bidirectional, allowing current flow in both directions without additional circuitry?
Diode
Thyristor
MOSFET
IGBT
Explanation - MOSFETs conduct in either direction when gated on, unlike diodes which allow only one direction.
Correct answer is: MOSFET
Q.112 In a three‑phase inverter, the minimum number of switches required to generate a balanced three‑phase output is:
3
6
9
12
Explanation - A three‑phase bridge requires six switches (two per phase) to produce a balanced set of three line‑to‑line voltages.
Correct answer is: 6
Q.113 The term ‘load‑line regulation’ in a power converter refers to:
The ability of the converter to maintain output voltage over variations in load current
The regulation of the input voltage
The regulation of the switching frequency
The regulation of the gate drive voltage
Explanation - Load‑line regulation quantifies how much the output voltage changes when the load varies.
Correct answer is: The ability of the converter to maintain output voltage over variations in load current
Q.114 Which loss mechanism is most affected by the parasitic capacitance between the drain and source (Cds) of a MOSFET?
Conduction loss
Switching loss
Reverse recovery loss
Thermal loss
Explanation - During switching, Cds must be charged and discharged, contributing to switching energy loss.
Correct answer is: Switching loss
Q.115 In a buck‑boost converter, the voltage conversion ratio is given by Vout/Vin = – D/(1‑D). What does the negative sign indicate?
The converter is inefficient
The output voltage is inverted relative to the input
The duty cycle is greater than 1
The output voltage is always lower than the input
Explanation - The classic buck‑boost topology yields an output voltage opposite in polarity to the input.
Correct answer is: The output voltage is inverted relative to the input
Q.116 Which of the following is a primary benefit of using a digital signal processor (DSP) for controlling power converters?
Lower cost than analog controllers
Higher switching frequency capability
Flexibility to implement complex control algorithms and easy re‑programming
No need for gate drivers
Explanation - DSPs allow rapid development, modification, and implementation of advanced control strategies.
Correct answer is: Flexibility to implement complex control algorithms and easy re‑programming
Q.117 In a three‑phase inverter, the line‑to‑line voltage RMS value is related to the phase‑to‑neutral voltage RMS value by:
V_LL = √3 × V_PH
V_LL = V_PH / √3
V_LL = 2 × V_PH
V_LL = V_PH
Explanation - For a balanced system, the line‑to‑line RMS voltage equals √3 times the phase‑to‑neutral RMS voltage.
Correct answer is: V_LL = √3 × V_PH
Q.118 The main reason for employing a ‘current‑mode control’ in a buck converter is:
To eliminate the need for an output capacitor
To provide inherent over‑current protection and improve response
To increase the switching frequency
To reduce gate driver voltage
Explanation - Current‑mode control directly monitors inductor current, allowing fast protection and better dynamic performance.
Correct answer is: To provide inherent over‑current protection and improve response
Q.119 In a power electronics system, the term ‘thermal resistance’ (Rθ) is measured in:
°C/W
W/°C
Ω
V/A
Explanation - Thermal resistance indicates the temperature rise per watt of power dissipated.
Correct answer is: °C/W
Q.120 Which of the following is a typical consequence of operating a MOSFET at a gate‑drive voltage that is too high?
Reduced on‑resistance
Increased gate oxide stress leading to reliability issues
Lower switching speed
Higher reverse recovery loss
Explanation - Exceeding the maximum gate‑drive voltage can damage the thin gate oxide, reducing device reliability.
Correct answer is: Increased gate oxide stress leading to reliability issues
Q.121 In a three‑phase PWM inverter using sinusoidal PWM, the modulation index must stay below 1 to avoid:
Over‑modulation and possible over‑voltage on the output
Undervoltage of the DC bus
Increase in dead‑time
Decrease in switching frequency
Explanation - An index >1 causes the reference sinusoid to exceed the carrier, leading to over‑modulation and distortion.
Correct answer is: Over‑modulation and possible over‑voltage on the output
Q.122 Which component in a power electronic converter directly determines the rate of change of current (di/dt) during switching?
Capacitor ESR
Inductor value
Gate resistor
Snubber resistor
Explanation - di/dt = V/L; thus the inductance determines how quickly current can change for a given voltage.
Correct answer is: Inductor value
Q.123 A common technique to reduce the size of the output filter in a high‑frequency inverter is:
Increasing the switching frequency
Decreasing the input voltage
Using a larger inductor
Operating in discontinuous‑conduction mode
Explanation - Higher switching frequency pushes harmonic energy higher, allowing smaller inductors and capacitors to filter the output.
Correct answer is: Increasing the switching frequency
Q.124 In a buck‑boost converter, the output voltage ripple is most strongly affected by:
Switching frequency
Input voltage magnitude
Gate drive resistance
Snubber capacitance
Explanation - Higher switching frequency reduces the time between charge/discharge cycles, lowering voltage ripple.
Correct answer is: Switching frequency
Q.125 Which of the following devices is typically used as a free‑wheeling element in high‑current DC‑DC converters?
Schottky diode
Silicon controlled rectifier (SCR)
Triac
Zener diode
Explanation - Schottky diodes have low forward voltage drop and fast recovery, ideal for free‑wheeling in high‑current paths.
Correct answer is: Schottky diode
Q.126 In a three‑phase inverter, the term ‘space vector’ refers to:
A vector representation of the three phase voltages in the complex plane
The physical location of the inverter
The frequency of the carrier signal
The gate driver voltage
Explanation - Space vector theory treats the three phase quantities as a single rotating vector, simplifying analysis and control.
Correct answer is: A vector representation of the three phase voltages in the complex plane
Q.127 The main function of a ‘common‑mode choke’ in a power inverter is to:
Increase the output voltage
Suppress common‑mode noise and reduce EMI
Provide current sensing
Regulate the gate drive voltage
Explanation - Common‑mode chokes attenuate noise that appears equally on all phases, improving EMI performance.
Correct answer is: Suppress common‑mode noise and reduce EMI
Q.128 Which of the following is the most suitable semiconductor material for ultra‑high‑frequency (GHz) RF power amplifiers?
Silicon
GaN (Gallium Nitride)
IGBT
Silicon Carbide (SiC)
Explanation - GaN offers high electron mobility and high breakdown fields, enabling efficient operation at GHz frequencies.
Correct answer is: GaN (Gallium Nitride)