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Power Electronics MCQs

Practice Power Electronics MCQs from fundamental concepts through intermediate, advanced and application-oriented questions. Each page contains up to 100 questions with answers and explanations where available.

Question 1

Power ElectronicsPower Semiconductor Devices

Which device is commonly used as a controlled switch in power-electronic circuits?

ASCR
BLDR
CZener diode only
DPhotodiode

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Correct Answer: A. SCR

Explanation: An SCR is a thyristor widely used as a controlled switching device in power-electronic converters.

Reference: Power electronics fundamentals

Question 2

Power ElectronicsSCR

The three terminals of an SCR are?

AAnode, cathode and gate
BCollector, base and emitter
CDrain, source and gate
DAnode, base and drain

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Correct Answer: A. Anode, cathode and gate

Explanation: The terminals of an SCR are anode (A), cathode (K) and gate (G).

Reference: Standard power electronics: scr

Question 3

Power ElectronicsSCR

For normal forward turn-on of an SCR by gate triggering, the anode should be?

APositive with respect to cathode
BNegative with respect to cathode
CAt exactly zero potential in every case
DDisconnected from the supply

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Correct Answer: A. Positive with respect to cathode

Explanation: For forward blocking and gate-triggered turn-on, the anode is positive relative to the cathode.

Reference: Standard power electronics: scr

Question 4

Power ElectronicsSCR

Once a conventional SCR has latched ON, removal of the gate signal normally?

ADoes not turn it OFF
BImmediately turns it OFF
CReverses its anode current
DMakes its holding current infinite

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Correct Answer: A. Does not turn it OFF

Explanation: After latching, the gate loses control over turn-off; anode current must be reduced below the holding current or reverse voltage applied.

Reference: Standard power electronics: scr

Question 5

Power ElectronicsSCR

The minimum anode current required to keep an SCR conducting after it is already ON is called?

AHolding current
BGate current
CLeakage current
DBreakover current

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Correct Answer: A. Holding current

Explanation: Holding current is the minimum anode current below which the SCR returns to its blocking state.

Reference: Standard power electronics: scr

Question 6

Power ElectronicsSCR

The minimum anode current needed immediately after triggering so that an SCR remains ON when the gate pulse is removed is called?

ALatching current
BReverse leakage current
CDisplacement current
DLoad current rating

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Correct Answer: A. Latching current

Explanation: Latching current is the minimum anode current required to establish regenerative conduction after the triggering signal is removed.

Reference: Standard power electronics: scr

Question 7

Power ElectronicsSCR

For an SCR, the latching current is generally?

AGreater than the holding current
BLess than the holding current in every device
CExactly zero
DAlways equal to gate current

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Correct Answer: A. Greater than the holding current

Explanation: Typically the latching current is higher than the holding current because stronger current is required just after turn-on to establish conduction.

Reference: Standard power electronics: scr

Question 8

Power ElectronicsTRIAC

A TRIAC can conduct main current in?

ABoth directions
BForward direction only
CReverse direction only
DNeither direction

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Correct Answer: A. Both directions

Explanation: A TRIAC is a bidirectional thyristor used for AC power control.

Reference: Standard power electronics: triac

Question 9

Power ElectronicsDIAC & TRIAC

A DIAC is commonly used to?

AProvide bidirectional triggering for a TRIAC
BRectify high-power three-phase DC
CAct as a controlled DC motor
DStore magnetic energy

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Correct Answer: A. Provide bidirectional triggering for a TRIAC

Explanation: A DIAC has bidirectional breakover behavior and is commonly used to produce symmetrical TRIAC gate triggering.

Reference: Standard power electronics: diac & triac

Question 10

Power ElectronicsPower Electronics Fundamentals

Power electronics primarily deals with the conversion and control of electrical power using?

ASemiconductor switching devices
BOnly mechanical switches
COnly passive resistors
DElectrochemical cells only

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Correct Answer: A. Semiconductor switching devices

Explanation: Power electronics uses semiconductor devices operated mainly as switches to efficiently convert and control electrical energy.

Reference: Standard power electronics: power electronics fundamentals

Question 11

Power ElectronicsPower Semiconductor Devices

An ideal power semiconductor switch in the ON state should have?

AZero voltage drop
BInfinite voltage drop
CZero current capability
DInfinite switching loss

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Correct Answer: A. Zero voltage drop

Explanation: An ideal closed switch conducts current with zero voltage across it, so its conduction loss is zero.

Reference: Standard power electronics: power semiconductor devices

Question 12

Power ElectronicsPower Semiconductor Devices

An ideal power semiconductor switch in the OFF state should have?

AZero current
BMaximum current
CZero blocking voltage
DInfinite conduction loss

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Correct Answer: A. Zero current

Explanation: An ideal open switch blocks voltage while carrying zero current.

Reference: Standard power electronics: power semiconductor devices

Question 13

Power ElectronicsSCR

SCR stands for?

ASilicon Controlled Rectifier
BSeries Current Regulator
CSwitching Capacitor Rectifier
DSilicon Current Resistor

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Correct Answer: A. Silicon Controlled Rectifier

Explanation: SCR is the standard abbreviation for Silicon Controlled Rectifier, a thyristor device.

Reference: Standard power electronics: scr

Question 14

Power ElectronicsSCR

A conventional SCR has how many semiconductor layers?

ATwo
BThree
CFour
DFive

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Correct Answer: C. Four

Explanation: An SCR is a four-layer PNPN semiconductor device.

Reference: Standard power electronics: scr

Question 15

Power ElectronicsCycloconverters

A cycloconverter directly converts?

AAC at one frequency to AC at another frequency
BDC to AC only
CAC to DC only
DDC to DC only

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Correct Answer: A. AC at one frequency to AC at another frequency

Explanation: A cycloconverter performs direct AC-to-AC frequency conversion without an intermediate DC link.

Reference: Standard power electronics: cycloconverters

Question 16

Power ElectronicsFreewheeling Diode

A freewheeling diode connected across an inductive DC load mainly provides?

AA path for load current when the main switch or rectifier device stops supplying it
BA permanent short circuit across the source
CGate current for an SCR
DVoltage amplification

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Correct Answer: A. A path for load current when the main switch or rectifier device stops supplying it

Explanation: Inductive current cannot change instantaneously, so the freewheeling diode provides a path for continuing current and helps limit load voltage excursions.

Reference: Standard power electronics: freewheeling diode

Question 17

Power ElectronicsControlled Rectifiers

In phase-controlled rectifiers, firing angle alpha is measured from?

AThe reference point where the device would naturally become forward-biased in the supply cycle
BThe instant of maximum load current only
CThe end of every negative half-cycle only
DThe gate-current peak regardless of supply waveform

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Correct Answer: A. The reference point where the device would naturally become forward-biased in the supply cycle

Explanation: The firing delay angle alpha specifies how long triggering is delayed from the natural forward-bias reference in each applicable cycle.

Reference: Standard power electronics: controlled rectifiers

Question 18

Power ElectronicsControlled Rectifiers

For a single-phase half-wave controlled rectifier with a purely resistive load, increasing firing angle from 0 toward 180 degrees generally causes the average output voltage to?

ADecrease
BIncrease
CRemain constant
DBecome independent of the supply

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Correct Answer: A. Decrease

Explanation: Delaying SCR turn-on shortens the conduction interval, reducing the average output voltage for a resistive load.

Reference: Standard power electronics: controlled rectifiers

Question 19

Power ElectronicsPower Diode

A power diode is primarily an?

AUncontrolled unidirectional power semiconductor device
BFully gate-controlled bidirectional device
CVoltage-controlled transistor only
DMechanical relay

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Correct Answer: A. Uncontrolled unidirectional power semiconductor device

Explanation: A diode's conduction is determined by circuit voltage/current conditions; it has no control terminal.

Reference: Standard power electronics: power diode

Question 20

Power ElectronicsPower MOSFET

A power MOSFET is primarily a?

AVoltage-controlled device
BCurrent-controlled device requiring continuous base current
CFour-layer latching thyristor
DBidirectional breakover diode

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Correct Answer: A. Voltage-controlled device

Explanation: The insulated gate of a MOSFET is controlled mainly by gate-to-source voltage, with negligible steady-state gate current ideally.

Reference: Standard power electronics: power mosfet

Question 21

Power ElectronicsIGBT

An IGBT combines the insulated-gate control characteristics of a MOSFET with?

ABipolar conduction characteristics
BMechanical switching
CDIAC breakover only
DA vacuum-tube cathode

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Correct Answer: A. Bipolar conduction characteristics

Explanation: An IGBT uses a MOS insulated gate with bipolar carrier conduction in its power path.

Reference: Standard power electronics: igbt

Question 22

Power ElectronicsPower MOSFET

Compared with an SCR, a power MOSFET can normally be turned OFF by?

ARemoving or appropriately changing its gate-to-source drive
BReducing load current below holding current only
CApplying a DIAC pulse only
DWaiting for AC current zero in every application

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Correct Answer: A. Removing or appropriately changing its gate-to-source drive

Explanation: A MOSFET is a fully controlled switch: its channel can be turned on and off by controlling gate voltage.

Reference: Standard power electronics: power mosfet

Question 23

Power ElectronicsRectifiers

A rectifier converts electrical power from?

AAC to DC
BDC to AC
CDC to DC only
DAC to AC only

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Correct Answer: A. AC to DC

Explanation: A rectifier converts alternating input power into a unidirectional/DC output.

Reference: Standard power electronics: rectifiers

Question 24

Power ElectronicsInverters

An inverter converts electrical power from?

ADC to AC
BAC to DC
CAC to AC only
DMechanical to electrical

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Correct Answer: A. DC to AC

Explanation: An inverter synthesizes an AC output from a DC source.

Reference: Standard power electronics: inverters

Question 25

Power ElectronicsDC Choppers

A DC chopper is principally a?

ADC-to-DC converter
BAC-to-DC converter
CDC-to-AC converter
DAC-to-AC transformer only

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Correct Answer: A. DC-to-DC converter

Explanation: A chopper controls the average DC output obtained from a DC input through high-frequency switching.

Reference: Standard power electronics: dc choppers

Question 26

Power ElectronicsAC Voltage Controllers

An AC voltage controller converts?

AFixed-frequency AC to variable-magnitude AC at the same basic frequency
BDC to AC
CAC directly to pure DC only
DDC to DC

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Correct Answer: A. Fixed-frequency AC to variable-magnitude AC at the same basic frequency

Explanation: An AC voltage controller regulates the RMS AC output, commonly by phase or integral-cycle control, while retaining the supply's fundamental frequency.

Reference: Standard power electronics: ac voltage controllers

Question 27

Power ElectronicsRectifier Numericals

A single-phase half-wave controlled rectifier has Vm=100 V and alpha=60 degrees with an R load. Its average output voltage is approximately?

A11.9 V
B23.9 V
C47.7 V
D75 V

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Correct Answer: B. 23.9 V

Explanation: Vdc=100(1+cos60°)/(2pi)=150/(2pi)≈23.9 V.

Reference: Standard power electronics: rectifier numericals

Question 28

Power ElectronicsSCR Characteristics

In the forward-blocking region of an SCR, the device is forward biased but remains OFF until?

AIt is triggered or forward breakover is reached
BThe anode becomes negative
CThe load current becomes zero only
DThe cathode is disconnected

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Correct Answer: A. It is triggered or forward breakover is reached

Explanation: In forward blocking, an SCR supports positive anode-to-cathode voltage until gate triggering or breakover initiates regenerative turn-on.

Reference: Standard power electronics: scr characteristics

Question 29

Power ElectronicsSCR Characteristics

The forward breakover voltage of an SCR is the anode-to-cathode voltage at which it turns ON without?

AA gate signal
BAn anode supply
CA cathode connection
DForward bias

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Correct Answer: A. A gate signal

Explanation: If forward voltage reaches the breakover level, an SCR can switch into conduction even without a gate trigger.

Reference: Standard power electronics: scr characteristics

Question 30

Power ElectronicsSCR Characteristics

Increasing the gate current of an SCR generally causes its forward breakover voltage to?

ADecrease
BIncrease without limit
CRemain exactly unchanged
DBecome negative

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Correct Answer: A. Decrease

Explanation: Greater gate drive assists regenerative turn-on, so less anode-to-cathode voltage is required to trigger the SCR.

Reference: Standard power electronics: scr characteristics

Question 31

Power ElectronicsSCR Triggering

Which SCR triggering method is normally preferred because it provides controlled turn-on at the desired instant?

AGate triggering
BForward breakover triggering
CThermal triggering
DUncontrolled dv/dt triggering

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Correct Answer: A. Gate triggering

Explanation: Gate triggering allows deliberate, low-power control of the SCR firing instant.

Reference: Standard power electronics: scr triggering

Question 32

Power ElectronicsSCR Protection

A high rate of rise of forward voltage across an OFF SCR may cause unwanted turn-on due to?

AJunction capacitance current
BHolding current becoming zero
CLoad resistance becoming infinite
DGate resistance alone

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Correct Answer: A. Junction capacitance current

Explanation: Internal junction capacitances carry i=C dv/dt; excessive dv/dt can create sufficient internal current to trigger the SCR.

Reference: Standard power electronics: scr protection

Question 33

Power ElectronicsSCR Protection

A snubber circuit connected across an SCR is commonly used to limit?

Adv/dt
BLoad frequency to zero
CGate current to exactly zero
DDC output power

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Correct Answer: A. dv/dt

Explanation: An RC snubber reduces the rate of rise of voltage across the SCR and helps prevent false triggering.

Reference: Standard power electronics: scr protection

Question 34

Power ElectronicsSCR Protection

A small series inductor is often used with an SCR to limit excessive?

Adi/dt during turn-on
Bdv/dt in the OFF state only
CHolding current
DGate voltage ripple only

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Correct Answer: A. di/dt during turn-on

Explanation: An inductor opposes rapid current change and can keep SCR turn-on di/dt within its safe rating.

Reference: Standard power electronics: scr protection

Question 35

Power ElectronicsSCR Commutation

SCR commutation means the process of?

ATurning the SCR OFF
BTurning the SCR ON by gate current
CIncreasing its breakover voltage
DMeasuring its gate resistance

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Correct Answer: A. Turning the SCR OFF

Explanation: Commutation reduces SCR current below the holding level and provides sufficient recovery conditions for the device to regain blocking capability.

Reference: Standard power electronics: scr commutation

Question 36

Power ElectronicsSCR Commutation

In a line-commutated AC rectifier, an SCR can turn OFF naturally when?

AIts current falls to zero and reverse voltage is available
BIts gate pulse is removed while current remains high
CIts gate is made more positive
DThe supply frequency becomes zero

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Correct Answer: A. Its current falls to zero and reverse voltage is available

Explanation: Natural commutation uses the AC source to bring current to zero and reverse-bias the outgoing SCR.

Reference: Standard power electronics: scr commutation

Question 37

Power ElectronicsSCR Commutation

Forced commutation is especially required for a conventional SCR in many?

ADC circuits
BPurely line-commutated AC circuits only
CPassive resistor networks with no source
DUncontrolled diode rectifiers

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Correct Answer: A. DC circuits

Explanation: A DC source has no natural current zero, so auxiliary circuitry is often required to force SCR current below holding current.

Reference: Standard power electronics: scr commutation

Question 38

Power ElectronicsSingle-Phase Controlled Rectifiers

For a single-phase half-wave controlled rectifier with a resistive load, the SCR conducts from firing angle alpha to?

Api radians
B2pi radians in every case
Calpha/2
Dpi/2 regardless of alpha

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Correct Answer: A. pi radians

Explanation: With a resistive load, current follows voltage and becomes zero at the supply zero crossing at omega t=pi.

Reference: Standard power electronics: single-phase controlled rectifiers

Question 39

Power ElectronicsRectifier Numericals

For a single-phase half-wave controlled rectifier with resistive load and supply peak Vm, the average output voltage is?

AVm(1+cos alpha)/(2pi)
B2Vm cos alpha/pi
CVm sin alpha/pi
DVm(1-cos alpha)/pi

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Correct Answer: A. Vm(1+cos alpha)/(2pi)

Explanation: Averaging Vm sin(theta) from alpha to pi over a full 2pi period gives Vdc=Vm(1+cos alpha)/(2pi).

Reference: Standard power electronics: rectifier numericals

Question 40

Power ElectronicsFull Converters

For a single-phase fully controlled bridge rectifier with continuous load current, the ideal average DC output voltage is?

A2Vm cos alpha/pi
BVm(1+cos alpha)/(2pi)
CVm cos alpha/(2pi)
D2Vm sin alpha/pi

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Correct Answer: A. 2Vm cos alpha/pi

Explanation: For continuous current and ideal devices, a single-phase full converter gives Vdc=(2Vm/pi)cos alpha.

Reference: Standard power electronics: full converters

Question 41

Power ElectronicsRectifier Numericals

A single-phase full converter has Vm=200 V and firing angle alpha=60 degrees. With continuous current, its average output voltage is approximately?

A31.8 V
B63.7 V
C100 V
D127.3 V

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Correct Answer: B. 63.7 V

Explanation: Vdc=(2×200/pi)cos60°=200/pi≈63.7 V.

Reference: Standard power electronics: rectifier numericals

Question 42

Power ElectronicsFull Converters

For an ideal single-phase fully controlled bridge with continuous current, alpha=90 degrees gives average output voltage of?

AMaximum positive value
BZero
CMaximum negative value
DVm

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Correct Answer: B. Zero

Explanation: Vdc=(2Vm/pi)cos90°=0.

Reference: Standard power electronics: full converters

Question 43

Power ElectronicsFull Converters

For a single-phase fully controlled bridge with continuous current, increasing alpha beyond 90 degrees makes the average DC-side voltage?

ANegative
BAlways positive
CExactly zero for all alpha
DIndependent of alpha

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Correct Answer: A. Negative

Explanation: Since cos alpha becomes negative for 90°<alpha<180°, the converter's average DC voltage becomes negative; with suitable DC-side conditions this corresponds to inversion/regeneration.

Reference: Standard power electronics: full converters

Question 44

Power ElectronicsSemi-Converters

For a single-phase semi-converter with continuous load current, the average output voltage is?

AVm(1+cos alpha)/pi
B2Vm cos alpha/pi
CVm(1-cos alpha)/(2pi)
D2Vm/pi independent of alpha

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Correct Answer: A. Vm(1+cos alpha)/pi

Explanation: The ideal single-phase semi-converter average output is Vdc=Vm(1+cos alpha)/pi for continuous current.

Reference: Standard power electronics: semi-converters

Question 45

Power ElectronicsRectifier Numericals

A single-phase semi-converter has Vm=100 V and alpha=60 degrees. Its average output voltage is approximately?

A23.9 V
B31.8 V
C47.7 V
D63.7 V

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Correct Answer: C. 47.7 V

Explanation: Vdc=100(1+0.5)/pi=150/pi≈47.7 V.

Reference: Standard power electronics: rectifier numericals

Question 46

Power ElectronicsSemi-Converters

Compared with a fully controlled bridge, a single-phase semi-converter uses?

ATwo SCRs and two diodes
BFour SCRs
CFour MOSFETs only
DOne SCR and three DIACs

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Correct Answer: A. Two SCRs and two diodes

Explanation: A conventional single-phase semi-converter is a half-controlled bridge containing two thyristors and two diodes.

Reference: Standard power electronics: semi-converters

Question 47

Power ElectronicsFreewheeling Operation

For an RL load, a freewheeling diode can improve load-current continuity by conducting when?

AThe main rectifier path no longer supplies the inductive current
BThe load inductance becomes zero
CThe AC source is permanently disconnected
DThe gate pulse is at its maximum only

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Correct Answer: A. The main rectifier path no longer supplies the inductive current

Explanation: Stored magnetic energy in the load inductance maintains current through the freewheeling diode when the source-side device stops conducting.

Reference: Standard power electronics: freewheeling operation

Question 48

Power ElectronicsFreewheeling Operation

When an ideal freewheeling diode conducts across an RL load, the load voltage is approximately?

AZero
BEqual to the negative supply peak
CTwice the supply peak
DInfinite

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Correct Answer: A. Zero

Explanation: The conducting ideal diode effectively shorts the load terminals for the freewheeling interval, making load voltage approximately zero while current circulates.

Reference: Standard power electronics: freewheeling operation

Question 49

Power ElectronicsFreewheeling Operation

One important benefit of a freewheeling diode in an inductive rectifier load is reduction of?

ANegative load-voltage excursions
BAverage load current to zero
CLoad inductance
DSupply frequency

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Correct Answer: A. Negative load-voltage excursions

Explanation: The diode provides a local current path and clamps the load voltage near zero during freewheeling, preventing substantial negative load voltage.

Reference: Standard power electronics: freewheeling operation

Question 50

Power ElectronicsRectifier Numericals

If the average output voltage of an ideal single-phase full converter is 100 V and the continuous DC load current is 5 A, the average DC output power is?

A20 W
B100 W
C500 W
D1000 W

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Correct Answer: C. 500 W

Explanation: For constant/continuous DC-side current, average output power P=Vdc Id=100×5=500 W.

Reference: Standard power electronics: rectifier numericals

Question 51

Power ElectronicsControlled Rectifiers

In a controlled rectifier, delaying the firing angle generally provides control over the?

AAverage output voltage
BSupply frequency
CNumber of semiconductor layers
DPhysical transformer turns ratio

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Correct Answer: A. Average output voltage

Explanation: Phase control changes the portion and polarity-weighted interval of the AC waveform applied to the load, thereby controlling average output voltage.

Reference: Standard power electronics: controlled rectifiers

Question 52

Power ElectronicsChopper Numericals

If the switching period of a chopper is 100 microseconds, its switching frequency is?

A1 kHz
B5 kHz
C10 kHz
D100 kHz

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Correct Answer: C. 10 kHz

Explanation: f=1/T=1/(100×10^-6)=10 kHz.

Reference: Standard power electronics: chopper numericals

Question 53

Power ElectronicsDC-DC Converters

An ideal boost converter is used when the required DC output voltage is generally?

AGreater than the DC input voltage
BAlways zero
CLess than the input in every case
DAC rather than DC

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Correct Answer: A. Greater than the DC input voltage

Explanation: A boost converter raises the DC voltage magnitude under ideal continuous-conduction operation.

Reference: Standard power electronics: dc-dc converters

Question 54

Power ElectronicsDC-DC Converters

For an ideal boost converter operating in continuous conduction, the voltage conversion ratio is?

AVo/Vs = 1/(1-D)
BVo/Vs = D
CVo/Vs = 1-D
DVo/Vs = -D/(1-D)

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Correct Answer: A. Vo/Vs = 1/(1-D)

Explanation: The ideal boost relation is Vo=Vs/(1-D).

Reference: Standard power electronics: dc-dc converters

Question 55

Power ElectronicsConverter Numericals

An ideal boost converter has Vs=100 V and duty ratio D=0.5. Its output voltage is?

A50 V
B100 V
C150 V
D200 V

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Correct Answer: D. 200 V

Explanation: Vo=100/(1-0.5)=200 V.

Reference: Standard power electronics: converter numericals

Question 56

Power ElectronicsDC Choppers

For an ideal step-down DC chopper with continuous current, the average output voltage is?

AD Vs
BVs/D
C(1-D)Vs
D-D Vs

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Correct Answer: A. D Vs

Explanation: For an ideal buck-type chopper, the average load voltage is Vo=DVs, where D is the duty ratio.

Reference: Standard power electronics: dc choppers

Question 57

Power ElectronicsChopper Numericals

A chopper has an ON time of 4 ms and an OFF time of 6 ms. Its duty ratio is?

A0.4
B0.6
C0.67
D1.5

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Correct Answer: A. 0.4

Explanation: D=Ton/(Ton+Toff)=4/(4+6)=0.4.

Reference: Standard power electronics: chopper numericals

Question 58

Power ElectronicsChopper Numericals

An ideal step-down chopper operates from 200 V with duty ratio 0.25. Its average output voltage is?

A25 V
B50 V
C100 V
D150 V

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Correct Answer: B. 50 V

Explanation: Vo=DVs=0.25×200=50 V.

Reference: Standard power electronics: chopper numericals

Question 59

Power ElectronicsPWM Inverters

Sinusoidal PWM in an inverter is commonly obtained by comparing a low-frequency sinusoidal reference with a?

AHigh-frequency triangular carrier
BDC resistance
CSquare-wave load current only
DMechanical timing disk

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Correct Answer: A. High-frequency triangular carrier

Explanation: SPWM compares a sinusoidal modulating/reference signal with a higher-frequency triangular carrier to determine switching instants.

Reference: Standard power electronics: pwm inverters

Question 60

Power ElectronicsPWM Inverters

In sinusoidal PWM, increasing the carrier frequency while keeping the fundamental reference frequency fixed generally moves dominant switching harmonics?

ATo higher frequencies
BTo DC
CExactly onto the fundamental
DTo zero frequency

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Correct Answer: A. To higher frequencies

Explanation: Higher carrier frequency shifts switching-related harmonic groups upward, which can simplify output filtering at the cost of higher switching losses.

Reference: Standard power electronics: pwm inverters

Question 61

Power ElectronicsAC Voltage Controllers

A single-phase AC voltage controller with a resistive load commonly uses two antiparallel SCRs or a?

ATRIAC
BPower diode only
CZener diode
DBJT operated as a linear amplifier only

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Correct Answer: A. TRIAC

Explanation: A TRIAC provides bidirectional controlled conduction and can replace two antiparallel SCRs in many single-phase AC controllers.

Reference: Standard power electronics: ac voltage controllers

Question 62

Power ElectronicsAC Voltage Controllers

With a purely resistive load in an AC voltage controller, increasing the firing angle generally causes the RMS load voltage to?

ADecrease
BIncrease above the source RMS without a transformer
CRemain constant
DBecome DC

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Correct Answer: A. Decrease

Explanation: A larger firing delay reduces the conduction interval in each half-cycle, lowering the RMS output voltage.

Reference: Standard power electronics: ac voltage controllers

Question 63

Power ElectronicsDC-DC Converters

The ideal inverting buck-boost converter output voltage is given by?

AVo/Vs = -D/(1-D)
BVo/Vs = D
CVo/Vs = 1/(1-D)
DVo/Vs = 1-D

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Correct Answer: A. Vo/Vs = -D/(1-D)

Explanation: The basic inverting buck-boost converter produces an output of opposite polarity with magnitude D/(1-D) times the input.

Reference: Standard power electronics: dc-dc converters

Question 64

Power ElectronicsConverter Numericals

An ideal inverting buck-boost converter operates at D=0.5. The magnitude of its output voltage is?

AHalf the input
BEqual to the input
CTwice the input
DFour times the input

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Correct Answer: B. Equal to the input

Explanation: At D=0.5, |Vo/Vs|=D/(1-D)=0.5/0.5=1.

Reference: Standard power electronics: converter numericals

Question 65

Power ElectronicsPWM Control

Pulse-width modulation in a DC converter controls average output mainly by varying the?

ASwitch duty ratio
BSemiconductor layer count
CSupply polarity every cycle only
DLoad resistance mechanically

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Correct Answer: A. Switch duty ratio

Explanation: PWM varies the fraction of each switching period for which the controlled switch is ON.

Reference: Standard power electronics: pwm control

Question 66

Power ElectronicsSwitching Devices

A major advantage of operating a power semiconductor as a switch rather than in its linear region is?

AHigher conversion efficiency
BGuaranteed zero electromagnetic interference
CInfinite switching frequency
DNo need for protection circuits

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Correct Answer: A. Higher conversion efficiency

Explanation: Switch-mode operation ideally keeps either voltage or current small in the device, reducing power dissipation compared with linear control.

Reference: Standard power electronics: switching devices

Question 67

Power ElectronicsPower MOSFET

For high-frequency low-to-medium voltage switching, a power MOSFET is often preferred because of its?

AFast switching capability
BNeed for continuous gate current
CLatching behavior like an SCR
DVery slow turn-off

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Correct Answer: A. Fast switching capability

Explanation: Power MOSFETs are majority-carrier devices and can switch rapidly, making them useful at relatively high switching frequencies.

Reference: Standard power electronics: power mosfet

Question 68

Power ElectronicsIGBT

Compared with a power MOSFET, an IGBT is commonly attractive in applications requiring?

AHigher voltage and substantial power handling at moderate switching frequencies
BOnly microampere signal amplification
CNo gate drive at all
DBidirectional blocking without any circuit consideration

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Correct Answer: A. Higher voltage and substantial power handling at moderate switching frequencies

Explanation: IGBTs combine voltage-controlled gate drive with strong high-voltage/current capability, though typically at lower switching speeds than MOSFETs.

Reference: Standard power electronics: igbt

Question 69

Power ElectronicsSwitching Losses

Switching loss in a practical semiconductor device occurs mainly during?

ATurn-on and turn-off transitions
BThe ideal OFF state only
CThe ideal ON state with zero voltage drop
DMechanical rotation

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Correct Answer: A. Turn-on and turn-off transitions

Explanation: During switching transitions, appreciable voltage and current coexist for finite time, producing energy loss.

Reference: Standard power electronics: switching losses

Question 70

Power ElectronicsInverters

A single-phase voltage-source inverter normally requires a relatively stiff?

ADC voltage source
BAC current source only
CMechanical source
DOpen-circuit input

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Correct Answer: A. DC voltage source

Explanation: A voltage-source inverter is supplied from a DC link whose voltage is maintained comparatively stiff.

Reference: Standard power electronics: inverters

Question 71

Power ElectronicsSingle-Phase Inverter

A single-phase full-bridge inverter commonly uses how many controlled power switches?

ATwo
BThree
CFour
DSix

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Correct Answer: C. Four

Explanation: An H-bridge/full-bridge single-phase inverter uses four controlled switches.

Reference: Standard power electronics: single-phase inverter

Question 72

Power ElectronicsSingle-Phase Inverter

In an ideal single-phase full-bridge square-wave inverter supplied by Vdc, the load voltage switches between?

A0 and Vdc only
B-Vdc and +Vdc
C+2Vdc and -2Vdc
D0 and 2Vdc only

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Correct Answer: B. -Vdc and +Vdc

Explanation: With diagonal switch pairs conducting alternately, the full bridge applies +Vdc and -Vdc across the load.

Reference: Standard power electronics: single-phase inverter

Question 73

Power ElectronicsInverter Numericals

The RMS value of an ideal bipolar square wave that alternates between +Vdc and -Vdc is?

AVdc/2
BVdc/sqrt(2)
CVdc
Dsqrt(2)Vdc

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Correct Answer: C. Vdc

Explanation: The magnitude is Vdc throughout the cycle, so RMS=sqrt(mean(Vdc²))=Vdc.

Reference: Standard power electronics: inverter numericals

Question 74

Power ElectronicsInverter Harmonics

For a symmetrical square-wave inverter output with half-wave symmetry, which harmonics are ideally absent?

AEven harmonics
BAll odd harmonics
CFundamental component
DThird harmonic only

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Correct Answer: A. Even harmonics

Explanation: Half-wave symmetry eliminates even harmonics; the waveform contains the fundamental and odd harmonics.

Reference: Standard power electronics: inverter harmonics

Question 75

Power ElectronicsAC Power Control

Integral-cycle control regulates AC power by applying?

AComplete supply cycles to the load for some intervals and blocking complete cycles for others
BOnly one microsecond pulse per year
CA continuously variable DC voltage
DA different transformer turns ratio every half-cycle

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Correct Answer: A. Complete supply cycles to the load for some intervals and blocking complete cycles for others

Explanation: Integral-cycle or on-off control passes whole AC cycles during ON intervals and suppresses whole cycles during OFF intervals.

Reference: Standard power electronics: ac power control

Question 76

Power ElectronicsAC Power Control

Integral-cycle control is particularly suitable for loads with relatively large thermal time constants, such as?

AElectric heaters
BHigh-frequency RF mixers
CSmall-signal microphone preamplifiers
DCrystal oscillators

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Correct Answer: A. Electric heaters

Explanation: Thermal loads respond slowly, so groups of complete AC cycles can regulate average heating power without requiring rapid output response.

Reference: Standard power electronics: ac power control

Question 77

Power ElectronicsInverter Protection

In a voltage-source inverter, shoot-through of one bridge leg means?

AThe upper and lower switches of the same leg conduct simultaneously
BAll switches remain OFF
COnly the load current becomes zero
DThe DC source is disconnected safely

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Correct Answer: A. The upper and lower switches of the same leg conduct simultaneously

Explanation: Simultaneous conduction of both devices in one leg creates a very low-impedance path across the DC link and can damage the switches.

Reference: Standard power electronics: inverter protection

Question 78

Power ElectronicsThree-Phase Rectifiers

For an ideal three-phase fully controlled bridge rectifier with continuous DC current, the average output voltage is commonly written as?

A1.35 VLL cos alpha
B0.45 VLL cos alpha
C2.34 VLL sin alpha
DVLL/(1-D)

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Correct Answer: A. 1.35 VLL cos alpha

Explanation: For a six-pulse three-phase full converter with continuous current and negligible overlap, Vdc=1.35 VLL cos alpha, where VLL is the RMS line-to-line input voltage.

Reference: Standard power electronics: three-phase rectifiers

Question 79

Power ElectronicsRectifier Numericals

A three-phase full converter is supplied from 400 V line-to-line RMS with alpha=0 degrees. Its ideal average DC output is approximately?

A270 V
B400 V
C540 V
D660 V

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Correct Answer: C. 540 V

Explanation: Vdc=1.35×400×cos0°=540 V.

Reference: Standard power electronics: rectifier numericals

Question 80

Power ElectronicsRectifier Numericals

The same 400 V three-phase full converter operated at alpha=60 degrees has ideal average DC output of approximately?

A135 V
B270 V
C400 V
D540 V

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Correct Answer: B. 270 V

Explanation: Vdc=1.35×400×cos60°=270 V.

Reference: Standard power electronics: rectifier numericals

Question 81

Power ElectronicsThree-Phase Rectifiers

An ideal three-phase fully controlled bridge uses how many SCRs?

AThree
BFour
CSix
DTwelve

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Correct Answer: C. Six

Explanation: A conventional three-phase full-controlled bridge is a six-pulse converter containing six SCRs.

Reference: Standard power electronics: three-phase rectifiers

Question 82

Power ElectronicsThree-Phase Rectifiers

The ripple frequency at the DC output of a six-pulse three-phase bridge supplied from 50 Hz is?

A50 Hz
B100 Hz
C300 Hz
D600 Hz

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Correct Answer: C. 300 Hz

Explanation: A six-pulse bridge produces six output pulses per AC cycle, so ripple frequency=6×50=300 Hz.

Reference: Standard power electronics: three-phase rectifiers

Question 83

Power ElectronicsThree-Phase Rectifiers

Compared with a single-phase rectifier, a three-phase bridge generally provides?

ALower output ripple for comparable conditions
BOnly one output pulse per cycle
CNo DC component
DZero power capability

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Correct Answer: A. Lower output ripple for comparable conditions

Explanation: The higher pulse number of a three-phase bridge produces smoother DC and makes filtering easier.

Reference: Standard power electronics: three-phase rectifiers

Question 84

Power ElectronicsCommutation Overlap

Source inductance in a practical controlled rectifier causes commutation overlap because?

ACurrent transfer between devices cannot occur instantaneously
BGate pulses become DC
CLoad resistance becomes infinite
DSupply frequency becomes zero

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Correct Answer: A. Current transfer between devices cannot occur instantaneously

Explanation: Inductance opposes instantaneous current change, so outgoing and incoming devices conduct simultaneously for a finite overlap interval.

Reference: Standard power electronics: commutation overlap

Question 85

Power ElectronicsCommutation Overlap

Increasing commutation overlap in a line-commutated converter generally causes the average DC output voltage to?

ADecrease
BIncrease without limit
CRemain exactly unchanged
DBecome equal to peak line voltage

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Correct Answer: A. Decrease

Explanation: Commutation overlap introduces an additional voltage drop/loss of average output voltage.

Reference: Standard power electronics: commutation overlap

Question 86

Power ElectronicsCycloconverters

A cycloconverter is most commonly used to obtain an output frequency that is?

ALower than the input frequency
BAlways much higher than input
CExactly DC only
DIndependent of switching sequence

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Correct Answer: A. Lower than the input frequency

Explanation: Line-commutated cycloconverters are particularly suited to direct conversion from fixed-frequency AC to lower-frequency AC.

Reference: Standard power electronics: cycloconverters

Question 87

Power ElectronicsCycloconverters

A major application of high-power cycloconverters is?

ALow-speed large AC motor drives
BSmall battery-powered calculators
CAudio microphone biasing
DDC resistance measurement only

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Correct Answer: A. Low-speed large AC motor drives

Explanation: Cycloconverters have historically been used in high-power, low-speed variable-frequency motor-drive applications.

Reference: Standard power electronics: cycloconverters

Question 88

Power ElectronicsInverter Protection

Dead time in an inverter bridge is intentionally inserted mainly to prevent?

AShoot-through
BFundamental output voltage
CPWM operation
DLoad current flow in every interval

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Correct Answer: A. Shoot-through

Explanation: A short blanking interval between turning one switch OFF and its complementary switch ON prevents cross-conduction.

Reference: Standard power electronics: inverter protection

Question 89

Power ElectronicsPWM Inverters

In sinusoidal PWM, the amplitude modulation index in the linear region is commonly the ratio of?

AReference sine amplitude to carrier amplitude
BCarrier frequency to reference frequency
CDC-link voltage to load current
DSwitching loss to conduction loss

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Correct Answer: A. Reference sine amplitude to carrier amplitude

Explanation: For carrier-based SPWM, amplitude modulation index ma is the peak reference/modulating signal divided by the peak carrier signal.

Reference: Standard power electronics: pwm inverters

Question 90

Power ElectronicsPWM Numericals

If the sinusoidal reference peak is 4 V and triangular carrier peak is 5 V, the amplitude modulation index is?

A0.4
B0.8
C1.0
D1.25

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Correct Answer: B. 0.8

Explanation: ma=4/5=0.8.

Reference: Standard power electronics: pwm numericals

Question 91

Power ElectronicsPWM Inverters

In sinusoidal PWM, the frequency modulation ratio is commonly defined as?

ACarrier frequency divided by reference frequency
BReference amplitude divided by carrier amplitude
COutput voltage divided by input voltage
DLoad current divided by switch current

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Correct Answer: A. Carrier frequency divided by reference frequency

Explanation: mf=fc/fm, where fc is carrier frequency and fm is the desired fundamental/reference frequency.

Reference: Standard power electronics: pwm inverters

Question 92

Power ElectronicsPWM Numericals

An SPWM inverter uses a 10 kHz carrier and a 50 Hz sinusoidal reference. Its frequency modulation ratio is?

A50
B100
C200
D500

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Correct Answer: C. 200

Explanation: mf=10000/50=200.

Reference: Standard power electronics: pwm numericals

Question 93

Power ElectronicsInverter Numericals

For an ideal single-phase full-bridge square-wave inverter with DC input Vdc, the RMS value of the fundamental output-voltage component is approximately?

A0.45 Vdc
B0.90 Vdc
C1.11 Vdc
D1.27 Vdc

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Correct Answer: B. 0.90 Vdc

Explanation: The fundamental peak is 4Vdc/pi; its RMS value is 4Vdc/(pi sqrt2)≈0.900Vdc.

Reference: Standard power electronics: inverter numericals

Question 94

Power ElectronicsInverter Numericals

For Vdc=200 V, the fundamental RMS output of an ideal single-phase full-bridge square-wave inverter is approximately?

A90 V
B127 V
C180 V
D200 V

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Correct Answer: C. 180 V

Explanation: V1,rms≈0.900×200=180 V.

Reference: Standard power electronics: inverter numericals

Question 95

Power ElectronicsHarmonics

Total harmonic distortion (THD) is a measure of?

AHarmonic content relative to the fundamental component
BOnly DC input voltage
CSemiconductor junction temperature
DGate resistance

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Correct Answer: A. Harmonic content relative to the fundamental component

Explanation: THD quantifies the RMS contribution of harmonics relative to the RMS fundamental component.

Reference: Standard power electronics: harmonics

Question 96

Power ElectronicsPWM Inverters

An output filter at a PWM inverter is mainly used to?

AAttenuate switching-frequency harmonics
BIncrease semiconductor layer count
CGenerate gate pulses
DEliminate the DC source

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Correct Answer: A. Attenuate switching-frequency harmonics

Explanation: The filter passes the desired low-frequency fundamental while reducing high-frequency switching components.

Reference: Standard power electronics: pwm inverters

Question 97

Power ElectronicsEfficiency & Losses

If a converter delivers 900 W output while drawing 1000 W input, its efficiency is?

A10%
B90%
C100%
D111%

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Correct Answer: B. 90%

Explanation: Efficiency=Pout/Pin=900/1000=0.9=90%.

Reference: Standard power electronics: efficiency & losses

Question 98

Power ElectronicsDevice Losses

A switch carries 20 A while its ON-state voltage drop is 2 V. Its instantaneous conduction loss during the ON interval is?

A10 W
B20 W
C40 W
D400 W

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Correct Answer: C. 40 W

Explanation: Conduction power loss is approximately VON×I=2×20=40 W while the device is ON.

Reference: Standard power electronics: device losses

Question 99

Power ElectronicsDevice Losses

If a switch dissipates 40 W during its ON interval and its duty ratio is 0.25, neglecting other losses its average conduction loss is?

A10 W
B20 W
C40 W
D160 W

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Correct Answer: A. 10 W

Explanation: Average conduction loss=40×0.25=10 W.

Reference: Standard power electronics: device losses

Question 100

Power ElectronicsDevice Selection

For high-voltage, high-power switching at moderate switching frequency, which device is commonly a suitable fully controllable choice?

AIGBT
BDIAC
CUncontrolled power diode
DConventional SCR with no turn-off circuit in every case

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Correct Answer: A. IGBT

Explanation: IGBTs are widely used where substantial voltage/current capability and gate-controlled turn-on/turn-off are required at moderate switching frequencies.

Reference: Standard power electronics: device selection

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