Power Electronics MCQs 2026

Step-down (buck) chopper output voltage is

1. A lower than input DC voltage ✓
2. B always higher than input without exception ✓
3. C equal to input always without switching ✓
4. D unrelated to duty cycle always ✓

Step-up (boost) chopper can produce output voltage

1. A only lower than input always ✓
2. B higher than input DC source ✓
3. C exactly zero without inductor ever in all operating modes ✓
4. D without any inductor or energy storage element ever in ideal boost ✓

Duty cycle D of chopper is defined as

1. A OFF time to period only ✓
2. B input voltage to output voltage always without switching ✓
3. C peak current to average current only without time definition ✓
4. D ratio of ON time to total switching period ✓

Average output voltage of ideal buck chopper is

1. A Vin/D always ✓
2. B (1-D)×Vin for buck—note boost is different ✓
3. C D × Vin ✓
4. D zero regardless of D always ✓

Average output voltage of ideal boost chopper is

1. A Vin / (1 − D) ✓
2. B D × Vin which is buck relation not boost ✓
3. C Vin × (1 − D) always ✓
4. D independent of D always ✓

Freewheeling diode in chopper circuit

1. A blocks inductor current always when switch OFF causing infinite voltage spike always without diode ✓
2. B provides path for inductor current when switch is OFF ✓
3. C replaces main switching device entirely without switch ✓
4. D measures fault impedance for distance relay only ✓

Chopper used in DC motor speed control varies

1. A average armature voltage by changing duty cycle ✓
2. B field flux only without armature voltage effect ever for speed control in shunt motor context both matter but armature chopper is classic ✓
3. C air gap length physically without electronics ✓
4. D number of poles without electrical control ✓

Regenerative chopper operation returns energy when

1. A motor always motoring without reversal of power flow ever ✓
2. B duty cycle is zero always with switch permanently open without regeneration path design ✓
3. C fuse blows on overload only without energy return ✓
4. D load drives current back to source during braking ✓

Current ripple in chopper output depends on

1. A only input DC voltage magnitude without L or fsw ✓
2. B only PID proportional band without power stage ✓
3. C inductance, switching frequency and load current ✓
4. D only insulator creepage distance only ✓

Discontinuous conduction mode in buck chopper occurs when

1. A inductor current never reaches zero ever in all loads without exception ✓
2. B inductor current falls to zero before next switching cycle ✓
3. C input voltage equals output always ✓
4. D duty cycle equals unity always in DCM definition boundary only at boundary case ✓

Thyristor chopper requires

1. A only natural commutation from AC line zero always on DC bus chopper without AC zero ✓
2. B no commutation because SCR turns off automatically on DC without design ever ✓
3. C only gate pulse removal without commutation network on DC ✓
4. D forced commutation circuit to turn off SCR ✓

IGBT/MOSFET choppers operate at

1. A higher switching frequencies than SCR choppers with simpler control ✓
2. B only line frequency 50 Hz without PWM ever as defining advantage of modern devices ✓
3. C zero frequency DC without switching ever ✓
4. D only sub-hertz without filters ever as typical modern design point ✓

Input filter on chopper reduces

1. A output voltage ripple only at load without affecting input ✓
2. B motor slip without electrical connection ✓
3. C current ripple reflected to DC source ✓
4. D symmetrical component unbalance only on AC lines without DC chopper relevance ✓

Buck-boost chopper output voltage polarity is

1. A same polarity always as input without inversion ever in ideal buck-boost ✓
2. B always positive without inversion regardless of topology name ✓
3. C independent of duty cycle always ✓
4. D inverted compared to input and magnitude depends on D ✓

Silicon controlled rectifier (SCR) is a

1. A two-terminal diode only without control ✓
2. B four-layer PNPN device with three terminals anode, cathode and gate ✓
3. C vacuum triode only ✓
4. D transformer core only ✓

SCR turns ON when

1. A cathode is positive with respect to anode always ✓
2. B gate alone turns off device in DC circuit without commutation ✓
3. C reverse voltage alone triggers conduction ✓
4. D gate pulse is applied while anode is positive with respect to cathode and forward biased ✓

Latching current of SCR is

1. A minimum anode current to maintain ON state after gate pulse removal ✓
2. B gate current only ✓
3. C reverse leakage only ✓
4. D holding current always greater than latching ✓

Holding current is

1. A always equal to latching current exactly in all datasheets without exception ✓
2. B gate trigger current only ✓
3. C minimum anode current below which SCR turns OFF ✓
4. D peak repetitive forward voltage only ✓

In forward blocking state SCR

1. A conducts heavily without gate ✓
2. B blocks current with anode positive until triggered ✓
3. C conducts in reverse direction freely ✓
4. D behaves as short circuit always ✓

SCR in reverse bias

1. A blocks current like a reverse-biased diode until breakdown rating exceeded ✓
2. B conducts like a forward diode always ✓
3. C turns on with gate pulse in reverse conduction mode normally ✓
4. D has no peak inverse voltage rating ✓

SCR cannot be turned OFF by removing gate signal alone in DC circuit because

1. A gate has no role in turn-on ✓
2. B anode-cathode becomes superconducting permanently ✓
3. C device is unipolar only ✓
4. D regenerative internal feedback sustains conduction once latched ✓

Natural commutation of SCR occurs in

1. A AC circuits when current passes through zero ✓
2. B pure DC bus without auxiliary circuit always ✓
3. C only open-loop control systems ✓
4. D only transformer oil only ✓

Isolation transformer at inverter output provides

1. A galvanic isolation and voltage matching ✓
2. B forced commutation for SCR always without electronics ✓
3. C symmetrical component transformation only without galvanic isolation benefit ✓
4. D distance protection reach setting only ✓

Overmodulation in PWM inverter occurs when

1. A firing angle is below zero in rectifier only without PWM context ✓
2. B DC link is unregulated without any voltage ripple ever ✓
3. C reference exceeds linear range and low-order harmonics increase ✓
4. D gate dead time is infinite always ✓

UPS inverter must maintain

1. A only rectifier operation without battery ever for offline UPS definition ✓
2. B stable voltage and frequency during supply outage using stored energy ✓
3. C only line sag calculation on EHV lines without battery ✓
4. D only Buchholz alarm without static switch ✓

Multilevel inverter reduces

1. A need for any switching devices entirely without semiconductors ✓
2. B dv/dt stress and harmonic content by stepping output voltage levels ✓
3. C DC link voltage to zero always without capacitors ✓
4. D motor slip to zero always without load torque consideration ✓

DC chopper converts

1. A fixed DC voltage to variable average DC voltage by switching ✓
2. B AC to fixed DC only without switching ✓
3. C DC to AC directly without inversion stage terminology confusion—chopper is DC-DC ✓
4. D AC to AC at same frequency without electronic switching only using transformer taps without switches ✓

Forced commutation is required in

1. A AC diode rectifier on resistive load only without SCR ✓
2. B passive RC snubber only without switches ✓
3. C DC choppers and inverters using SCRs ✓
4. D only Buchholz relay operation ✓

dv/dt rating of SCR specifies

1. A maximum rate of rise of anode voltage without spurious turn-on ✓
2. B maximum gate current only ✓
3. C maximum on-state voltage drop only ✓
4. D PID derivative gain only ✓

di/dt rating limits

1. A only reverse recovery of diode without relation ✓
2. B only line sag calculation ✓
3. C only distance relay reach only ✓
4. D rate of rise of anode current at turn-on to protect device ✓

Gate trigger current is

1. A same as holding current always ✓
2. B equal to anode latching current always ✓
3. C minimum gate current to switch SCR from off to on state ✓
4. D independent of junction temperature always without derating ✓

Two SCRs in anti-parallel on AC switch line allow

1. A only half-wave rectification without diodes ✓
2. B only DC chopper operation without commutation ✓
3. C only open-loop step response measurement only ✓
4. D control of power in both half cycles ✓

RC snubber across SCR protects against

1. A only di/dt at turn-on without capacitance ✓
2. B dv/dt and voltage transients ✓
3. C only gate oxide breakdown from overcurrent only without voltage effect ✓
4. D only motor slip measurement ✓

On-state voltage drop of high-power SCR is typically

1. A about 1 to 2 V depending on current rating ✓
2. B hundreds of volts always when conducting ✓
3. C zero always ✓
4. D equal to reverse peak voltage always ✓

SCR is preferred over transistor in high-power AC line applications when

1. A switching frequency exceeds 1 MHz always without IGBT ✓
2. B only microampere currents are switched ✓
3. C line commutation is available and high voltage/current rating needed economically ✓
4. D only battery voltage below 1 V ✓

Reverse conducting SCR integrates

1. A anti-parallel diode in same package for commutation and freewheeling ✓
2. B IGBT with anti-parallel SCR only without diode ✓
3. C only vacuum interrupter contacts ✓
4. D only PID integral windup clamp only ✓

Light activated SCR (LASCR) is triggered by

1. A only mechanical vibration of heat sink without photons ✓
2. B only distance relay carrier signal ✓
3. C only symmetrical component calculator only ✓
4. D incident light on sensitive gate region ✓

Half-wave rectifier with resistive load produces

1. A output with one conduction half cycle per input cycle ✓
2. B full DC without ripple without filter ✓
3. C pure sinusoidal AC at output always ✓
4. D three-phase balanced output always ✓

Full-wave centre-tap rectifier uses

1. A one diode only on three-phase supply without transformer ✓
2. B four SCRs in bridge without AC input ✓
3. C only inductor filter without diodes ✓
4. D two diodes and centre-tapped transformer secondary ✓

Bridge rectifier with four diodes gives

1. A half-wave only always ✓
2. B DC with frequency equal to input without ripple ✓
3. C requires forced commutation for diodes always ✓
4. D full-wave output without centre-tapped transformer ✓

Average DC voltage of single-phase full-wave bridge (ideal) is

1. A Vm/π only ✓
2. B 2Vm/π where Vm is peak AC voltage ✓
3. C 2Vm only without division ✓
4. D Vm/2 only ✓

Three-phase half-wave rectifier (three diodes) has ripple frequency

1. A equal to supply frequency only ✓
2. B six times supply frequency always ✓
3. C three times supply frequency ✓
4. D zero ripple without filter always ✓

Three-phase full-wave bridge rectifier uses

1. A two diodes only on single phase always ✓
2. B one SCR without diodes on DC bus only ✓
3. C six diodes or SCRs for two pulses per phase per cycle ✓
4. D only RC snubber without switches ✓

Output ripple frequency of six-pulse rectifier is

1. A three times only always ✓
2. B six times the supply frequency ✓
3. C same as supply without harmonics ✓
4. D zero with any load always ✓

Power factor of uncontrolled three-phase rectifier is

1. A less than unity and depends on load and firing angle ✓
2. B always unity without harmonics ✓
3. C zero always ✓
4. D independent of load current always ✓

Controlled rectifier uses SCRs to adjust

1. A only frequency of AC input without inversion ✓
2. B average output voltage by varying firing angle α ✓
3. C only power factor to unity always without control ✓
4. D only filter inductance without delay angle ✓

As firing angle α increases in controlled rectifier, average output voltage

1. A increases linearly without limit above input peak always ✓
2. B decreases ✓
3. C remains constant always ✓
4. D becomes independent of AC supply ✓

Extinction angle β in line-commutated inverter mode must be

1. A zero for safe operation always ✓
2. B negative without limit ✓
3. C equal to firing angle only without overlap ✓
4. D greater than recovery time of devices plus safety margin ✓

Overlap angle μ in rectifiers occurs due to

1. A source inductance causing simultaneous conduction of phases ✓
2. B only load capacitance without source impedance ✓
3. C only PID derivative kick ✓
4. D only Buchholz gas pressure alone ✓

Interphase transformer in dual three-phase rectifiers helps

1. A eliminate all harmonics completely without any remaining ripple ✓
2. B replace all SCRs with diodes only on DC side without AC ✓
3. C measure distance to fault on line only ✓
4. D operate two 6-pulse bridges with 30° phase shift for 12-pulse operation ✓

Filter inductor on DC side of rectifier

1. A increases ripple amplitude always ✓
2. B blocks DC while passing only AC ripple without inductance effect ✓
3. C smooths current and reduces ripple ✓
4. D replaces need for diodes entirely ✓

Filter capacitor on DC side of rectifier

1. A smooths only current without affecting voltage ripple ✓
2. B eliminates need for transformer always ✓
3. C smooths voltage ripple ✓
4. D provides forced commutation for SCR in all DC circuits automatically without design ✓

Diode rectifier does not require

1. A AC input voltage ever ✓
2. B transformer or source impedance ever ✓
3. C any heat sinking on diodes ever ✓
4. D gate firing or commutation circuit ✓

Harmonics generated by rectifiers can cause

1. A improved power factor without filters always ✓
2. B distortion, heating and relay misoperation on the supply system ✓
3. C zero neutral current in all cases always ✓
4. D automatic voltage regulation without STATCOM always ✓

Dual converter uses two bridges to provide

1. A only half-wave rectification without reversal ✓
2. B regenerative four-quadrant DC control ✓
3. C only open-loop motor speed without feedback ✓
4. D only fuse protection without thyristors ✓

Voltage source inverter (VSI) has

1. A DC voltage source with capacitor at input and switches create AC output ✓
2. B DC current source at input with large inductor always as defining feature without alternative ✓
3. C no switching devices ✓
4. D only diode rectifier without switches on AC side ✓

Current source inverter (CSI) has

1. A capacitive DC link without inductor as defining element ✓
2. B large DC inductor maintaining nearly constant DC current ✓
3. C only passive diodes without controlled switches on output always ✓
4. D only SCR without commutation in all modern CSI designs without exception ✓

PWM inverter controls output voltage and harmonic content by

1. A fixed square wave only at 50 Hz without modulation ever in all drives ✓
2. B varying pulse widths of switched DC voltage ✓
3. C analog potentiometer on DC link only without switches ✓
4. D only transformer tap changer without switching ✓

Single-phase H-bridge inverter uses

1. A two diodes only without switches ✓
2. B one SCR only on DC positive rail without return path switch ✓
3. C four switches to synthesize AC from DC ✓
4. D six diodes in three-phase bridge only without inversion capability ✓

Third harmonic injection in three-phase inverter PWM helps

1. A eliminate all switching losses completely without trade-off ✓
2. B increase DC bus utilization by extending linear modulation range ✓
3. C replace PID controller in outer loop always ✓
4. D measure symmetrical components only without PWM change ✓

Dead time in inverter gate signals prevents

1. A overmodulation only without shoot-through risk ever ✓
2. B natural commutation in AC line only ✓
3. C shoot-through short circuit of DC bus through upper and lower switches ✓
4. D Ferranti effect on transmission line only ✓

Line-commutated inverter (LCI) feeds power from DC to AC when

1. A firing angle is zero always in rectifier only without reversal ✓
2. B DC voltage is zero always ✓
3. C SCR firing angle exceeds 90° into inverter mode with sufficient extinction angle ✓
4. D load is purely resistive without AC source ✓

Forced-commutated inverter (e.g., IGBT based) can

1. A only work with synchronous motor at fixed speed without control ✓
2. B not control frequency or voltage ✓
3. C not use PWM ever ✓
4. D operate from DC source without relying on AC line for turn-off ✓

Output frequency of variable frequency drive inverter is controlled by

1. A only DC link voltage without changing switch rate ever for frequency change ✓
2. B only transformer turns ratio on output without electronics ✓
3. C only fuse rating without switches ✓
4. D modulating switch timing / PWM carrier and reference frequency ✓

Harmonic filter at inverter output may be required to

1. A meet grid code THD limits ✓
2. B increase THD deliberately ✓
3. C eliminate DC link capacitor always ✓
4. D replace all IGBTs with SCRs without commutation ever in all cases ✓

Space vector modulation (SVM) in three-phase inverters

1. A uses only single-phase H-bridge always without three phases ✓
2. B optimizes switching to maximize DC bus usage and reduce harmonics ✓
3. C eliminates all switching losses without heat sink ever ✓
4. D replaces motor windings entirely ✓

Regenerative braking with inverter drive returns energy by

1. A inverting DC link power back to AC supply when motor acts as generator ✓
2. B dissipating only in rotor copper without electronics always ✓
3. C opening fuse links only ✓
4. D increasing line sag only on overhead conductors ✓

Square wave inverter output contains

1. A only DC without any AC component ever ✓
2. B only even harmonics without fundamental ever ✓
3. C fundamental plus odd harmonics ✓
4. D pure sinusoid without harmonics always without filter ✓

Chopper efficiency is improved by

1. A maximizing dead time without limit always causing shoot-through if too small but excessive dead time hurts output—still minimizing losses is key ✓
2. B using only linear regulators without switching always for high power efficiency claim ✓
3. C opening fuse on every cycle for current limit as efficiency strategy ✓
4. D minimizing switching and conduction losses in devices ✓

Soft switching techniques in choppers aim to

1. A increase dv/dt stress on SCR always deliberately ✓
2. B eliminate inductor from circuit always without replacement ✓
3. C switch at zero voltage or zero current to reduce losses ✓
4. D replace PID with bang-bang only without semiconductor benefit ✓