What is the main role of a power electronic converter?
A Store mechanical energy
B Control electrical power
C Measure insulation resistance
D Convert heat directly
Answer: B. Control electrical power. Power electronic converters use controlled semiconductor switches to change voltage, current, frequency or power flow.
How do switching converters reduce conduction losses?
A Operate mainly as switches
B Use resistors continuously
C Require no control
D Eliminate all losses
Answer: A. Operate mainly as switches. An ideal switch has either low on-state voltage or low off-state current; real devices still have conduction and switching losses.
Which terminals belong to a conventional SCR?
A Drain, source, gate
B Anode, cathode, gate
C Emitter, base, collector
D Line, neutral, earth
Answer: B. Anode, cathode, gate. An SCR has an anode and cathode in its main path and a gate control terminal.
How is a forward-biased SCR normally triggered?
A Reverse its terminals
B Apply a gate pulse
C Remove its heat sink
D Open its anode path
Answer: B. Apply a gate pulse. A suitable gate-current pulse triggers a forward-biased SCR into conduction.
After gate removal, what keeps an SCR latched on?
A A permanent gate pulse
B A reverse gate voltage
C Current above latching current
D Zero anode current
Answer: C. Current above latching current. The anode current must rise above the latching current before the gate pulse is removed.
What is SCR holding current?
A Maximum gate current
B Peak reverse current
C Rated leakage current
D Minimum sustaining anode current
Answer: D. Minimum sustaining anode current. If the anode current falls below holding current, the SCR turns off after its recovery interval.
Which device conducts one way without gate control?
A Power diode
B GTO thyristor
C IGBT module
D MOSFET switch
Answer: A. Power diode. A power diode conducts when forward biased and blocks reverse voltage; it has no gate terminal.
Which converter changes AC into DC?
A Inverter
B Chopper
C Rectifier
D Cycloconverter
Answer: C. Rectifier. A rectifier converts an alternating input into a unidirectional output.
Which converter changes DC into AC?
A Inverter
B Rectifier
C Chopper
D AC regulator
Answer: A. Inverter. An inverter synthesizes an alternating output from a direct-current source.
Which converter changes one DC level to another?
A Line commutator
B DC chopper
C Diode bridge
D AC regulator
Answer: B. DC chopper. A chopper switches a DC source to produce a controlled DC output.
An AC voltage controller usually keeps which output quantity unchanged?
A RMS voltage
B Load current
C Firing angle
D Supply frequency
Answer: D. Supply frequency. Phase control changes the RMS voltage while the output frequency remains the supply frequency.
Which converter directly produces lower-frequency AC?
A Cycloconverter
B Buck converter
C Diode rectifier
D Boost converter
Answer: A. Cycloconverter. A cycloconverter directly converts AC at one frequency to AC at a lower or otherwise controlled frequency.
How many diodes are used in a single-phase half-wave rectifier?
A Two diodes
B Four diodes
C Six diodes
D One diode
Answer: D. One diode. A single diode conducts on one input half-cycle in the basic half-wave rectifier.
How many diodes form a single-phase bridge rectifier?
A One diode
B Two diodes
C Six diodes
D Four diodes
Answer: D. Four diodes. The basic single-phase bridge uses four diodes, with two conducting in each half-cycle.
What is the usual output of a diode rectifier?
A Pure sinusoidal AC
B Constant-frequency AC
C Pulsating DC
D Negative-only AC
Answer: C. Pulsating DC. Rectification makes the output unidirectional, though it contains ripple unless filtered.
How can a conventional SCR be turned off?
A Reduce current below holding
B Increase gate current
C Raise forward voltage
D Short its gate
Answer: A. Reduce current below holding. A conventional SCR is turned off by circuit commutation that reduces its anode current below holding current for long enough.
Which thyristor can be turned off by its gate?
A Ordinary SCR
B Power diode
C GTO thyristor
D DIAC device
Answer: C. GTO thyristor. A gate turn-off thyristor can be turned off by applying an appropriate negative gate-current pulse.
Which device combines MOS gate control and bipolar conduction?
A Power diode
B DIAC
C IGBT
D Zener diode
Answer: C. IGBT. An IGBT has a voltage-controlled insulated gate and bipolar conduction in its main path.
Which terminal voltage controls a power MOSFET?
A Drain-gate voltage
B Collector-emitter voltage
C Anode-cathode voltage
D Gate-source voltage
Answer: D. Gate-source voltage. A MOSFET channel is controlled by the gate-to-source voltage.
Why is an antiparallel diode used with an inverter switch?
A Carry inductive load current
B Set the carrier frequency
C Measure DC-link voltage
D Block every reverse current
Answer: A. Carry inductive load current. An antiparallel diode provides a path for inductive current when the controlled switch is off.
What does firing angle measure in a line-controlled converter?
A Load power factor
B Delay from reference crossing
C Switching frequency
D DC ripple amplitude
Answer: B. Delay from reference crossing. The firing angle is the delay between a chosen AC reference point and the thyristor gate pulse.
With continuous current, what happens as firing angle rises in a full converter?
A Average output rises
B Frequency doubles
C Ripple disappears
D Average output voltage falls
Answer: D. Average output voltage falls. For a fully controlled line-commutated converter, the average DC voltage varies with the cosine of firing angle.
What provides a path for inductive current after switch turn-off?
A Gate resistor
B Freewheeling diode
C Bleeder resistor
D Line fuse
Answer: B. Freewheeling diode. An inductive load resists abrupt current change, so a freewheeling path carries current during the switch-off interval.
Why isolate a gate-drive circuit from control logic?
A Increase load frequency
B Remove device losses
C Protect low-voltage controls
D Rectify the output
Answer: C. Protect low-voltage controls. Galvanic isolation helps protect low-voltage control circuits from high-voltage power stages and switching transients.
What is a common switching-frequency tradeoff?
A Smaller filters, higher switching loss
B Larger filters, zero losses
C Lower ripple, no heat
D Higher ripple, lower voltage
Answer: A. Smaller filters, higher switching loss. A higher switching frequency can reduce passive filter size and ripple, but usually increases switching losses.