Answer: D. Vout equals D Vin. In an ideal buck converter operating in continuous conduction, the average output voltage is the duty ratio times the input voltage.
What does an ideal boost converter do?
A Lowers the DC voltage
B Reverses AC phase
C Raises the DC voltage
D Changes DC to AC
Answer: C. Raises the DC voltage. A boost converter transfers energy from a lower DC input to a higher DC output in its ideal operating range.
How is switch duty ratio defined?
A Off-time divided by voltage
B On-time divided by period
C Current divided by power
D Frequency divided by time
Answer: B. On-time divided by period. Duty ratio is the fraction of each switching period for which the controlled switch is on.
What equals one switching period?
A On-time plus off-time
B Input plus output voltage
C Current plus resistance
D Ripple plus average power
Answer: A. On-time plus off-time. Each switching period consists of the switch on interval and the switch off interval.
In continuous conduction, what happens to inductor current?
A It never reaches zero
B It stays perfectly constant
C It reverses every cycle
D It equals output voltage
Answer: A. It never reaches zero. Continuous conduction mode means the inductor current remains above zero throughout the switching cycle.
What defines discontinuous conduction mode?
A Inductor current reaches zero
B Output voltage reaches zero
C Switch stays always on
D Input current is sinusoidal
Answer: A. Inductor current reaches zero. In discontinuous conduction mode, the inductor current falls to zero for part of each switching period.
In a buck converter, what happens during switch on-time?
A Inductor stores input energy
B Diode shorts the source
C Output capacitor is removed
D Inductor current must stop
Answer: A. Inductor stores input energy. During the on interval, the input supplies the load and increases the buck inductor current.
In a boost converter, when does its inductor store energy?
A Only while its diode conducts
B Only at zero current
C During gate isolation
D While its switch conducts
Answer: D. While its switch conducts. With the boost switch on, the diode is reverse biased and the input builds energy in the inductor.
During buck switch off-time, where can inductor current flow?
A Through the open switch
B Through the freewheel diode
C Into the gate circuit
D Only through earth
Answer: B. Through the freewheel diode. The freewheel diode carries the inductor current when the buck switch turns off.
During boost switch off-time, where does inductor energy go?
A Back through the gate
B Into an open circuit
C Through the diode to output
D Only into the switch
Answer: C. Through the diode to output. When the boost switch turns off, the inductor voltage forward biases the diode and supplies the output.
What is a common role of the output capacitor?
A Set SCR holding current
B Reduce output voltage ripple
C Create gate pulses
D Increase switching frequency
Answer: B. Reduce output voltage ripple. The output capacitor supplies or absorbs ripple current to keep the output voltage smoother.
For an ideal CCM buck, what is the output at D equals 0.25?
A Four times input voltage
B One quarter input voltage
C Half input voltage
D Input plus 25 volts
Answer: B. One quarter input voltage. The ideal buck relation is Vout equals D times Vin, so a duty ratio of 0.25 gives one quarter of the input.
What is the ideal CCM boost gain?
A Vin times one minus D
B D divided by Vin
C Vin/(1-D)
D Vin divided by D
Answer: C. Vin/(1-D). For an ideal boost converter in CCM, Vout equals Vin divided by one minus the duty ratio.
What output polarity does a classical buck-boost have?
A Same polarity always
B Opposite input polarity
C Three-phase polarity
D No output polarity
Answer: B. Opposite input polarity. The basic inverting buck-boost topology produces an output voltage with polarity opposite to its input reference.
What does an isolated DC-DC converter use for galvanic isolation?
A Freewheeling diode
B Output fuse
C High-frequency transformer
D Current shunt
Answer: C. High-frequency transformer. An isolated converter transfers energy magnetically through a transformer while separating input and output grounds.
In a flyback converter, when is energy transferred to its output?
A During switch off-time
B Only during switch on-time
C Before the input is applied
D Only at startup
Answer: A. During switch off-time. The flyback transformer stores energy while the primary switch is on and transfers it to the output when the switch turns off.
How many controlled switches are in a basic single-phase full bridge?
A One switch
B Two switches
C Four switches
D Six switches
Answer: C. Four switches. A single-phase full-bridge inverter uses two switching devices in each of its two legs.
What can happen if both switches in one inverter leg conduct?
A DC-link shoot-through
B Output filtering
C Safe freewheeling
D Zero gate current
Answer: A. DC-link shoot-through. Simultaneous conduction of the upper and lower switches in one leg can short the DC link.
Why is dead time inserted between leg switches?
A Raise the DC voltage
B Increase load frequency
C Remove diode current
D Prevent simultaneous conduction
Answer: D. Prevent simultaneous conduction. Dead time prevents the upper and lower devices in one inverter leg from being commanded on together.
In sine-triangle PWM, what sets carrier frequency?
A Load resistance
B DC-link current
C Output capacitor value
D Triangle-wave frequency
Answer: D. Triangle-wave frequency. The triangular carrier frequency sets the switching frequency in conventional sine-triangle PWM.
In linear sinusoidal PWM, what mainly controls fundamental amplitude?
A Load inductance
B Dead-time duration
C Gate resistance only
D Modulation index
Answer: D. Modulation index. Within the linear modulation range, increasing the modulation index increases the inverter fundamental voltage.
What mainly sets inverter fundamental frequency in sine PWM?
A Reference sine frequency
B Carrier amplitude
C DC-link capacitance
D Switching-device area
Answer: A. Reference sine frequency. The sinusoidal reference sets the fundamental output frequency while the carrier sets the switching pattern.
What is a defining DC-link element in a current-source inverter?
A Large shunt capacitor
B Large series inductor
C Output transformer only
D No energy storage
Answer: B. Large series inductor. A current-source inverter uses a large DC-link inductance to keep its input current comparatively continuous.
What is a defining DC-link element in a voltage-source inverter?
A Large series resistor
B Open DC circuit
C Constant gate current
D Stiff DC voltage source
Answer: D. Stiff DC voltage source. A voltage-source inverter is supplied by a relatively stiff voltage source, often supported by a DC-link capacitor.
What does an AC voltage controller primarily vary?
A Supply frequency
B Number of phases
C Load RMS voltage
D DC-link polarity
Answer: C. Load RMS voltage. An AC voltage controller changes the RMS voltage applied to a load, commonly by phase control or integral-cycle control.