A buck has Vin 200 V and D 0.4. What is ideal Vout?
A 50 V
B 80 V
C 120 V
D 500 V
Answer: B. 80 V. For an ideal buck in CCM, Vout equals D times Vin; 0.4 times 200 V is 80 V.
A boost has Vin 100 V and D 0.6. Find ideal Vout.
A 160 V
B 60 V
C 600 V
D 250 V
Answer: D. 250 V. The ideal CCM boost gain is 1 divided by one minus D, so 100 divided by 0.4 is 250 V.
A full converter has Vm 100 V and α 60°. Find average output, continuous current.
A -31.8 V
B 63.7 V
C 31.8 V
D 86.6 V
Answer: C. 31.8 V. For a single-phase fully controlled converter with continuous current, Vdc equals 2Vm cos α divided by π; here it is about 31.8 V.
A three-phase diode bridge has VLL 400 V. Estimate average DC voltage.
A 540 V
B 400 V
C 360 V
D 692 V
Answer: A. 540 V. For an ideal three-phase six-pulse diode bridge with continuous current, the average output is about 1.35 times the line-to-line RMS voltage.
What is the ripple frequency of a six-pulse bridge on 50 Hz?
A 50 Hz
B 100 Hz
C 250 Hz
D 300 Hz
Answer: D. 300 Hz. A six-pulse bridge produces six main output ripple pulses per AC cycle, so 6 times 50 Hz is 300 Hz.
Which term sets buck inductor ripple during the on interval?
A Vin minus Vout
B Vout plus Vin
C Input current only
D Output resistance
Answer: A. Vin minus Vout. During buck switch on-time, the inductor voltage is Vin minus Vout, which determines its rising current slope.
For an ideal boost, what is inductor voltage during switch on?
A Output voltage only
B Zero volts
C Input voltage
D Negative output voltage
Answer: C. Input voltage. When the boost switch conducts, the diode is off and the input voltage is applied across the inductor.
At fixed inductance and duty ratio, what does higher fs usually do to ripple?
A Increases current ripple
B Reduces current ripple
C Makes current discontinuous
D Reverses average current
Answer: B. Reduces current ripple. For a fixed converter operating point, inductor ripple is inversely proportional to switching frequency.
What causes commutation overlap in a line converter?
A Gate isolation
B Output capacitance only
C Low switching frequency
D Source-side inductance
Answer: D. Source-side inductance. Source inductance prevents current from transferring instantaneously between outgoing and incoming thyristors.
What is a usual effect of overlap on average converter voltage?
A It raises average voltage
B It reduces average voltage
C It removes input harmonics
D It reverses load current
Answer: B. It reduces average voltage. During overlap, two phases share current transfer and the average DC output falls below the zero-source-inductance value.
In a diode bridge with a large input capacitor, what is line current like?
A Narrow harmonic-rich pulses
B Pure sinusoidal current
C Constant DC current
D Zero crossing current
Answer: A. Narrow harmonic-rich pulses. The capacitor draws short charging pulses near voltage peaks, creating harmonic-rich input current.
What does active power-factor correction shape?
A Input current near sinusoidal
B Output voltage into pulses
C Gate voltage into DC
D Load resistance into zero
Answer: A. Input current near sinusoidal. An active PFC stage controls its input current to follow the supply voltage waveform, improving power factor and reducing harmonics.
What can diode reverse recovery cause in a hard-switched bridge?
A Current spikes and losses
B Zero voltage switching
C Lower gate charge
D Perfect unity power factor
Answer: A. Current spikes and losses. Reverse recovery briefly drives current through a diode as it turns off, increasing switching stress, loss and EMI.
Why can excessive SCR dv/dt trigger it?
A Low gate resistance only
B Junction displacement current
C Excessive holding current
D Anode heating only
Answer: B. Junction displacement current. A rapid voltage rise creates capacitive current inside the SCR that can trigger unintended conduction.
What does a gate resistor primarily set in a driven switch?
A Load current frequency
B DC-link ripple count
C Gate current level
D Thermal resistance
Answer: C. Gate current level. The gate resistor limits or shapes the current delivered by the gate driver, subject to device and driver requirements.
What happens when an inductor core saturates?
A Inductance becomes infinite
B Switch frequency becomes zero
C Current stops instantly
D Inductance drops sharply
Answer: D. Inductance drops sharply. Core saturation reduces incremental inductance, so current can rise rapidly and overstress the switch.
What is a common purpose of an RCD snubber?
A Absorb turn-off energy
B Set PWM frequency
C Increase gate threshold
D Measure load power
Answer: A. Absorb turn-off energy. An RCD snubber captures switching energy and limits voltage overshoot across a device.
Which expression estimates hard-switching overlap loss per cycle?
A V divided by I
B I squared times frequency
C V times duty ratio
D Half V I transition time
Answer: D. Half V I transition time. A simple estimate is one half times voltage times current times the combined rise and fall time; multiply by switching frequency for power.
What contributes to IGBT turn-off tail loss?
A Gate oxide current
B Output capacitor ESR
C Stored minority carriers
D Diode forward drop only
Answer: C. Stored minority carriers. Stored minority carriers continue conducting briefly after gate turn-off, creating a current tail and additional loss.
What approximates MOSFET conduction loss?
A Vdc divided by fs
B Gate charge times duty
C Irms squared Rds(on)
D Frequency divided by current
Answer: C. Irms squared Rds(on). When the MOSFET is on, its channel loss is approximately its RMS current squared times on-resistance.
A device dissipates 20 W with RθJC 0.5 °C/W. Find junction rise above case.
A 20 °C
B 40 °C
C 0.5 °C
D 10 °C
Answer: D. 10 °C. The junction-to-case temperature rise is power times thermal resistance: 20 W times 0.5 °C/W equals 10 °C.
What does a freewheel interval do in a half-controlled bridge?
A Stops all load current
B Raises supply frequency
C Maintains inductive load current
D Adds gate isolation
Answer: C. Maintains inductive load current. The bridge provides a freewheeling path so an inductive load current can continue when the supply path is not conducting.
As firing angle increases, what happens to displacement factor in a full converter?
A It becomes unity
B It generally decreases
C It doubles
D It is unchanged always
Answer: B. It generally decreases. For continuous current, the fundamental input current displacement angle tracks the firing delay, so its cosine generally decreases as α rises.
In linear SPWM, what mainly changes inverter fundamental amplitude?
A Load inductance
B Modulation index
C Dead-time polarity
D Device package size
Answer: B. Modulation index. In the linear modulation region, the fundamental output voltage is approximately proportional to the modulation index.
What improves when an inductive converter load current is kept continuous?
A Lower current ripple
B Higher switching frequency
C Zero device loss
D No gate pulses
Answer: A. Lower current ripple. A sufficiently large inductance smooths load current and reduces its ripple between switching or commutation intervals.