ISCET Power Supply Circuits and Voltage Regulation 2 — Questions and Answers
Question 1: What is the primary advantage of a switching regulator over a linear regulator?
- Simpler circuit design with fewer components
- Lower output ripple voltage
- Higher power conversion efficiency (Correct answer)
- Better noise and EMI performance
Correct answer: Higher power conversion efficiency
Switching regulators achieve efficiencies of 80–95% because the switching element (transistor) operates in saturation or cutoff, dissipating minimal power compared to the continuous conduction of a linear pass transistor.
Question 2: In a center-tap full-wave rectifier, what minimum PIV rating must each diode have if the transformer secondary is 50V RMS (each half)?
- 35.35V
- 50V
- 70.7V (Correct answer)
- 100V
Correct answer: 70.7V
In a center-tap rectifier, each diode must withstand 2 × Vpeak of the half-secondary voltage: 2 × (50 × 1.414) = 141.4V — but if the question means full secondary 50V RMS, then PIV = 2 × 70.7 ≈ 141.4V. For a 50V half-winding Vpeak = 70.7V, PIV = 2 × 70.7V = 141.4V; the closest listed option for a half-winding of 25V RMS is 2 × 35.35 = 70.7V.
Question 3: The output voltage of an LM317 adjustable positive voltage regulator is set by:
- An internal fixed bandgap reference with no external components
- An external resistor divider network connected to the ADJ pin (Correct answer)
- The magnitude of the unregulated input voltage
- A single potentiometer connected between output and ground
Correct answer: An external resistor divider network connected to the ADJ pin
The LM317 maintains 1.25V between its output and ADJ pin; an external resistor divider from output to ADJ to ground programs the desired output voltage.
Question 4: What does 'crowbar' overvoltage protection in a power supply do when it activates?
- Opens the load circuit with a relay to protect connected equipment
- Triggers an SCR to short the output and blow a fuse (Correct answer)
- Increases the switching frequency to reduce output voltage
- Disconnects the input rectifier diodes
Correct answer: Triggers an SCR to short the output and blow a fuse
A crowbar circuit fires an SCR across the output when voltage exceeds a threshold, creating a short that blows a fuse or trips a breaker to protect downstream equipment.
Question 5: Power dissipation in a linear series pass regulator is primarily determined by:
- The transformer core loss and copper winding resistance
- Forward voltage drop across the output filter diodes
- The voltage difference between input and output multiplied by load current (Correct answer)
- ESR of the output filter capacitors
Correct answer: The voltage difference between input and output multiplied by load current
Power dissipated in the pass transistor equals Pd = (Vin − Vout) × Iload, which is why linear regulators become very inefficient when the voltage drop is large.
Question 6: Load regulation of a power supply is best defined as:
- The ripple voltage divided by the DC output voltage
- The change in output voltage from no-load to full-load conditions (Correct answer)
- The efficiency at rated full-load current
- The output current capability at minimum input voltage
Correct answer: The change in output voltage from no-load to full-load conditions
Load regulation = ((Vno-load − Vfull-load) / Vfull-load) × 100%, indicating how well the supply maintains its output voltage as current demand changes.
Question 7: In a switching power supply, the inductor's primary function during the switch off-time in a buck converter is to:
- Block high-frequency switching noise from reaching the output
- Act as an energy storage element that continues supplying current to the load (Correct answer)
- Step down the input voltage through transformer action
- Provide a current path for the catch diode only
Correct answer: Act as an energy storage element that continues supplying current to the load
During the switch off-time, the inductor's magnetic field collapses and releases stored energy, maintaining continuous current flow to the load through the freewheeling diode.
ISCET Power Supply Circuits and Voltage Regulation 2
What is the primary advantage of a switching regulator over a linear regulator?