Interview questions & answers
Q1. What is the difference between a linear power supply and a switching power supply (SMPS)?
A linear power supply uses a series pass transistor (like in the LM7805) operating in the active region to drop excess voltage as heat, giving clean output but typical efficiency of 40–60% since all input-output voltage difference is dissipated in the regulator. An SMPS switches a transistor at high frequency (50 kHz–1 MHz), uses inductors and capacitors for energy storage, and achieves 85–95% efficiency because the switching element dissipates minimal power when fully on or off. For a 5 V, 1 A supply from 12 V: the LM7805 dissipates 7 W as heat; a comparable buck converter IC like the LM2596 dissipates only ~0.5 W, a 14× efficiency improvement.
Follow-up: When would you choose a linear supply over an SMPS despite the efficiency penalty?
Q2. Explain the working of an LM7805 linear voltage regulator.
The LM7805 contains a reference voltage (bandgap ~1.25 V), an error amplifier, and a PNP pass transistor; the error amp drives the pass transistor to maintain the output at exactly 5 V by comparing a divided-down output to the bandgap reference. It requires a minimum 3 V input-output differential (dropout voltage) to keep the pass transistor in saturation, so the input must be at least 8 V for a regulated 5 V output. The LM7805 includes thermal shutdown (trips at ~150°C) and current limiting (~750 mA), making it essentially short-circuit proof, which is why it is universally used in simple 5 V regulation for microcontroller boards.
Follow-up: What is the dropout voltage and how does an LDO (Low Dropout regulator) improve on the LM7805?
Q3. What is ripple voltage in a rectifier output and how do you reduce it?
Ripple voltage is the AC variation remaining on the DC output after rectification, approximated by Vripple = IL/(f×C) for a full-wave rectifier where IL is load current, f is ripple frequency (100 Hz for full-wave on 50 Hz supply), and C is the filter capacitor. For IL = 500 mA and C = 1000 µF: Vripple = 0.5/(100 × 1000µ) = 5 V peak-to-peak — unacceptably large. Using C = 10,000 µF reduces ripple to 0.5 V; a subsequent LM7805 regulator reduces it to <10 mV at the output. Ripple is reduced by increasing capacitance, using full-wave rectification, increasing supply frequency, or using an active filter.
Follow-up: Why does the ripple voltage increase with increasing load current?
Q4. What is a buck (step-down) converter and how does it work?
A buck converter uses a high-side MOSFET switch, a catch diode (or synchronous MOSFET), and an LC output filter to step down input voltage by controlling the duty cycle: Vout = D × Vin, where D = ton/T. The LM2596 with Vin = 12 V and D = 41.7% produces Vout = 5 V; the inductor (68 µH typical) stores energy during the on time and releases it during the off time, keeping output current continuous. Because the MOSFET only dissipates power during switching transitions (not when fully on or off), efficiencies of 85–92% are achievable at 150 kHz switching frequency, compared to 40% for a linear regulator in the same application.
Follow-up: What is the role of the output inductor in a buck converter?
Q5. What is a boost converter and when is it used?
A boost converter steps voltage upward by storing energy in an inductor during the MOSFET on-time and releasing it to the output (in series with the input) during the off-time: Vout = Vin/(1-D). The MT3608 boost converter with Vin = 3.3 V and D = 0.6 steps up to Vout = 3.3/(1-0.6) = 8.25 V, used to power 7-segment LED displays from a single lithium cell at 3.7 V. Boost converters are used in battery-powered devices where the load voltage exceeds the declining battery voltage, backup lighting circuits, LCD bias generators, and LED driver supply rails.
Follow-up: What is the voltage conversion ratio of an ideal boost converter?
Q6. What is load regulation and line regulation in a power supply?
Load regulation is the change in output voltage when load current changes from no load to full load, expressed as a percentage: (VNL - VFL)/VFL × 100%. Line regulation is the change in output voltage for a specified change in input voltage, in mV/V or percentage. The LM7805 has load regulation of 100 mV (from 5 mA to 1.5 A) and line regulation of 3 mV (from 7 V to 25 V input), meaning output stays within 100 mV of 5 V even as load swings fully. An unregulated rectifier supply powering a circuit through an LM7805 thus provides a stable 5 V to the circuit regardless of whether it draws 10 mA or 1 A.
Follow-up: What is the difference between load regulation and line regulation from a circuit design perspective?
Q7. What is a flyback converter and why is it used for multiple output power supplies?
A flyback converter is an isolated SMPS topology where a transformer (actually a coupled inductor) stores energy in the primary during the MOSFET on-time and transfers it to the secondary during the off-time — the transformer provides both isolation and voltage conversion with turns ratio selection. A flyback using the TNY268P in a 5 W universal input (85–265 VAC) to 5 V/1 A supply provides 3 kV galvanic isolation required by safety standards for mains-connected equipment. Multiple secondary windings can produce different voltages (e.g., +5 V, +12 V, -12 V) simultaneously, making flyback the dominant topology in off-line (mains-input) SMPS for consumer electronics and industrial equipment.
Follow-up: What is the role of the snubber circuit in a flyback converter?
Q8. What is a crowbar circuit in power supply protection?
A crowbar circuit is an overvoltage protection mechanism that uses an SCR (silicon controlled rectifier) to short-circuit the power supply output when voltage exceeds a set threshold, blowing the fuse and disconnecting power before the downstream load is damaged. A TL431-based crowbar set at 5.5 V in a 5 V supply fires the 2N6397 SCR if output rises above 5.5 V, drawing hundreds of amperes and blowing the F1 fuse in microseconds. Unlike a simple Zener clamp which only limits voltage while the overvoltage source persists, the crowbar 'latches' — it stays shorted until power is removed and the fuse replaced, ensuring the fault is investigated.
Follow-up: What is the difference between a crowbar circuit and a foldback current limiter?
Q9. What is power factor and why does it matter in AC power supplies?
Power factor is the ratio of real power (W) to apparent power (VA), equal to cos(φ) for purely sinusoidal current, and it represents how efficiently the AC supply is used — a PF of 0.6 means 60% of the current drawn does useful work. An SMPS without power factor correction (PFC) draws pulsed current only near the AC voltage peaks, giving PF ≈ 0.6 and generating large harmonic currents that cause transformer heating and voltage distortion in the grid. The IEC 61000-3-2 standard requires PF > 0.9 for equipment above 75 W, which is why modern laptop adapters and PC power supplies include a PFC boost stage (like the NCP1654) before the main DC-DC stage.
Follow-up: What is the difference between displacement power factor and distortion power factor?
Q10. What is the role of the output filter capacitor in an SMPS?
The output filter capacitor (with its associated inductor in LC filter) reduces the switching ripple at the converter output, smoothing the pulsed inductor current into a near-DC voltage with small AC ripple. A 220 µF/16 V electrolytic capacitor in a 150 kHz buck converter reduces output ripple to approximately ΔI_L/(8×f×C) = 50mA/(8×150kHz×220µF) ≈ 0.19 mV_pp. The capacitor's equivalent series resistance (ESR) is critical: high ESR in a cheap electrolytic causes I×ESR ripple voltage that can equal or exceed the capacitive ripple, which is why low-ESR polymer capacitors (Nichicon HV series) are specified in quality SMPS designs.
Follow-up: What is ESR and how does it affect output ripple in a switching regulator?
Q11. What is the difference between a half-wave and full-wave rectifier in a power supply design?
A half-wave rectifier uses one diode and conducts on only one half-cycle, giving 40.6% efficiency, double the ripple of full-wave, and poor transformer utilization; a bridge rectifier uses four diodes (MB6F package) and conducts both half-cycles, giving 81.2% efficiency and 100 Hz ripple on a 50 Hz supply. For a 12 V/1 A supply, the half-wave rectifier needs a 10,000 µF filter capacitor to achieve <1 V ripple; the bridge rectifier needs only 2,500 µF for the same ripple, using a physically smaller, cheaper capacitor. Full-wave bridge rectification is standard in virtually all practical linear and SMPS pre-regulation stages.
Follow-up: What is the peak inverse voltage (PIV) requirement for diodes in each rectifier topology?
Q12. What is an LDO (Low Dropout) regulator and when is it preferred over the LM7805?
An LDO regulator uses a PMOS pass transistor instead of NPN to achieve a very low dropout voltage of 100–400 mV at full load, compared to 2–3 V for the LM7805, allowing regulation from input voltages only slightly above the output. The LP2985-3.3 LDO produces 3.3 V with only 300 mV dropout (input can be as low as 3.6 V), while an LM7833 (3.3 V series) needs 5.5 V input minimum. LDOs are essential in battery-powered devices where the supply voltage is close to the regulated output — a lithium cell at 3.6 V end-of-life cannot use an LM7833 to generate 3.3 V because the 2.3 V headroom is not available.
Follow-up: Why does a PMOS pass transistor achieve lower dropout than an NPN transistor?
Q13. What are common protection features in a power supply IC?
Standard protection features include over-current protection (OCP, foldback or constant current limiting), over-voltage protection (OVP, shutting down or crowbarring above set threshold), over-temperature protection (OTP, thermal shutdown with hysteresis around 150–160°C), and under-voltage lockout (UVLO, preventing operation below minimum input voltage). The LM3481 boost controller IC integrates OCP, OVP, and UVLO in a single 8-pin package, eliminating the need for external protection circuits. Understanding these protection mechanisms is critical for designing industrial power supplies at L&T and ABB that must survive fault conditions without damaging connected equipment.
Follow-up: What is foldback current limiting and how does it differ from constant current limiting?
Q14. What is efficiency of a power supply and how do you calculate it?
Power supply efficiency η = (Pout/Pin) × 100% = (Vout × Iout) / (Vin × Iin) × 100%, representing the fraction of input power delivered to the load. For an LM2596 buck converter with Vin = 12 V, Iin = 460 mA and Vout = 5 V, Iout = 1 A: η = (5×1)/(12×0.46) × 100% = 90.6%. Efficiency is important because wasted power becomes heat: a 100 W supply at 80% efficiency dissipates 25 W as heat, requiring significant heatsinking; at 95% efficiency, only 5.3 W is dissipated, needing a much smaller heatsink or even passive cooling.
Follow-up: What are the main loss mechanisms in a buck converter?
Q15. What is isolation in a power supply and why is it required in some applications?
Isolation means there is no direct electrical connection between the input (primary) and output (secondary) of the power supply, implemented using a transformer or optocoupler in the feedback path, providing safety and breaking ground loops. IEC 60950-1 (now IEC 62368-1) requires 3 kV isolation for mains-powered equipment to prevent shock hazard to users touching the secondary-referenced output. The HCPL-314J gate driver with 1500 V RMS isolation is used in ABB's motor drive inverter to control high-voltage IGBTs from a low-voltage microcontroller without risk of HV appearing on the control board.
Follow-up: What is the difference between basic insulation and reinforced insulation in safety standards?
Common misconceptions
Misconception: SMPS is always better than a linear power supply because it is more efficient.
Correct: SMPS generates significant conducted and radiated EMI due to high-frequency switching, and linear supplies are preferred for sensitive analog circuits like ADC references, audio preamps, and RF systems where SMPS noise would corrupt the signal.
Misconception: A larger filter capacitor always gives a better power supply.
Correct: Very large capacitors cause extremely high inrush current at power-on that can damage rectifier diodes and blow fuses; a thermistor NTC or soft-start circuit must be added when using large bulk capacitors.
Misconception: The output voltage of a Zener-based power supply is rock-steady like an LM7805.
Correct: A Zener regulator has poor load regulation because finite dynamic impedance (Zz) causes output voltage to change with load current; the LM7805 uses an error amplifier with high loop gain to correct any output deviation immediately.
Misconception: An SMPS can be designed to any efficiency by simply choosing a more efficient topology.
Correct: Real efficiency is limited by conduction losses (I²×Rds_on), switching losses (proportional to frequency and gate charge), magnetic core losses, and PCB resistance — 95–97% is typically the practical upper bound even with the best components.