Power Supplies and Electrical Safety¶
Possible Exam Questions¶
Exam Questions and Answer Map
Questions marked [PYQ paper/year] were directly observed in past papers; [likely] means pattern-based prediction.
- Compare an SMPS with a linear power supply. [10] — [PYQ 2079]
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Model answer: SMPS and Linear Power-Supply Comparison
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How is electronic equipment protected from electrical hazards? [5] — [PYQ 2079]
- Model answer: Protection from Electrical Hazards
5. SMPS vs Linear Power Supply¶
Likely Exam Questions
"State the advantages and disadvantages of SMPS over a linear power supply."
SMPS and Linear-Supply Definitions¶
A Switched-Mode Power Supply (SMPS) converts AC mains to regulated DC by first rectifying the input, then switching the DC at high frequency (50–500 kHz) through a transformer, rectifying and filtering the secondary output, and regulating via a feedback loop that controls the switching duty cycle.
A linear power supply uses a mains-frequency (50/60 Hz) transformer followed by a rectifier, filter and a series-pass linear regulator (e.g. 7805) that drops excess voltage as heat.
Comparison — SMPS vs Linear Supply¶
| Parameter | SMPS | Linear Power Supply |
|---|---|---|
| Efficiency | High (80–95%) | Low (30–50%) |
| Size and weight | Compact and lightweight (high-frequency transformer is small) | Bulky and heavy (50 Hz transformer) |
| Input voltage range | Wide (e.g. 90–264 V AC universal) | Narrow (designed for specific mains voltage) |
| Output ripple | Higher switching noise/ripple | Very low ripple, clean DC |
| EMI/RFI | Generates significant electromagnetic interference (requires shielding/filtering) | Very low EMI |
| Complexity | Complex (controller IC, MOSFET, feedback loop, magnetics design) | Simple (transformer + regulator IC) |
| Cost | Higher initial design cost; lower running cost | Lower design cost; higher running cost (heat) |
| Regulation | Excellent (via PWM feedback) | Excellent (via linear regulator) |
| Heat dissipation | Low (less waste) | High (excess voltage dropped as heat in pass transistor) |
| Applications | Computers, phones, TVs, telecom equipment | Audio amplifiers, laboratory instruments, low-noise medical devices |
Key Points¶
- SMPS is preferred when efficiency, size and weight are critical (portable/telecom equipment).
- Linear supply is preferred when low noise and simplicity are critical (sensitive analog/audio circuits).
- Modern SMPS topologies include buck, boost, buck-boost, flyback and forward converters.
6. Electrical Safety¶
Likely Exam Question (5 marks)
"How do you make equipment safe from electrical hazards?"
Electrical-Safety Definition¶
Electrical safety encompasses the design practices, protective measures and installation standards that prevent electric shock, fire and equipment damage from electrical energy.
Methods to Make Equipment Safe from Electrical Hazards¶
| Method | How It Protects |
|---|---|
| Protective earthing (grounding) | Metal enclosure connected to earth via low-resistance conductor; fault current flows to earth and trips the protective device, not through the user |
| Fuses and circuit breakers | Disconnect supply when current exceeds rated value; protect against overload and short circuit |
| Residual Current Device (RCD/ELCB) | Detects imbalance between live and neutral (≥30 mA leakage) and disconnects in <30 ms — primary shock protection |
| Double insulation (Class II) | Two independent layers of insulation; no earth conductor needed; marked with □ inside □ symbol |
| Isolation transformers | Galvanically isolate the equipment from mains; no direct return path through earth in case of single fault |
| Proper cable sizing and strain relief | Prevents overheating and mechanical damage to conductors |
| Surge protection (SPDs/varistors) | Clamps transient overvoltages from lightning or switching |
| Enclosure protection (IP rating) | Prevents ingress of fingers, tools, water, dust into live parts |
| Lockout/tagout (LOTO) | Ensures equipment is de-energised and isolated during maintenance |
| Regular inspection and testing | PAT testing, insulation resistance checks, earth continuity verification |
Safety Standards¶
- IEC 62368-1 (IT/AV equipment safety — replaced IEC 60950 and IEC 60065)
- IEC 61010 (laboratory/measurement equipment)
- IS 732 / Nepal Electricity Rules (wiring and installation)
Key Exam Points — Electrical Safety
- Protective earthing + RCD (30 mA) is the primary defence against electric shock.
- Double insulation eliminates need for earth but requires two independent insulation layers.
- Fuses/MCBs protect against overcurrent; RCDs protect against earth-leakage/shock.
- Isolation transformers remove direct earth return path.
- Proper IP-rated enclosures prevent contact with live parts.
Model Answer — SMPS and Linear Power-Supply Comparison [10 marks, PYQ 2079]¶
Exam-ready answer
A linear supply normally uses a 50/60-Hz transformer, rectifier and reservoir, followed by a series-pass transistor operated in its active region. It regulates by dropping \(V_{in}-V_o\) continuously, so pass-device loss is approximately
An SMPS first rectifies/filter mains where applicable, chops DC with a MOSFET at tens or hundreds of kilohertz, transfers/stores energy in an inductor or high-frequency transformer, rectifies/filters the output, and adjusts duty ratio \(D\) through negative feedback.
Common ideal switching relations are
and, for an isolated flyback in continuous ideal operation,
Forward converters transfer energy while the switch is on; flyback converters store it in magnetising inductance and deliver it while off. Buck/boost/buck-boost are normally non-isolated DC-DC topologies; flyback/forward/bridge topologies can provide galvanic isolation.
| Property | SMPS | Linear supply |
|---|---|---|
| Efficiency | Typically 80–95% or higher | Often 30–60%, especially for large voltage drop |
| Magnetics/size | Small high-frequency magnetics; light | Large mains-frequency transformer; heavy |
| Input/output flexibility | Wide input; step-down, step-up, invert, multiple isolated outputs | Primarily step-down; limited by dropout |
| Heat | Low switching/conduction loss when well designed | High series-pass dissipation |
| Ripple/noise | Switching ripple, common/differential EMI | Very low noise and ripple |
| Complexity | Controller, compensation, gate drive, magnetics, protection | Simple regulator and easy debugging |
| Transient/loop design | Compensation and layout are critical | Usually straightforward |
| Cost | Economical at medium/high power, more parts/design effort | Attractive at low power and low-noise use |
Advantages of SMPS: high efficiency, compact size, lower cooling requirement, universal mains capability, electrical isolation and multiple rails, and high power density. Disadvantages: conducted/radiated EMI, switching ripple and spikes, acoustic/magnetic noise, more difficult control-loop stability, component stress, poorer light-load behavior in some modes and more complex fault diagnosis.
Worked comparison: a linear regulator producing \(5\,\text{V},2\,\text{A}\) from \(12\,\text{V}\) delivers \(10\,\text{W}\) but loses \((12-5)2=14\,\text{W}\), only \(41.7\%\) efficient. A 90%-efficient SMPS delivering the same \(10\,\text{W}\) draws \(11.11\,\text{W}\) and loses only \(1.11\,\text{W}\).
Practical SMPS design adds input fuse/MOV/inrush limiting, EMI filters, adequate creepage/clearance, reinforced isolation where required, optocoupler or isolated feedback, current limiting, overvoltage/thermal shutdown, snubbers/clamps and safe discharge of bulk capacitors. A linear supply remains preferable for precision references, low-noise analog/audio stages or very small loads; SMPS dominates computers, chargers, telecom and efficient power conversion.
Practice target: 18 minutes; describe the energy path, state three topology equations, reproduce the comparison and calculate the loss example.
Model Answer — Protection from Electrical Hazards [5 marks, PYQ 2079]¶
Exam-ready answer
Electrical equipment must prevent shock, burns, fire and unsafe stored energy under normal use and a credible single fault. Protection uses independent layers:
- Basic/reinforced insulation and enclosure: cover live parts, provide suitable creepage and clearance, an IP-rated case, barriers, strain relief and correctly rated flame-resistant wiring. Class II equipment uses double/reinforced insulation and does not rely on earth.
- Protective earthing: bond accessible metal in Class I equipment to PE with a low-resistance conductor. An insulation fault then produces a large fault current that operates the fuse/MCB instead of raising the case to dangerous voltage.
- Overcurrent protection: a correctly rated fuse or MCB disconnects overload/short-circuit current and protects wiring from fire. It is not sensitive enough to replace shock protection.
- Residual-current protection: an RCD compares live and neutral currents and trips on earth leakage; \(30\,\text{mA}\) devices are commonly used for additional personal protection. An RCD does not replace overcurrent protection or safe isolation.
- Isolation and energy limiting: safety transformers/SELV remove a direct mains-earth path; current limiting, thermal cut-outs, MOV/surge protection, bleeder resistors and guarded high-voltage capacitors control other hazards.
Before maintenance: de-energise, isolate, lock out/tag out, prove dead with a verified tester, discharge stored energy, and use suitable PPE/tools. Periodic earth-continuity, insulation-resistance, leakage/RCD and visual tests reveal deterioration. Design and test against the applicable standard, such as IEC 62368-1 or IEC 61010, rather than relying on warning labels alone.
Practice target: 8 minutes; distinguish earth, fuse/MCB, RCD, double insulation and isolation by the fault each one clears.