SMPS is the acronym for

Correct — C, (c) Switch Mode Power Supply.

SMPS STANDS FOR SWITCH MODE POWER SUPPLY — more often written 'switched-mode power supply', which is the same device. It is the metal box with a fan inside a desktop computer's cabinet, and it is also, in miniature, the laptop adapter, the mobile charger and the driver inside an LED bulb.

THE NAME DESCRIBES THE METHOD, which is why the first word is the one to remember. The supply works by SWITCHING a semiconductor device rapidly on and off — tens of thousands to hundreds of thousands of times a second — rather than by dropping the unwanted voltage across a resistive element. The steps, in order:

the alternating mains supply is rectified and smoothed into a high-voltage direct current; a fast switching transistor chops that direct current into a high-frequency square wave; the square wave is fed to a small high-frequency transformer, which steps it down and isolates the output from the mains; the transformer's output is rectified and filtered back to direct current; a feedback loop measures the output voltage and adjusts the fraction of each cycle for which the switch is on — the DUTY CYCLE — to hold the output steady. This adjustment is pulse width modulation.

WHY THIS DESIGN WON, against the older LINEAR power supply that used a heavy mains-frequency transformer and a regulator that burned off the surplus as heat. Two advantages, and both are worth stating as reasons rather than as facts:

EFFICIENCY. A switch that is fully ON has almost no voltage across it and a switch that is fully OFF carries almost no current, and power is the product of the two — so in either state the switch dissipates very little. A linear regulator, by contrast, sits permanently in a partly conducting state and turns the difference between input and output voltage into heat. Switch-mode supplies commonly run above eighty per cent efficient, where a linear supply may waste half its input. SIZE AND WEIGHT. The size of a transformer needed for a given power falls as the frequency rises. Working at tens of kilohertz instead of the mains fifty hertz allows a transformer a small fraction of the size and weight of the linear equivalent. That is why a modern laptop charger is a small brick and its 1970s counterpart would have been a lump of iron.

WHAT IT COSTS. Switching at high speed generates electromagnetic interference and output ripple, so the design needs filtering and shielding; the circuit is far more complex than a linear one; and the output is electrically noisier, which is why sensitive analogue and laboratory instruments sometimes still use linear supplies.

THE OPTION SET IS BUILT ON A SINGLE WORD. All four choices read '... Mode Power Supply' and differ only in the first word, so the entire question is whether the candidate knows that the S stands for SWITCH. The device is defined by what it does — it switches — and no other reading of the first letter describes any real class of power supply.

The stem ends on the words 'acronym for' with no punctuation, as this booklet sets a stem that runs into its options.

Why the others are wrong

• (a)Store Mode Power Supply — There is no such class of power supply, and 'store mode' is not a term of art in power electronics at all. The word may sound plausible because energy storage genuinely is part of how these circuits work — an inductor stores energy in its magnetic field during the part of the cycle when the switch is on and releases it when the switch turns off, and capacitors store charge to smooth the output — but that is a description of a component's behaviour inside the supply, not a name for the supply. Naming a device after an incidental internal process rather than its governing principle is the standard way a wrong expansion is built for an acronym question. The device that stores energy to supply a load when the mains fails is an uninterruptible power supply, a UPS, which is a different thing with a different acronym and is worth keeping distinct from SMPS in the same revision note.
• (b)Single Mode Power Supply — 'Single mode' is a genuine technical term, which is what makes this the most tempting of the three wrong expansions — but it belongs to optical fibre, not to power electronics. SINGLE-MODE FIBRE has a very narrow core, of the order of nine micrometres, so that light can travel along essentially one path; this eliminates the modal dispersion that limits multi-mode fibre and allows far greater distances and bandwidth, which is why long-haul and undersea cables use it. MULTI-MODE FIBRE has a wider core, allows many light paths, and is cheaper and easier to couple, so it is used over short runs within a building. None of that has anything to do with converting mains electricity into low-voltage direct current, and there is no power supply described as single mode. A candidate meeting a familiar-sounding phrase in an unfamiliar setting should ask which subject the phrase actually comes from before accepting it.
• (d)Start Mode Power Supply — Another expansion with no referent. There is no category of power supply called a start mode supply. As with option (a), the phrase borrows a real word from the neighbourhood of the subject: switch-mode supplies genuinely do have a start-up sequence, in which a small auxiliary circuit powers the controller until the main output comes up and can sustain it, and a supply that fails to complete that sequence is said to fail on start-up. But 'start-up circuit' names a stage inside the device, not the device. The general lesson for acronym questions of this shape, where three of the four expansions differ from the answer by exactly one word, is to ask what the object DOES and then check which first word describes that. A power supply of this kind is defined by rapidly switching a transistor on and off, so the S is for switch, and the other three fall away together.

A POWER SUPPLY converts electrical energy from the form in which it arrives into the form a device needs — typically from alternating mains voltage to one or more low direct-current voltages. There are two families, and the difference between them is the whole of this topic.

A LINEAR POWER SUPPLY uses a transformer operating at the mains frequency of fifty hertz to step the voltage down, rectifies it, smooths it with capacitors, and then holds it steady with a regulator that behaves like a variable resistance. Whatever voltage the regulator drops is turned into heat. The design is simple, quiet and electrically clean, but it is heavy, bulky and inefficient, and it needs a large heat sink.

A SWITCH MODE POWER SUPPLY, or SMPS, replaces the mains-frequency transformer and the dissipative regulator with a high-frequency switching circuit: rectification and smoothing of the incoming mains into high-voltage direct current; a switching transistor, usually a MOSFET, chopping that direct current at a frequency of tens to hundreds of kilohertz; a small high-frequency transformer providing the step-down and the isolation from the mains; output rectification and filtering; a feedback loop adjusting the duty cycle of the switch by pulse width modulation to keep the output voltage constant as the load changes.

WHY IT IS EFFICIENT. A switch dissipates power only when there is both a voltage across it and a current through it. Fully on, the voltage across it is near zero; fully off, the current through it is near zero. Losses are therefore confined largely to the brief transitions between the two states, and typical efficiencies run from eighty to well over ninety per cent, against something between forty and sixty for a linear supply.

WHY IT IS SMALL. The core size a transformer needs for a given power is inversely related to the frequency at which it operates. Moving from fifty hertz to fifty kilohertz shrinks the magnetics by orders of magnitude, which is the reason a modern charger fits in a pocket.

WHAT IT SACRIFICES. Rapid switching radiates and conducts electromagnetic interference, so filtering and shielding are required and regulatory limits apply. The output carries switching ripple. The circuit is complex, with more components to fail. For very low-noise analogue work a linear supply is still sometimes preferred.

WHERE SMPS IS FOUND. The power supply unit of every desktop computer, which typically delivers 3.3, 5 and 12 volt rails; laptop and mobile chargers; television and monitor supplies; LED lamp drivers; and industrial power conversion generally.

ACRONYMS WORTH KEEPING DISTINCT, since these are what an option set will be built from. SMPS — switch mode power supply, the converter described here. UPS — uninterruptible power supply, which carries a battery and keeps a load running when the mains fails. PSU — power supply unit, the general term for the assembled component in a computer. AVR — automatic voltage regulator, which corrects mains voltage variation. Inverter — converts direct current to alternating current, the reverse direction from a supply.

Key facts

• SMPS stands for Switch Mode Power Supply, also written switched-mode power supply.
• It converts power by rapidly switching a transistor on and off at high frequency, rather than by dissipating surplus voltage as heat.
• The stages are rectification of the mains, high-frequency chopping by a switching transistor, a small high-frequency transformer, output rectification and filtering, and feedback regulation.
• The output is held steady by pulse width modulation, which varies the duty cycle of the switch.
• It is efficient because a switch dissipates little power when fully on or fully off; efficiencies of eighty to ninety per cent and above are usual.
• It is small because transformer size falls as operating frequency rises, so a kilohertz-range transformer replaces a bulky fifty-hertz one.
• A linear power supply uses a mains-frequency transformer and a dissipative regulator: simple and electrically quiet, but heavy and inefficient.
• The drawbacks of switch mode operation are electromagnetic interference, output ripple and circuit complexity.
• SMPS is found in desktop computer power supply units, laptop and mobile chargers, television supplies and LED drivers.
• A UPS, uninterruptible power supply, is a different device: it carries a battery and keeps a load running when the mains fails.
• Single-mode and multi-mode are terms belonging to optical fibre, not to power supplies.

Common traps

• Confusing SMPS with UPS. One converts and regulates power; the other carries a battery and rides through a mains failure.
• Taking 'single mode' as a power-supply term. It belongs to optical fibre, where it describes a narrow-core cable carrying essentially one light path.
• Assuming a switch mode supply is inefficient because it works so fast. The opposite is true: a switch loses little power in either of its two states.
• Thinking the high-frequency transformer is there to change the frequency. It is there to step the voltage down and isolate the output, and it can be small only because the frequency is high.
• Expecting to reason out an acronym from the options when three of them share every word but the first.
• Believing a switch mode supply is always better. Its electrical noise is a real reason that some precision instruments still use linear supplies.

• practice — not a real PYQ

  The principal reason a switch mode power supply is smaller and lighter than a linear power supply of the same rating is that

  • (a)it uses no transformer at all
  • (b)its transformer operates at a high frequency and can therefore be much smaller
  • (c)it produces a lower output voltage
  • (d)it dissipates its surplus power as heat instead of storing it

  Answer(b) its transformer operates at a high frequency and can therefore be much smaller — the core size needed for a given power falls as the operating frequency rises, so a transformer running at tens of kilohertz replaces a bulky fifty-hertz one. A switch mode supply does use a transformer, its output voltage is not necessarily lower, and it is the linear supply that dissipates surplus power as heat.

• practice — not a real PYQ

  Which one of the following devices is designed to keep a connected load running during a failure of the mains supply ?

  • (a)SMPS
  • (b)AVR
  • (c)UPS
  • (d)Inverter transformer

  Answer(c) UPS — an uninterruptible power supply carries a battery and continues to feed the load when the mains fails. An SMPS converts and regulates power but stores none, and an automatic voltage regulator corrects variation in mains voltage while the mains is still present.