Switched-mode power supply

A switched-mode power supply, or SMPS, is an electronic power supply unit (PSU) that incorporates a switching regulator - an internal control circuit that switches the load current rapidly on and off in order to stabilise the output voltage. Switching regulators are used as replacements for simpler linear regulators when higher efficiency, smaller size or lighter weight are required. They are, however, more complicated and more expensive, their switching currents can cause noise problems if not carefully suppressed, and simple designs can have a poor power factor.

SMPS can also be classified into four types according to the input and output waveforms, as follows.

AC in, DC out: rectifier, off-line converter
DC in, DC out: voltage converter, or current converter
AC in, AC out: frequency changer, cycloconverter
DC in, AC out: inverter

AC and DC are abbreviations for alternating current and direct current.

Switched-mode PSUs in domestic products such as personal computers often have universal inputs, meaning that they can accept power from most mains supplies throughout the world, with frequencies from 50 Hz to 60 Hz and voltages from 100 V to 240 V.

SMPS compared with linear PSUs

Two main types of regulated power supply are available: SMPS and linear. The reasons for choosing one type or the other can be summarised as follows.

Size and weight. Linear power supplies use a line-(mains-)frequency transformer (if they are isolating types), and line-frequency smoothing filters. These components are larger and heavier than the corresponding parts of a SMPS, which works at a higher frequency.
Efficiency. Linear power supplies regulate their output by generating a higher voltage than needed at the output, then reducing it by converting some of the electrical power to heat. This loss is a necessary part of the operation of the circuit, and cannot be eliminated by improving the design. SMPS generate no more voltage than they require, and only a small amount of energy is wasted.
Heat output. This is determined by the efficiency, above. Linear PSUs output much more heat than SMPS.
Complexity. Linear PSUs are simple enough to be designed by beginners. SMPS are complicated, difficult to design well, and the higher number of components makes them more expensive to assemble and to repair.
Noise. The switched currents in an SMPS contain much more energy at high frequencies than those in a linear PSU. This high-frequency energy can be transmitted by electromagnetic induction to other nearby equipment, or as radio waves over long distances, causing interference. Care is therefore needed to eliminate this energy at source, or to contain it by screening.
Power factor. If the current drawn by a load such as a SMPS from an AC supply is nonsinusoidal and/or out of phase with the supply voltage waveform, the power factor will be less than unity and the efficiency, capacity and reliability of generating plants and the transmission grid can be substantially decreased. The simplest and most common SMPS designs have a power factor of about 0.6, and their use in increasingly popular personal computers and compact fluorescent lamps presents a growing problem. Power factor correction (PFC) circuits can reduce this problem, and are required in some countries by regulation. Notably, power factor correction is not yet widely required or used in North America.

How an SMPS works

Rectifier stage

If the SMPS has an AC input, then its first job is to convert the input to DC. This is called rectification. The rectifier circuit is often the same as that in a linear power supply, and produces an unregulated DC. An SMPS with a DC input does not require this stage. An SMPS designed for AC input can often be run from a DC supply, as the DC passes through the rectifier stage unchanged. (The user should check the manual before trying this!)

Inverter stage

The inverter stage converts DC, whether directly from the input or from the rectifier stage described above, to AC by switching it on and off ('chopping') at a frequency of tens or hundreds of kilohertz (kHz). The frequency is usually chosen to be above 20 kHz, to make it inaudible to humans. The switching is done by MOSFETs, which are a type of transistor with a low on-resistance and a high current-handling capacity.

Voltage converter

If the output is required to be isolated from the input, as is usually the case in mains power supplies, the inverted AC is used to drive the primary winding of a high-frequency transformer. This converts the voltage up or down to the required output level on its secondary winding.

If a DC output is required, the AC output from the transformer is rectified and smoothed by a filter consisting of inductors and capacitors. The higher the switching frequency, the smaller these components can be made.

Simpler, non-isolated power supplies contain an inductor instead of a transformer. This type includes boost regulators, whose output voltage is greater than the input, and buck regulators, whose output voltage is less than the input. These designs have another disadvantage over transformer-based designs: each power transfer cycle is asymmetric, because the inductor charges at one rate when connected to the DC source, and then discharges at a different rate into the load. This makes the output difficult to regulate accurately.

Other types of SMPS use capacitors instead of inductors and transformers. These are mostly used for generating high voltages at low currents, although the Cuk converter can compete with inductive SMPS designs.

Regulation

A feedback circuits monitors the output voltage and compares it with a reference voltage, which is set manually or electronically to the desired output. If there is an error in the output voltage, the feedback circuit compensates by adjusts the timing with which the MOSFETs are switched on and off. This part of the power supply is called the switching regulator.

Open-loop regulators do not have a feedback circuit. Instead, they rely on feeding a constant voltage to the input of the transformer or inductor, and assume that the output will be correct.

Power factor

Unlike most other appliances, switched mode power supplies tend to be constant power devices, drawing more current as the line voltage reduces. Also, in common with many static rectifiers, maximum current draw occurs at the peaks of the waveform cycle. This means that basic switched mode power supplies tend to produce more harmonics and have a worse power factor than other types of appliances. This may cause stability problems in some situations such as emergency generator systems. However, higher-quality switched-mode power supplies with power-factor correction (PFC) are available, which are designed to present close to a resistive load to the mains.

Types

Switched-mode power supplies can classified according to the circuit topology.

buck regulator (single inductor; output voltage < input voltage)
boost regulator (single inductor; output voltage > input voltage)
buckboost regulator (single inductor; output voltage can be more or less than the input voltage)
flyback regulator (uses output transformer; allows multiple outputs and input-to-output isolation)
forward regulator (uses output transformer; allows multiple outputs and input-to-output isolation)
Cuk converter (uses a capacitor for energy storage; produces negative voltage for positive input)

de:Schaltnetzteil da:Switching-mode power supply

Retrieved from "http://www.mywiseowl.com/articles/Switched-mode_power_supply"

This page has been accessed 518 times. This page was last modified 16:05, 28 Sep 2004. All text is available under the terms of the GNU Free Documentation License (see Copyrights for details).