Interior view of an ATX SMPS:
Below A - input EMI filtering
A - Bridge rectifier
B - Input filter capacitors
Between B and C - Primary side heat sink
C - Transformer
Between C and D - Secondary side heat sink
D - Output filter coil
E - Output filter capacitors
The coil and large yellow capacitor below E are additional input filtering components that are mounted directly on the power input connector and are not part of the main circuit board.

An adjustable switched-mode power supply for laboratory use

A switched-mode power supply (also switching-mode power supply, SMPS, or simply switcher) is an electronic power supply unit (PSU) that incorporates a switching regulator in order to provide the required output voltage. An SMPS is actually a power converter that transmits power from a source (e.g., a battery or the electrical power grid) to a load (e.g., a personal computer) with ideally no losses. The function of the converter is to provide a reliable output voltage often at a different level than the input voltage.

When mechanical shafts are rotating, a simple gear train can deliver power to a shaft at one speed from a shaft at a different speed. However, fluid power can be converted from a source with one pressure–flow ratio to another pressure–flow level without rotation by using the switching action of a hydraulic ram. Similarly, when AC power is being delivered from an AC source, a simple transformer can be used to convert power at one voltage level to power at another voltage level with low losses. Likewise, the switched action of an SMPS can convert DC power with low losses.

[edit] Explanation

A linear regulator maintains the desired output voltage by dissipating excess power in Ohmic losses (e.g., in a resistor or in the collector–emitter region of a pass transistor in its active mode). A linear regulator regulates output current by dissipating power, and hence its maximum power efficiency is 50%. In contrast, a switched-mode power supply regulates output current by switching ideal storage elements, like inductors and capacitors, into and out of different configurations. Ideal switching elements (e.g., transistors operated outside of their active mode) have no resistance when "closed" and carry no current when "open", and so the converters can theoretically operate with 100% efficiency (i.e., all input power is delivered to the load; no power is wasted as dissipated heat).

For example, the DC component (i.e., the time average) at one terminal of an inductor will match the DC component at the other terminal. If a DC source, an inductor, a switch, and the corresponding electrical ground are placed in series and the switch is driven by a square wave, the voltage waveform measured across the switch will also be a square wave. Because the inductor ensures that the average value of the output waveform matches the DC source voltage, the peak amplitude of the voltage across the switch will be twice the voltage of the input. If a diode-and-capacitor combination are placed in parallel to the switch, the peak voltage can be stored in the capacitor, and the capacitor can be used as a DC source with voltage higher than the DC voltage driving the circuit. This so-called boost converter acts like a step-up transformer for DC signals.

In an SMPS, the output current flow depends on the input power signal, the storage elements and circuit topologies used, and also on the pattern used (e.g., PWM with an adjustable duty cycle) to drive the switching elements. Typically, the spectral density of these switching waveforms has energy concentrated at relatively high frequencies. As such, switching transients, like ripple, introduced onto the output waveforms can be filtered with small LC filters.

[edit] Advantages and Disadvantages

The main advantage of this method is greater efficiency because the switching transistor dissipates little power when it is outside of its active region (i.e., when the transistor acts like a switch and either has a negligible voltage drop across it or a negligible current through it). Other advantages include smaller size and lighter weight (from the elimination of low frequency transformers which have a high weight) and lower heat generation due to higher efficiency. Disadvantages include greater complexity, the generation of high-amplitude, high-frequency energy that the low-pass filter must block to avoid electromagnetic interference (EMI), and a ripple voltage at the switching frequency and the harmonic frequencies thereof.

Very low cost SMPS may couple electrical switching noise back onto the mains power line, causing interference with A/V equipment connected to the same phase. Non power-factor-corrected SMPSs also cause harmonic distortion.

[edit] Classification

SMPS can be classified into four types according to the input and output waveforms:

• AC in, DC out: rectifier, off-line converter input stage
• DC in, DC out: voltage converter, or current converter, or DC to DC converter
• AC in, AC out: frequency changer, cycloconverter, transformer
• DC in, AC out: inverter

[edit] SMPS and linear power supply comparison

There are two main types of regulated power supplies available: SMPS and linear. The reasons for choosing one type or the other can be summarized as:

[edit] Theory of operation

Block diagram of a mains operated AC-DC SMPS with output voltage regulation

[edit] Input rectifier stage

AC, half-wave and full-wave rectified signals.

If the SMPS has an AC input, then the first stage is to convert the input to DC. This is called rectification. The rectifier circuit can be configured as a voltage doubler by the addition of a switch operated either manually or automatically. This is a feature of larger supplies to permit operation from nominally 120 V or 240 V supplies. The rectifier produces an unregulated DC voltage which is then sent to a large filter capacitor. The current drawn from the mains supply by this rectifier circuit occurs in short pulses around the AC voltage peaks. These pulses have significant high frequency energy which reduces the power factor. Special control techniques can be employed by the following SMPS to force the average input current to follow the sinusoidal shape of the AC input voltage thus the designer should try correcting the power factor. 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 (for 230 V AC this would be 330 V DC), as the DC passes through the rectifier stage unchanged. It's however advisable to consult the manual before trying this, though most supplies are quite capable of such operation even though nothing is mentioned in the documentation. However, this type of use may be harmful to the rectifier stage as it will only use half of diodes in the rectifier for the full load. This may result in overheating of these components, and cause them to fail prematurely. ^[3]

If an input range switch is used, the rectifier stage is usually configured to operate as a voltage doubler when operating on the low voltage (~120 V AC) range and as a straight rectifier when operating on the high voltage (~240 V AC) range. If an input range switch is not used, then a full-wave rectifier is usually used and the downstream inverter stage is simply designed to be flexible enough to accept the wide range of DC voltages that will be produced by the rectifier stage. In higher-power SMPSs, some form of automatic range switching may be used.

[edit] Inverter stage

The inverter stage converts DC, whether directly from the input or from the rectifier stage described above, to AC by running it through a power oscillator, whose output transformer is very small with few windings 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 output voltage is optically coupled to the input and thus very tightly controlled. The switching is implemented as a multistage (to achieve high gain) MOSFET amplifier. MOSFETs are a type of transistor with a low on-resistance and a high current-handling capacity. Since only the last stage has a large duty cycle, previous stages can be implemented by bipolar transistors leading to roughly the same efficiency. The second last stage needs to be of a complementary design, where one transistor charges the last MOSFET and another one discharges the MOSFET. A design using a resistor would run idle most of the time and reduce efficiency. All earlier stages do not weight into efficiency because power decreases by a factor of 10 for every stage (going backwards) and thus the earlier stages are responsible for at most 1% of the efficiency. This section refers to the block marked Chopper in the block diagram.

[edit] Voltage converter and output rectifier

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. The output transformer in the block diagram serves this purpose.

If a DC output is required, the AC output from the transformer is rectified. For output voltages above ten volts or so, ordinary silicon diodes are commonly used. For lower voltages, Schottky diodes are commonly used as the rectifier elements; they have the advantages of faster recovery times than silicon diodes (allowing low-loss operation at higher frequencies) and a lower voltage drop when conducting. For even lower output voltages, MOSFETs may be used as synchronous rectifiers; compared to Schottky diodes, these have even lower conducting state voltage drops.

The rectified output is then smoothed by a filter consisting of inductors and capacitors. For higher switching frequencies, components with lower capacitance and inductance are needed.

Simpler, non-isolated power supplies contain an inductor instead of a transformer. This type includes boost converters, buck converters, and the so called buck-boost converters. These belong to the simplest class of single input, single output converters which use one inductor and one active switch. The buck converter reduces the input voltage in direct proportion to the ratio of conductive time to the total switching period, called the duty cycle. For example an ideal buck converter with a 10 V input operating at a 50% duty cycle will produce an average output voltage of 5 V. A feedback control loop is employed to regulate the output voltage by varying the duty cycle to compensate for variations in input voltage. The output voltage of a boost converter is always greater than the input voltage and the buck-boost output voltage is inverted but can be greater than, equal to, or less than the magnitude of its input voltage. There are many variations and extensions to this class of converters but these three form the basis of almost all isolated and non-isolated DC to DC converters. By adding a second inductor the Ćuk and SEPIC converters can be implemented, or, by adding additional active switches, various bridge converters can be realised.

Other types of SMPSs use a capacitor-diode voltage multiplier instead of inductors and transformers. These are mostly used for generating high voltages at low currents (Cockcroft-Walton generator). The low voltage variant is called charge pump.

[edit] Regulation

A feedback circuit 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 adjusting the timing with which the MOSFETs are switched on and off. This part of the power supply is called the switching regulator. The Chopper controller shown in the block diagram serves this purpose. Depending on design/safety requirements, the controller may or may not contain an isolation mechanism (such as opto-couplers) to isolate it from the DC output. Switching supplies in computers, TVs and VCRs have these opto-couplers to tightly control the output voltage.

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. Regulated designs compensate for the parasitic capacitance of the transformer or coil. Monopolar designs also compensate for the magnetic hysteresis of the core.

The feedback circuit needs power to run before it can generate power, so an additional non-switching power-supply for stand-by is added.

[edit] Transformer design

SMPS transformers run at high frequency. Most of the cost savings (and space savings) in off-line power supplies come from the fact that a high frequency transformer is much smaller than the 50/60 Hz transformers formerly used.

There are several differences in the design of transformers for 50 Hz vs 500 kHz. Firstly a low frequency transformer usually transfers energy through its core (soft iron), while the (usually ferrite) core of a high frequency transformer limits leakage. Since the waveforms in a SMPS are generally high speed (PWM square waves), the wiring must be capable of supporting high harmonics of the base frequency due to the skin effect, which is a major source of power loss.

[edit] Power factor

Simple off-line switched mode power supplies incorporate a simple full wave rectifier connected to a large energy storing capacitor. Such SMPSs draw current from the AC line in short pulses when the mains instantaneous voltage exceeds the voltage across this capacitor. During the remaining portion of the AC cycle the capacitor provides energy to the power supply.

As a result, the input current of such basic switched mode power supplies has high harmonic content and relatively low power factor. This creates extra load on utility lines, increases heating of the utility transformers and standard AC electric motors, and may cause stability problems in some applications such as in emergency generator systems or aircraft generators. Harmonics can be removed through the use of filter banks but the filtering is expensive, and the power utility may require a business with a very low power factor to purchase and install the filtering onsite.

In 2001 the European Union put into effect the standard IEC/EN61000-3-2 to set limits on the harmonics of the AC input current up to the 40th harmonic for equipment above 75 W. The standard defines four classes of equipment depending on its type and current waveform. The most rigorous limits (class D) are established for personal computers, computer monitors, and TV receivers. In order to comply with these requirements modern switched-mode power supplies normally include an additional power factor correction (PFC) stage.

Putting a current regulated boost chopper stage after the off-line rectifier (to charge the storage capacitor) can help correct the power factor, but increases the complexity (and cost).

[edit] Types

Switched-mode power supplies can be classified according to the circuit topology. The most important distinction is between isolated converters and non-isolated ones.

[edit] Non-isolated topologies

Non-isolated converters are simplest, with the three basic types using a single inverter for energy storage. In the Voltage relation column, D is the duty cycle of the converter, and can vary from 0 to 1. Vin is assumed to be greater than zero; if it is negative, negate Vout to match.

When equipment is human-accessible, voltage and power limits of < 42.5 V and 8.0 A limit apply for UL, CSA, VDE approval.

The buck, boost, and buck-boost topologies are all strongly related. Input, output and ground come together at one point. One of the three passes through an inductor on the way, while the other two pass through switches. One of the two switches must be active (e.g. a transistor), while the other can be a diode. Sometimes, the topology can be changed simply by re-labeling the connections. A 12 V input, 5 V output buck converter can be converted to a 7 V input, −5 V output buck-boost by grounding the "output" and takin the output from the "ground" pin.

Likewise, SEPIC and Zeta converters are both minor rearrangements of the Cuk converter.

Switchers become less efficient as duty cycles become extremely short. For large voltage changes, a transformer (isolated) topology may be better.

[edit] Isolated topologies

All isolated topologies include a transformer, and thus can produce an output of higher or lower voltage than the input by adjusting the turns ratio.^[5][6] For some topologies, multiple windings can be placed on the transformer to produce multiple output voltages. ^[7] Some converters use the transformer for energy storage, while others use a separate inductor.

The forward converter has several variants, varying in how the transformer is "reset" to zero magnetic flux every cycle.

[edit] Quasi-resonant ZCS/ZVS

Quasi-resonant switching switches when the voltage is at a minimum and a valley is detected

A quasi-resonant ZCS/ZVS switch (Zero Current/Zero Voltage) where "each switch cycle delivers a quantized 'packet' of energy to the converter output, and switch turn-on and turn-off occurs at zero current and voltage, resulting in an essentially lossless switch." ^[10] Quasi-resonant switching, also known as 'valley switching', reduces EMI in the power supply by two methods:

1. By switching the bipolar switch when the voltage is at a minimum (in the valley) to minimize the hard switching effect that causes EMI.
2. By switching when a valley is detected, rather than at a fixed frequency, introduces a natural frequency jitter that spreads the RF emissions spectrum and reduces overall EMI.

[edit] Efficiency & EMI

Higher input voltage and synchronous rectification mode makes the conversion process more efficient; the power consumption of the controller also has to be taken into account. Higher switch frequency allows component sizes to be shrunk, but suffers from interfering radio frequency (RF) properties on the other hand. A resonant forward converter produces the lowest EMI of any SMPS approach because it uses a soft-switching resonant waveform compared with conventional hard switching topologies.

[edit] Applications

Switched mode mobile phone charger

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 rated frequencies from 50 Hz to 60 Hz and voltages from 100 V to 240 V (although a manual voltage range switch may be required). In practice they will operate from a much wider frequency range and often from a DC supply as well. In 2006, at an Intel Developers Forum, Google engineers proposed the use of a single 12 V supply inside PCs, due to the high efficiency of switch mode supplies directly on the PCB.^[11]

Most modern desktop and laptop computers also have a voltage regulator module -- a DC–DC converter on the motherboard to step down the voltage from the power supply or the battery to the CPU core voltage, which is as low as 0.8 V for a low voltage CPU to 1.2–1.5 V for a desktop CPU as of 2007. Some motherboards have a setting in the BIOS that allows overclockers to set a new CPU core voltage; other motherboards support dynamic voltage scaling which constantly adjust the CPU core voltage. Most laptop computers also have a DC–AC converter to step up the voltage from the battery to drive the backlight in the flat-screen monitor, which typically requires around 1000 Vrms.^[12]

Due to their high volumes mobile phone chargers have always been particularly cost sensitive. The first chargers were linear power supplies but they quickly moved to the cost effective Ringing Choke Converter (RCC) SMPS topology, when new levels of efficiency were required. Recently the demand for even lower no load power requirements in the application has meant that flyback topology is being used more widely; primary side sensing flyback controllers are also helping to cut the bill of material (BOM) by removing secondary-side sensing components such as optocouplers.

[edit] Terminology

The term switchmode was widely used until Motorola claimed ownership of (but did not register^[13]) the trademark SWITCHMODE, for products aimed at the switching-mode power supply market, and started to enforce their trademark.^[14] Switching-mode power supply, switching power supply, and switching regulator refer to this type of power supply.^[14]

[edit] See also

Energy portal

• Transformer
• Leakage inductance
• DC to DC converter
• Switching amplifier
• Conducted Electromagnetic Interference
• Vibrator (electronic)
• Autotransformer
• Joule thief

[edit] External links

Wikimedia Commons has media related to: Switched-mode power supplies

• Switching-Mode Power Supply Design
• Unitrode Power Supply Design Seminar Books Online
• Switching Power Supply design, PSpice simulation
• A general description of DC-DC converters
• Online Power Supplies manufacturers database
• Switching Power Supply, switch-mode supplies
• DC-DC Converter Tutorial This article outlines the different types of switching regulators used in DC-DC conversion.
• Introduction to power supplies – National Semiconductor
• Compendium and database of power supply efficiency regulations
• Power Supplies industry press releases, jobs, design discussions
• SMPS Topologies Poster from TI
• A useful Web calculator and theory text for various SMPS topologies
• Switching mode power supply's principle of operation and current formula.

[edit] Book references

This section includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations where appropriate. (January 2008)

• AN19, Application Notes , LT1070 design Manual, an extensive introduction in Buck, Boost, CUK , Inverter application with Integrated circuit. Carl Nelson (download as PDF from http://www.linear.com/designtools/app_notes.jsp)
• Abraham I. Pressman, Keith Billings, Taylor Morey (2009). Switching Power Supply Design, Third Edition. McGraw-Hill. ISBN 0-07-148272-5.
• Ned Mohan, Tore M. Undeland, William P. Robbins (2002). Power Electronics : Converters, Applications, and Design. Wiley. ISBN 0-471-22693-9.
• Muhammad H. Rashid (2003). Power Electronics : Circuits, Devices, and Applications. Prentice Hall. ISBN 0-13-122815-3.
• Fang Lin Luo, Hong Ye (2004). Advanced DC/DC Converters. CRC Press. ISBN 0-8493-1956-0.
• Mingliang Liu (2006). Demystifying Switched-Capacitor Circuits. Elsevier. ISBN 0-7506-7907-7.
• Fang Lin Luo, Hong Ye, Muhammad H. Rashid (2005). Power Digital Power Electronics and Applications. Elsevier. ISBN 0-12-088757-6.
• Robert W. Erickson & Dragan Maksimovic (2001). Fundamentals of Power Electronics. Second edition. ISBN 0-7923-7270-0.
• Marty Brown, Power Supply Cookbook. Newnes. 2nd ed 2001. ISBN 0-7506-7329-X.
• Christophe Basso (2008), Switch-Mode Power Supplies: SPICE Simulations and Practical Designs. McGraw-Hill. ISBN 0071508589.
• Sanjaya Maniktala (2004), Switching Power Supply Design and Optimization. McGraw-Hill. ISBN 0071434836.
• Sanjaya Maniktala (2006), Switching Power Supplies A to Z. Newnes/Elsevier. ISBN 0750679700.
• Sanjaya Maniktala (2007), Troubleshooting Switching Power Converters: A Hands-on Guide. Newnes/Elsevier. ISBN 0750684216.

[edit] References