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A switched-mode power supply (switching-mode power supply, switch-mode power supply, switched power supply, SMPS, or switcher) is an electronic
power supply A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to power the load. As ...
that incorporates a
switching regulator A voltage regulator is a system designed to automatically maintain a constant voltage. A voltage regulator may use a simple feed-forward design or may include negative feedback. It may use an electromechanical mechanism, or electronic components ...
to convert electrical power efficiently. Like other power supplies, an SMPS transfers power from a DC or AC source (often mains power, see
AC adapter An AC adapter or AC/DC adapter is a type of external power supply, often enclosed in a case similar to an AC plug. Other common names include wall wart, power brick, wall charger, and power adapter. Adapters for battery-powered equipment may ...
) to DC loads, such as a
personal computer A personal computer (PC) is a multi-purpose microcomputer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or te ...
, while converting
voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...
and current characteristics. Unlike a linear power supply, the pass transistor of a switching-mode supply continually switches between low-
dissipation In thermodynamics, dissipation is the result of an irreversible process that takes place in homogeneous thermodynamic systems. In a dissipative process, energy ( internal, bulk flow kinetic, or system potential) transforms from an initial form to ...
, full-on and full-off states, and spends very little time in the high dissipation transitions, which minimizes wasted energy. A hypothetical ideal switched-mode power supply dissipates no power. Voltage regulation is achieved by varying the ratio of on-to-off time (also known as
duty cycle A duty cycle or power cycle is the fraction of one period in which a signal or system is active. Duty cycle is commonly expressed as a percentage or a ratio. A period is the time it takes for a signal to complete an on-and-off cycle. As a form ...
s). In contrast, a linear power supply regulates the output voltage by continually dissipating power in the pass
transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...
. The switched-mode power supply's higher
electrical efficiency The efficiency of a system in electronics and electrical engineering is defined as useful power output divided by the total electrical power consumed (a vulgar fraction, fractional Expression (mathematics), expression), typically denoted by the Gr ...
is an important advantage. Switched-mode power supplies can also be substantially smaller and lighter than a linear supply because the transformer can be much smaller. This is because it operates at a high switching frequency which ranges from several hundred kHz to several MHz in contrast to the 50 or 60 Hz
mains frequency The utility frequency, (power) line frequency (American English) or mains frequency (British English) is the nominal frequency of the oscillations of alternating current (AC) in a wide area synchronous grid transmitted from a power station to th ...
. Despite the reduced transformer size, the power supply topology and the requirement for electromagnetic interference (EMI) suppression in commercial designs result in a usually much greater component count and corresponding circuit complexity. Switching regulators are used as replacements for linear regulators when higher efficiency, smaller size or lighter weight is required. They are, however, more complicated; switching currents can cause electrical noise problems if not carefully suppressed, and simple designs may have a poor
power factor In electrical engineering, the power factor of an AC power system is defined as the ratio of the ''real power'' absorbed by the load to the '' apparent power'' flowing in the circuit. Real power is the average of the instantaneous product of v ...
.


History

; 1836:
Induction coil An induction coil or "spark coil" ( archaically known as an inductorium or Ruhmkorff coil after Heinrich Rühmkorff) is a type of electrical transformer used to produce high-voltage pulses from a low-voltage direct current (DC) supply. p.98 ...
s use switches to generate high voltages. ; 1910:An inductive discharge ignition system invented by
Charles F. Kettering Charles Franklin Kettering (August 29, 1876 – November 25, 1958) sometimes known as Charles Fredrick Kettering was an American inventor, engineer, businessman, and the holder of 186 patents. For the list of patents issued to Kettering, see, Le ...
and his company Dayton Engineering Laboratories Company (Delco) goes into production for Cadillac. The Kettering ignition system is a mechanically switched version of a fly back boost converter; the transformer is the ignition coil. Variations of this ignition system were used in all non-diesel internal combustion engines until the 1960s when it began to be replaced first by solid-state electronically switched versions, then capacitive discharge ignition systems. ; 1926: On 23 June, British inventor Philip Ray Coursey applies for a patent in his country and United States, for his "Electrical Condenser". The patent mentions high frequency
welding Welding is a fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing fusion. Welding is distinct from lower temperature techniques such as b ...
and furnaces, among other uses. ; : Electromechanical relays are used to stabilize the voltage output of generators. See Voltage regulator#Electromechanical regulators. ; c. 1936: Car radios used
electromechanical In engineering, electromechanics combines processes and procedures drawn from electrical engineering and mechanical engineering. Electromechanics focuses on the interaction of electrical and mechanical systems as a whole and how the two systems ...
vibrators to transform the 6 V battery supply to a suitable B+ voltage for the vacuum tubes. ; 1959: The
MOSFET The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
(metal-oxide-semiconductor field-effect transistor) is invented by Mohamed M. Atalla and
Dawon Kahng Dawon Kahng ( ko, 강대원; May 4, 1931 – May 13, 1992) was a Korean-American electrical engineer and inventor, known for his work in solid-state electronics. He is best known for inventing the MOSFET (metal–oxide–semiconductor field-effe ...
at
Bell Labs Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984), then AT&T Bell Laboratories (1984–1996) and Bell Labs Innovations (1996–2007), is an American industrial research and scientific development company owned by mul ...
. The
power MOSFET A power MOSFET is a specific type of metal–oxide–semiconductor field-effect transistor (MOSFET) designed to handle significant power levels. Compared to the other power semiconductor devices, such as an insulated-gate bipolar transistor (IG ...
later became the most widely used power device for switching power supplies. ; 1959:
Transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...
oscillation and rectifying converter power supply system is filed by Joseph E. Murphy and Francis J. Starzec, from General Motors Company ; 1960s: The
Apollo Guidance Computer The Apollo Guidance Computer (AGC) was a digital computer produced for the Apollo program that was installed on board each Apollo command module (CM) and Apollo Lunar Module (LM). The AGC provided computation and electronic interfaces for guidanc ...
, developed in the early 1960s by the MIT Instrumentation Laboratory for NASA's ambitious moon missions (1966-1972), incorporated early switched mode power supplies. ; c. 1967:
Bob Widlar Robert John Widlar (pronounced ''wide-lar''; November 30, 1937 – February 27, 1991) was an American electronics engineer and a designer of linear integrated circuits (ICs). Early years Widlar was born November 30, 1937 in Cleveland to par ...
of
Fairchild Semiconductor Fairchild Semiconductor International, Inc. was an American semiconductor company based in San Jose, California. Founded in 1957 as a division of Fairchild Camera and Instrument, it became a pioneer in the manufacturing of transistors and of int ...
designs the µA723 IC voltage regulator. One of its applications is as a switched mode regulator. ; 1970:
Tektronix Tektronix, Inc., historically widely known as Tek, is an American company best known for manufacturing test and measurement devices such as oscilloscopes, logic analyzers, and video and mobile test protocol equipment. Originally an independent ...
starts using High-Efficiency Power Supply in its 7000-series oscilloscopes produced from about 1970 to 1995. ; 1970: Robert Boschert develops simpler, low cost circuits. By 1977, Boschert Inc. grows to a 650-person company. After a series of mergers, acquisitions, and spin offs (Computer Products, Zytec, Artesyn, Emerson Electric) the company is now part of
Advanced Energy Advanced Energy Industries, Inc. is an American multinational technology company headquartered in Denver, Colorado that develops precision power conversion, measurement and control technologies for the manufacture of semiconductors, flat panel ...
. ; 1972: HP-35,
Hewlett-Packard The Hewlett-Packard Company, commonly shortened to Hewlett-Packard ( ) or HP, was an American multinational information technology company headquartered in Palo Alto, California. HP developed and provided a wide variety of hardware components ...
's first pocket calculator, is introduced with transistor switching power supply for
light-emitting diode A light-emitting diode (LED) is a semiconductor Electronics, device that Light#Light sources, emits light when Electric current, current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy i ...
s, clocks, timing, ROM, and registers. ; 1973:
Xerox Xerox Holdings Corporation (; also known simply as Xerox) is an American corporation that sells print and digital document products and services in more than 160 countries. Xerox is headquartered in Norwalk, Connecticut (having moved from St ...
uses switching power supplies in the
Alto The musical term alto, meaning "high" in Italian ( Latin: ''altus''), historically refers to the contrapuntal part higher than the tenor and its associated vocal range. In 4-part voice leading alto is the second-highest part, sung in choruse ...
minicomputer ; 1976: Robert Mammano, a co-founder of Silicon General Semiconductors, develops the first integrated circuit for SMPS control, model SG1524. After a series of mergers and acquisitions (Linfinity, Symetricom,
Microsemi Microsemi Corporation was an Aliso Viejo, California-based provider of semiconductor and system solutions for aerospace & defense, communications, data center and industrial markets. In February 2018, it was announced that Chandler, Arizona-bas ...
), the company is now part of Microchip Technology. ; 1977:
Apple II The Apple II (stylized as ) is an 8-bit home computer and one of the world's first highly successful mass-produced microcomputer products. It was designed primarily by Steve Wozniak; Jerry Manock developed the design of Apple II's foam-m ...
is designed with a switching mode power supply. "'' Rod Holt ... created the switching power supply that allowed us to do a very lightweight computer''". ; 1980: The HP8662A
synthesized signal generator Synthesis or synthesize may refer to: Science Chemistry and biochemistry * Chemical synthesis, the execution of chemical reactions to form a more complex molecule from chemical precursors **Organic synthesis, the chemical synthesis of organ ...
went with a switched mode power supply.


Explanation

A linear power supply (non-SMPS) uses a linear regulator to provide the desired output
voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...
by dissipating 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 either output voltage or current by dissipating the electric power in the form of
heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
, and hence its maximum power efficiency is voltage-out/voltage-in since the volt difference is wasted. In contrast, a SMPS changes output voltage and current by switching ideally lossless storage elements, such as
inductor An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a c ...
s and
capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...
s, between different electrical configurations. Ideal switching elements (approximated by transistors operated outside of their active mode) have no resistance when "on" and carry no current when "off", and so converters with ideal components would operate with 100% efficiency (i.e., all input power is delivered to the load; no power is wasted as dissipated heat). In reality, these ideal components do not exist, so a switching power supply cannot be 100% efficient, but it is still a significant improvement in efficiency over a linear regulator. For example, if a DC source, an inductor, a switch, and the corresponding
electrical ground In electrical engineering, ground or earth is a reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the Earth. Electrical circuits may be co ...
are placed in series and the switch is driven by a
square wave A square wave is a non-sinusoidal periodic waveform in which the amplitude alternates at a steady frequency between fixed minimum and maximum values, with the same duration at minimum and maximum. In an ideal square wave, the transitions b ...
, the peak-to-peak voltage of the waveform measured across the switch can exceed the input voltage from the DC source. This is because the inductor responds to changes in current by inducing its own voltage to counter the change in current, and this voltage adds to the source voltage while the switch is open. If a diode-and-capacitor combination is 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 an output voltage greater than the DC voltage driving the circuit. This boost converter acts like a step-up transformer for DC signals. A
buck–boost converter The buck–boost converter is a type of DC-to-DC converter that has an output voltage magnitude that is either greater than or less than the input voltage magnitude. It is equivalent to a flyback converter using a single inductor instead of a t ...
works in a similar manner, but yields an output voltage which is opposite in polarity to the input voltage. Other buck circuits exist to boost the average output current with a reduction of voltage. In a 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.,
pulse-width modulation Pulse-width modulation (PWM), or pulse-duration modulation (PDM), is a method of reducing the average power delivered by an electrical signal, by effectively chopping it up into discrete parts. The average value of voltage (and current) fed ...
with an adjustable
duty cycle A duty cycle or power cycle is the fraction of one period in which a signal or system is active. Duty cycle is commonly expressed as a percentage or a ratio. A period is the time it takes for a signal to complete an on-and-off cycle. As a form ...
) to drive the switching elements. The
spectral density The power spectrum S_(f) of a time series x(t) describes the distribution of power into frequency components composing that signal. According to Fourier analysis, any physical signal can be decomposed into a number of discrete frequencies ...
of these switching waveforms has energy concentrated at relatively high frequencies. As such, switching transients and ripple introduced onto the output waveforms can be filtered with a small LC filter.


Advantages and disadvantages

The main advantage of the switching power supply is greater efficiency ( up to 96%) than linear regulators because the switching transistor dissipates little power when acting as a switch. Other advantages include smaller size, lower noise, and lighter weight from the elimination of heavy line-frequency transformers, and comparable heat generation. Standby power loss is often much less than transformers. The transformer in a switching power supply is also smaller than a traditional line frequency (50 Hz or 60 Hz depending on region) transformer, and therefore requires smaller amounts of expensive raw materials, like copper. Disadvantages include greater complexity, the generation of high-amplitude, high-frequency energy that the
low-pass filter A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filt ...
must block to avoid
electromagnetic interference Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrost ...
(EMI), a ripple voltage at the switching frequency and its harmonic frequencies. Very low cost SMPSs may couple electrical switching noise back onto the mains power line, causing interference with devices connected to the same phase, such as A/V equipment. Non- power-factor-corrected SMPSs also cause harmonic distortion.


SMPS and linear power supply comparison

There are two main types of regulated power supplies available: SMPS and linear. The following table compares linear with switching power supplies in general:


Theory of operation


Input rectifier stage

If the SMPS has an AC input, then the first stage is to convert the input to DC. This is called '
rectification Rectification has the following technical meanings: Mathematics * Rectification (geometry), truncating a polytope by marking the midpoints of all its edges, and cutting off its vertices at those points * Rectifiable curve, in mathematics * Recti ...
'. An SMPS with a DC input does not require this stage. In some power supplies (mostly computer ATX power supplies), the rectifier circuit can be configured as a voltage doubler by the addition of a switch operated either manually or automatically. This feature permits operation from power sources that are normally at 115 VAC or at 230 VAC. 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. To correct for this, many newer SMPS will use a special power factor correction (PFC) circuit to make the input current follow the sinusoidal shape of the AC input voltage, correcting the power factor. Power supplies that use
active PFC In electrical engineering, the power factor of an AC power system is defined as the ratio of the ''real power'' absorbed by the load to the ''apparent power'' flowing in the circuit. Real power is the average of the instantaneous product of vol ...
usually are auto-ranging, supporting input voltages from , with no input voltage selector switch. An SMPS designed for AC input can usually be run from a DC supply, because the DC would pass through the rectifier unchanged. If the power supply is designed for and has no voltage selector switch, the required DC voltage would be (115 × √2). This type of use may be harmful to the rectifier stage, however, as it will only use half of diodes in the rectifier for the full load. This could possibly result in overheating of these components, causing them to fail prematurely. On the other hand, if the power supply has a voltage selector switch, based on the Delon circuit, for 115/230 V (computer ATX power supplies typically are in this category), the selector switch would have to be put in the position, and the required voltage would be (230 × √2). The diodes in this type of power supply will handle the DC current just fine because they are rated to handle double the nominal input current when operated in the mode, due to the operation of the voltage doubler. This is because the doubler, when in operation, uses only half of the bridge rectifier and runs twice as much current through it.


Inverter stage

:This section refers to the block marked ''chopper'' in the diagram. 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 The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that on ...
. The frequency is usually chosen to be above 20 kHz, to make it inaudible to humans. The switching is implemented as a multistage (to achieve high gain)
MOSFET The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
amplifier. MOSFETs are a type of
transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...
with a low on- resistance and a high current-handling capacity.


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 A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the 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 diode The Schottky diode (named after the German physicist Walter H. Schottky), also known as Schottky barrier diode or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltage ...
s 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
inductor An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a c ...
s and
capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...
s. 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 converter A buck converter (step-down converter) is a DC-to-DC power converter which steps down voltage (while stepping up current) from its input (supply) to its output (load). It is a class of switched-mode power supply (SMPS) typically containing at ...
s'', and the ''
buck–boost converter The buck–boost converter is a type of DC-to-DC converter that has an output voltage magnitude that is either greater than or less than the input voltage magnitude. It is equivalent to a flyback converter using a single inductor instead of a t ...
s''. 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 The single-ended primary-inductor converter (SEPIC) is a type of DC/DC converter that allows the electrical potential (voltage) at its output to be greater than, less than, or equal to that at its input. The output of the SEPIC is controlled by t ...
converters can be implemented, or, by adding additional active switches, various bridge converters can be realized. Other types of SMPSs use a
capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...
diode A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A diod ...
voltage multiplier 280px, Villard cascade voltage multiplier. A voltage multiplier is an electrical circuit that converts AC electrical power from a lower voltage to a higher DC voltage, typically using a network of capacitors and diodes. Voltage multipliers can ...
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.


Regulation

A
feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...
circuit monitors the output voltage and compares it with a reference voltage. Depending on design and safety requirements, the controller may contain an isolation mechanism (such as an
opto-coupler An opto-isolator (also called an optocoupler, photocoupler, or optical isolator) is an electronic component that transfers electrical signals between two isolated circuits by using light. Opto-isolators prevent high voltages from affecting the ...
) 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 impedance 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.


Transformer design

Any switched-mode power supply that gets its power from an AC power line (called an "off-line" converter) requires a transformer for galvanic isolation. Some DC-to-DC converters may also include a transformer, although isolation may not be critical in these cases. SMPS transformers run at high frequency. Most of the cost savings (and space savings) in off-line power supplies result from the smaller size of the high frequency transformer compared to the 50/60 Hz transformers formerly used. There are additional design tradeoffs. The terminal voltage of a transformer is proportional to the product of the core area, magnetic flux, and frequency. By using a much higher frequency, the core area (and so the mass of the core) can be greatly reduced. However, core losses increase at higher frequencies. Cores generally use ferrite material which has a low loss at the high frequencies and high flux densities used. The laminated iron cores of lower-frequency (<400 Hz) transformers would be unacceptably lossy at switching frequencies of a few kilohertz. Also, more energy is lost during transitions of the switching semiconductor at higher frequencies. Furthermore, more attention to the physical layout of the circuit board is required as parasitics become more significant, and the amount of
electromagnetic interference Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrost ...
will be more pronounced.


Copper loss

At low frequencies (such as the line frequency of 50 or 60 Hz), designers can usually ignore the
skin effect Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases exponentially with greater depths in the co ...
. For these frequencies, the skin effect is only significant when the conductors are large, more than in diameter. Switching power supplies must pay more attention to the skin effect because it is a source of power loss. At 500 kHz, the skin depth in copper is about – a dimension smaller than the typical wires used in a power supply. The effective resistance of conductors increases, because current concentrates near the surface of the conductor and the inner portion carries less current than at low frequencies. The skin effect is exacerbated by the harmonics present in the high speed
pulse-width modulation Pulse-width modulation (PWM), or pulse-duration modulation (PDM), is a method of reducing the average power delivered by an electrical signal, by effectively chopping it up into discrete parts. The average value of voltage (and current) fed ...
(PWM) switching waveforms. The appropriate skin depth is not just the depth at the fundamental, but also the skin depths at the harmonics. In addition to the skin effect, there is also a
proximity effect Proximity effect may refer to: * Proximity effect (atomic physics) * Proximity effect (audio), an increase in bass or low frequency response when a sound source is close to a microphone * ''Proximity Effect'' (comics), a comic book series written by ...
, which is another source of power loss.


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 A harmonic is a wave with a frequency that is a positive integer multiple of the ''fundamental frequency'', the frequency of the original periodic signal, such as a sinusoidal wave. The original signal is also called the ''1st harmonic'', t ...
content and relatively low power factor. This creates extra load on utility lines, increases heating of building wiring, the utility
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s, 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 by filtering, but the filters are expensive. Unlike displacement power factor created by linear inductive or capacitive loads, this distortion cannot be corrected by addition of a single linear component. Additional circuits are required to counteract the effect of the brief current pulses. Putting a current regulated boost chopper stage after the off-line rectifier (to charge the storage capacitor) can correct the power factor, but increases the complexity and cost. In 2001, the European Union put into effect the standard
IEC 61000-3-2 IEC 61000-3-2 ''Electromagnetic compatibility (EMC) – Part 3-2: Limits – Limits for harmonic current emissions (equipment input current ≤ 16 A per phase)'' is an international standard that limits mains voltage distortion by prescribing the ma ...
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. To comply with these requirements, modern switched-mode power supplies normally include an additional power factor correction (PFC) stage.


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.


Non-isolated topologies

Non-isolated converters are simplest, with the three basic types using a single inductor for energy storage. In the voltage relation column, ''D'' is the duty cycle of the converter, and can vary from 0 to 1. The input voltage (V1) is assumed to be greater than zero; if it is negative, for consistency, negate the output voltage (V2). When equipment is human-accessible, voltage limits of ≤ 30 V (r.m.s.) AC or ≤ 42.4 V peak or ≤ 60 V DC and power limits of 250 VA apply for safety certification ( 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 taking the output from the ''ground'' pin. Likewise, SEPIC and Zeta converters are both minor rearrangements of the Ćuk converter. The ''neutral point clamped'' (NPC) topology is used in power supplies and active filters and is mentioned here for completeness. Switchers become less efficient as duty cycles become extremely short. For large voltage changes, a transformer (isolated) topology may be better.


Isolated topologies

All isolated topologies include a
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
, and thus can produce an output of higher or lower voltage than the input by adjusting the turns ratio. 090112 powerdesigners.com 090112 jaycar.com.au Page 4 For some topologies, multiple windings can be placed on the transformer to produce multiple output voltages. Some converters use the transformer for energy storage, while others use a separate inductor. * Flyback converter logarithmic control loop behavior might be harder to control than other types. 100517 powerelectronics.com * The forward converter has several variants, varying in how the transformer is "reset" to zero
magnetic flux In physics, specifically electromagnetism, the magnetic flux through a surface is the surface integral of the normal component of the magnetic field B over that surface. It is usually denoted or . The SI unit of magnetic flux is the weber ...
every cycle. Chopper controller: The output voltage is coupled to the input thus very tightly controlled


Quasi-resonant zero-current/zero-voltage switch

In a quasi-resonant zero-current/zero-voltage switch (ZCS/ZVS) "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." Quasi-resonant switching, also known as ''valley switching'', reduces EMI in the power supply by two methods: # By switching the bipolar switch when the voltage is at a minimum (in the valley) to minimize the hard switching effect that causes EMI. # 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.


Efficiency and 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 switching frequency allows component sizes to be shrunk, but can produce more RFI. 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.


Failure modes

''For failure in switching components, circuit board and so on read the failure modes of electronics article.'' Power supplies which use capacitors suffering from the
capacitor plague The capacitor plague was a problem related to a higher-than-expected failure rate of non-solid aluminium electrolytic capacitors, between 1999 and 2007, especially those from some Taiwanese manufacturers, due to faulty electrolyte composition t ...
may experience premature failure when the capacitance drops to 4% of the original value. This usually causes the switching semiconductor to fail in a conductive way. That may expose connected loads to the full input volt and current, and precipitate wild oscillations in output. 100211 lowyat.net Failure of the switching transistor is common. Due to the large switching voltages this transistor must handle (around for a mains supply), these transistors often short out, in turn immediately blowing the main internal power fuse.


Precautions

The main filter capacitor will often store up to long after the input power has been disconnected. Not all power supplies contain a small "bleeder" resistor which slowly discharges the capacitor. Contact with this capacitor can result in a severe electrical shock. The primary and secondary sides may be connected with a capacitor to reduce EMI and compensate for various capacitive couplings in the converter circuit, where the transformer is one. This may result in electric shock in some cases. The current flowing from
line Line most often refers to: * Line (geometry), object with zero thickness and curvature that stretches to infinity * Telephone line, a single-user circuit on a telephone communication system Line, lines, The Line, or LINE may also refer to: Art ...
or neutral through a resistor to any accessible part must, according to , be less than for IT equipment.


Applications

Switched-mode power supply units (PSUs) in domestic products such as
personal computer A personal computer (PC) is a multi-purpose microcomputer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or te ...
s often have universal inputs, meaning that they can accept power from mains supplies throughout the world, although a manual voltage range switch may be required. Switch-mode power supplies can tolerate a wide range of power frequencies and voltages. 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 materials A bill of materials or product structure (sometimes bill of material, BOM or associated list) is a list of the raw materials, sub-assemblies, intermediate assemblies, sub-components, parts, and the quantities of each needed to manufacture an end ...
(BOM) by removing secondary-side sensing components such as
optocouplers An opto-isolator (also called an optocoupler, photocoupler, or optical isolator) is an electronic component that transfers electrical signals between two isolated circuits by using light. Opto-isolators prevent high voltages from affecting the ...
. Switched-mode power supplies are used for DC to DC conversion as well. In heavy vehicles that use a nominal cranking supply, 12 V for accessories may be furnished through a DC/DC switch-mode supply. This has the advantage over tapping the battery at the 12 V position (using half the cells) that the entire 12 V load is evenly divided between all cells of the 24 V battery. In industrial settings such as telecommunications racks, bulk power may be distributed at a low DC voltage (e.g. from a battery back up system) and individual equipment items will have DC/DC switched-mode converters to supply required voltages. A common use for switched-mode power supplies is an extra-low-voltage source for lighting. For this application, they are often called "electronic transformers".


Terminology

The term ''switch mode'' was widely used until
Motorola Motorola, Inc. () was an American multinational telecommunications company based in Schaumburg, Illinois, United States. After having lost $4.3 billion from 2007 to 2009, the company split into two independent public companies, Motorola ...
claimed ownership of the trademark SWITCHMODE for products aimed at the switching-mode power supply market and started to enforce their trademark. ''Switching-mode power supply'', ''switching power supply'', and ''switching regulator'' refer to this type of power supply.


See also

* Auto transformer * Boost converter *
Buck converter A buck converter (step-down converter) is a DC-to-DC power converter which steps down voltage (while stepping up current) from its input (supply) to its output (load). It is a class of switched-mode power supply (SMPS) typically containing at ...
* Conducted electromagnetic interference *
DC to DC converter A DC-to-DC converter is an electronic circuit or electromechanical device that converts a source of direct current (DC) from one voltage level to another. It is a type of electric power converter. Power levels range from very low (small batteries) ...
* Inrush current * Joule thief *
Leakage inductance Leakage inductance derives from the electrical property of an imperfectly-coupled transformer whereby each winding behaves as a self-inductance in series with the winding's respective ohmic resistance constant. These four winding constants also ...
*
Resonant converter A resonant converter is a type of electric power converter that contains a network of inductors and capacitors called a "resonant tank", tuned to resonate at a specific frequency. They find applications in electronics, in integrated circuits. There ...
* Switching amplifier *
Transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
* Vibrator (electronic)


Notes


References

*


Further reading

* * * * * * * * * * * * Application Note giving an extensive introduction in Buck, Boost, CUK, Inverter applications. (download as PDF from http://www.linear.com/designtools/app_notes.php) * *


External links

*
Switching Power Supply Topologies Poster
- Texas Instruments
Load Power Sources for Peak Efficiency, by James Colotti, published in EDN 1979 October 5


{{DEFAULTSORT:Switched-Mode Power Supply Power supplies Power electronics Electric power conversion Voltage regulation