# Switched-mode power supply

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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 that incorporates a switching regulator to convert electrical power efficiently. Like other power supplies, an SMPS transfers power from a DC or AC source (often mains power) to DC loads, such as a personal computer, while converting voltage and current characteristics. Unlike a linear power supply, the pass transistor of a switching-mode supply continually switches between low-dissipation, 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 cycles ). In contrast, a linear power supply regulates the output voltage by continually dissipating power in the pass transistor. This higher power conversion efficiency is an important advantage of a switched-mode power supply. Switched-mode power supplies may also be substantially smaller and lighter than a linear supply due to the smaller transformer size and weight.

## Contents

Switching regulators are used as replacements for linear regulators when higher efficiency, smaller size or lighter weight are required. They are, however, more complicated; their switching currents can cause electrical noise problems if not carefully suppressed, and simple designs may have a poor power factor.

## History

1836
Induction coils use switches to generate high voltages.
1910
An inductive discharge ignition system invented by Charles F. Kettering and his company Dayton Engineering Laboratories Company (Delco) goes into production for Cadillac. [1] 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". [2] [3] The patent mentions high frequency welding [4] and furnaces, among other uses. [3]
c.1932
Electromechanical relays are used to stabilize the voltage output of generators. See Voltage regulator#Electromechanical regulators. [5] [6]
c. 1936
Car radios used electromechanical vibrators to transform the 6 V battery supply to a suitable B+ voltage for the vacuum tubes. [7]
1959
The MOSFET (metal-oxide-semiconductor field-effect transistor) is invented by Mohamed M. Atalla and Dawon Kahng at Bell Labs. [8] The power MOSFET later became the most widely used power device for switching power supplies. [9]
1959
Transistor oscillation and rectifying converter power supply system is filed by Joseph E. Murphy and Francis J. Starzec, from General Motors Company [10]
1960s
The Apollo Guidance Computer, developed in the early 1960s by the MIT Instrumentation Laboratory for NASA’s ambitious moon missions (1966-1972), incorporated early switched mode power supplies. [11]
c. 1967
Bob Widlar of Fairchild Semiconductor designs the µA723 IC voltage regulator. One of its applications is as a switched mode regulator. [12]
1970
Tektronix starts using High-Efficiency Power Supply in its 7000-series oscilloscopes produced from about 1970 to 1995. [13] [14] [15] [16]
1970
Robert Boschert develops simpler, low cost circuits. By 1977, Boschert Inc. grows to a 650-people company. [17] [18] After a series of mergers, acquisitions, and spin offs (Computer Products, Zytec, Artesyn, Emerson Electric) the company is now part of Advanced Energy. [19] [20] [21]
1972
HP-35, Hewlett-Packard's first pocket calculator, is introduced with transistor switching power supply for light-emitting diodes, clocks, timing, ROM, and registers. [22]
1973
Xerox uses switching power supplies in the Alto minicomputer [23]
1976
Robert Mammano, a co-founder of Silicon General Semiconductors, develops the first integrated circuit for SMPS control, model SG1524. [17] After a series of mergers and acquisitions (Linfinity, Symetricom, Microsemi), the company is now part of Microchip Technology. [24]
1977
Apple II is designed with a switching mode power supply. " Rod Holt was brought in as product engineer and there were several flaws in Apple II that were never publicized. One thing Holt has to his credit is that he created the switching power supply that allowed us to do a very lightweight computer". [25]
1980
The HP8662A 10 kHz – 1.28 GHz synthesized signal generator went with a switched mode power supply. [26]

## Explanation

A linear regulator provides 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 either output voltage or current by dissipating the excess electric power in the form of heat, 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 inductors and capacitors, 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 are placed in series and the switch is driven by a square wave, 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 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 with an adjustable duty cycle) to drive the switching elements. The spectral density 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.

The main advantage of the switching power supply is greater efficiency 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 must block to avoid electromagnetic interference (EMI), a ripple voltage at the switching frequency and the harmonic frequencies thereof.

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 regulated and unregulated AC-to-DC supplies with switching regulators in general:

Comparison of a linear power supply and a switched-mode power supply
Linear power supplySwitching power supplyNotes
Size and weightHeatsinks for high power linear regulators add size and weight. Transformers, if used, are large due to low operating frequency (mains power frequency is at 50 or 60 Hz); otherwise can be compact due to low component count.Smaller transformer (if used; else inductor) due to higher operating frequency (typically 50 kHz – 1 MHz). Size and weight of adequate RF shielding may be significant.A transformer's power handling capacity of given size and weight increases with frequency provided that hysteresis losses can be kept down. Therefore, higher operating frequency means either a higher capacity or smaller transformer.
Output voltageWith transformer used, any voltages available; if transformerless, limited to what can be achieved with a voltage doubler. If unregulated, voltage varies significantly with load.Any voltages available, limited only by transistor breakdown voltages in many circuits. Voltage varies little with load.An SMPS can usually cope with wider variation of input before the output voltage changes.
Efficiency, heat, and power dissipation If regulated: efficiency largely depends on voltage difference between input and output; output voltage is regulated by dissipating excess power as heat resulting in a typical efficiency of 30–40%. [27] If unregulated, transformer iron and copper losses may be the only significant sources of inefficiency.Output is regulated using duty cycle control; the transistors are switched fully on or fully off, so very little resistive losses between input and the load. The only heat generated is in the non-ideal aspects of the components and quiescent current in the control circuitry.Switching losses in the transistors (especially in the short part of each cycle when the device is partially on), on-resistance of the switching transistors, equivalent series resistance in the inductor and capacitors, and core losses in the inductor, and rectifier voltage drop contribute to a typical efficiency of 60–70%. However, by optimizing SMPS design (such as choosing the optimal switching frequency, avoiding saturation of inductors, and active rectification), the amount of power loss and heat can be minimized; a good design can have an efficiency of 95%.
ComplexityUnregulated may be simply a diode and capacitor; regulated has a voltage-regulating circuit and a noise-filtering capacitor; usually a simpler circuit (and simpler feedback loop stability criteria) than switched-mode circuits.Consists of a controller IC, one or several power transistors and diodes as well as a power transformer, inductors, and filter capacitors. Some design complexities present (reducing noise/interference; extra limitations on maximum ratings of transistors at high switching speeds) not found in linear regulator circuits.In switched-mode mains (AC-to-DC) supplies, multiple voltages can be generated by one transformer core, but that can introduce design/use complications: for example it may place minimum output current restrictions on one output. For this SMPSs have to use duty cycle control. One of the outputs has to be chosen to feed the voltage regulation feedback loop (usually 3.3 V or 5 V loads are more fussy about their supply voltages than the 12 V loads, so this drives the decision as to which feeds the feedback loop. The other outputs usually track the regulated one pretty well). Both need a careful selection of their transformers. Due to the high operating frequencies in SMPSs, the stray inductance and capacitance of the printed circuit board traces become important.
Radio frequency interference Mild high-frequency interference may be generated by AC rectifier diodes under heavy current loading, while most other supply types produce no high-frequency interference. Some mains hum induction into unshielded cables, problematical for low-signal audio. EMI/RFI produced due to the current being switched on and off sharply. Therefore, EMI filters and RF shielding are needed to reduce the disruptive interference.Long wires between the components may reduce the high frequency filter efficiency provided by the capacitors at the inlet and outlet. Stable switching frequency may be important.
Electronic noise at the output terminalsUnregulated PSUs may have a little AC ripple superimposed upon the DC component at twice mains frequency (100–120 Hz). It can cause audible mains hum in audio equipment, brightness ripples or banded distortions in analog security cameras.Noisier due to the switching frequency of the SMPS. An unfiltered output may cause glitches in digital circuits or noise in audio circuits.This can be suppressed with capacitors and other filtering circuitry in the output stage. With a switched mode PSU the switching frequency can be chosen to keep the noise out of the circuits working frequency band (e.g., for audio systems above the range of human hearing)
Electronic noise at the input terminalsCauses harmonic distortion to the input AC, but relatively little or no high frequency noise.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.This can be prevented if a (properly earthed) EMI/RFI filter is connected between the input terminals and the bridge rectifier.
Acoustic noise Faint, usually inaudible mains hum, usually due to vibration of windings in the transformer or magnetostriction.Usually inaudible to most humans, unless they have a fan or are unloaded/malfunctioning, or use a switching frequency within the audio range, or the laminations of the coil vibrate at a subharmonic of the operating frequency.The operating frequency of an unloaded SMPS is sometimes in the audible human range, and may sound subjectively quite loud for people whose hearing is very sensitive to the relevant frequency range.
Power factor Low for a regulated supply because current is drawn from the mains at the peaks of the voltage sinusoid, unless a choke-input or resistor-input circuit follows the rectifier (now rare).Ranging from very low to medium since a simple SMPS without PFC draws current spikes at the peaks of the AC sinusoid.Active/passive power factor correction in the SMPS can offset this problem and are even required by some electric regulation authorities, particularly in the EU. The internal resistance of low-power transformers in linear power supplies usually limits the peak current each cycle and thus gives a better power factor than many switched-mode power supplies that directly rectify the mains with little series resistance.
Inrush current Large current when mains-powered linear power supply equipment is switched on until magnetic flux of transformer stabilises and capacitors charge completely, unless a slow-start circuit is used.Extremely large peak "in-rush" surge current limited only by the impedance of the input supply and any series resistance to the filter capacitors.Empty filter capacitors initially draw large amounts of current as they charge up, with larger capacitors drawing larger amounts of peak current. Being many times above the normal operating current, this greatly stresses components subject to the surge, complicates fuse selection to avoid nuisance blowing and may cause problems with equipment employing overcurrent protection such as uninterruptible power supplies. Mitigated by use of a suitable soft-start circuit or series resistor.
Risk of electric shock Supplies with transformers isolate the incoming power supply from the powered device and so allow metalwork of the enclosure to be grounded safely. Dangerous if primary/secondary insulation breaks down, unlikely with reasonable design. Transformerless mains-operated supply dangerous. In both linear and switch-mode the mains, and possibly the output voltages, are hazardous and must be well-isolated.Common rail of equipment (including casing) is energized to half the mains voltage, but at high impedance, unless equipment is earthed/grounded or doesn't contain EMI/RFI filtering at the input terminals.Due to regulations concerning EMI/RFI radiation, many SMPS contain EMI/RFI filtering at the input stage consisting of capacitors and inductors before the bridge rectifier. Two capacitors are connected in series with the Live and Neutral rails with the Earth connection in between the two capacitors. This forms a capacitive divider that energizes the common rail at half mains voltage. Its high impedance current source can provide a tingling or a 'bite' to the operator or can be exploited to light an Earth Fault LED. However, this current may cause nuisance tripping on the most sensitive residual-current devices. In power supplies without a ground pin (like USB charger) there is EMI/RFI capacitor placed between primary and secondary side. [28] It can also provide some very mild tingling sensation but it's safe to the user. [29]
Risk of equipment damageVery low, unless a short occurs between the primary and secondary windings or the regulator fails by shorting internally.Can fail so as to make output voltage very high[ quantify ]. Stress on capacitors may cause them to explode. Can in some cases destroy input stages in amplifiers if floating voltage exceeds transistor base-emitter breakdown voltage, causing the transistor's gain to drop and noise levels to increase. [30] Mitigated by good failsafe design. Failure of a component in the SMPS itself can cause further damage to other PSU components; can be difficult to troubleshoot.The floating voltage is caused by capacitors bridging the primary and secondary sides of the power supply. Connection to earthed equipment will cause a momentary (and potentially destructive) spike in current at the connector as the voltage at the secondary side of the capacitor equalizes to earth potential.

## 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 . 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 V or at 230 V. 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 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 usually are auto-ranging, supporting input voltages from ~100 VAC – 250 VAC, 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. [31] If the power supply is designed for 115 VAC and has no voltage selector switch, the required DC voltage would be 163 VDC (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 230 V position, and the required voltage would be 325 VDC (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 115 V 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. [32]

### 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 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 amplifier. MOSFETs are a type of transistor 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. 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 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 realized.

Other types of SMPSs use a capacitordiode 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.

### Regulation

A feedback 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) 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 [33] ) 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 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. For these frequencies, the skin effect is only significant when the conductors are large, more than 0.3 inches (7.6 mm) 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 0.003 inches (0.076 mm) – 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 (PWM) switching waveforms. The appropriate skin depth is not just the depth at the fundamental, but also the skin depths at the harmonics. [34]

In addition to the skin effect, there is also a proximity effect, 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 content and relatively low power factor. This creates extra load on utility lines, increases heating of building wiring, 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 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/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. 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).

Type [35] Typical Power [ W ]Relative costEnergy storageVoltage relationFeatures
Buck 0–1,0001.0Single inductor 0 ≤ Out ≤ In, ${\displaystyle \scriptstyle V_{2}=DV_{1}}$Current is continuous at output.
Boost 0–5,0001.0Single inductorOut ≥ In, ${\displaystyle \scriptstyle V_{2}={\frac {1}{1-D}}V_{1}}$Current is continuous at input.
Buck–boost 0–1501.0Single inductorOut ≤ 0, ${\displaystyle \scriptstyle V_{2}=-{\frac {D}{1-D}}V_{1}}$Current is discontinuous at both input and output.
Split-pi (or, boost–buck)0–4,500>2.0Two inductors and three capacitorsUp or downBidirectional power control; in or out.
Ćuk Capacitor and two inductorsAny inverted, ${\displaystyle \scriptstyle V_{2}=-{\frac {D}{1-D}}V_{1}}$Current is continuous at input and output.
SEPIC Capacitor and two inductorsAny, ${\displaystyle \scriptstyle V_{2}={\frac {D}{1-D}}V_{1}}$Current is continuous at input.
ZetaCapacitor and two inductorsAny, ${\displaystyle \scriptstyle V_{2}={\frac {D}{1-D}}V_{1}}$Current is continuous at output.
Charge pump / switched capacitor Capacitors onlyNo magnetic energy storage is needed to achieve conversion, however high efficiency power processing is normally limited to a discrete set of conversion ratios.

When equipment is human-accessible, voltage and power limits of <=42.4 V peak/60 V DV and 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. [36]

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, and thus can produce an output of higher or lower voltage than the input by adjusting the turns ratio. [37] [38] For some topologies, multiple windings can be placed on the transformer to produce multiple output voltages. [39] Some converters use the transformer for energy storage, while others use a separate inductor.

Type [35] Power
[ W ]
Relative costInput range
[ V ]
Energy storageFeatures
Flyback 0–2501.05–600Mutual inductorsIsolated form of the buck–boost converter 1
Ringing choke converter (RCC)0–1501.05–600TransformerLow-cost self-oscillating flyback variant [40]
Half-forward0–2501.25–500Inductor
Forward 2 100–20060–200InductorIsolated form of buck converter
Resonant forward0–601.060–400Inductor and capacitorSingle rail input, unregulated output, high efficiency, low EMI. [41]
Push-pull 100–1,0001.7550–1,000Inductor
Half-bridge0–2,0001.950–1,000Inductor
Full-bridge400–5,000>2.050–1,000InductorVery efficient use of transformer, used for highest powers
Resonant, zero voltage switched>1,000>2.0Inductor and capacitor
Isolated Ćuk Two capacitors and two inductors
• ^1 Flyback converter logarithmic control loop behavior might be harder to control than other types. [42]
• ^2 The forward converter has several variants, varying in how the transformer is "reset" to zero magnetic flux 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." [43] 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.

## 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 may experience premature failure when the capacitance drops to 4% of the original value.[ failed verification ] 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. [44]

Failure of the switching transistor is common. Due to the large switching voltages this transistor must handle (around 325 V for a 230 VAC 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 325 volts long after the power cord has been removed from the wall. Not all power supplies contain a small "bleeder" resistor to slowly discharge this capacitor. Any contact with this capacitor may 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 or neutral through a 2 kΩ resistor to any accessible part must, according to IEC 60950, be less than 250 μA for IT equipment. [45]

## Applications

Switched-mode power supply units (PSUs) in domestic products such as personal computers 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 (BOM) by removing secondary-side sensing components such as optocouplers.[ citation needed ]

Switched-mode power supplies are used for DC to DC conversion as well. In automobiles where heavy vehicles use a nominal 24 VDC 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 all the 12 V load is evenly divided over all cells of the 24 V battery. In industrial settings such as telecommunications racks, bulk power may be distributed at a low DC voltage (from a battery back up system, for example) and individual equipment items will have DC/DC switched-mode converters to supply whatever voltages are needed.

A common use for switched-mode power supplies is as extra-low-voltage sources for lighting, and for this application they are often called "electronic transformers".

## Terminology

The term switch mode was widely used until Motorola claimed ownership of the trademark SWITCHMODE for products aimed at the switching-mode power supply market and started to enforce their trademark. [33] Switching-mode power supply, switching power supply, and switching regulator refer to this type of power supply. [33]

## Notes

1. US 1037492, Kettering, Charles F.,"Ignition system",published 2 November 1910,issued 3 September 1912
2. US 1754265,Coursey, Philip Ray,"Electrical Condenser",published 23 June 1926,issued 15 April 1930
3. "When was the SMPS power supply invented?". electronicspoint.com.
4. "Electrical condensers (Open Library)". openlibrary.org.
5. "First-Hand:The Story of the Automobile Voltage Regulator - Engineering and Technology History Wiki". ethw.org. Retrieved 21 March 2018.
6. US 2014869,Teare Jr., Benjamin R.&Max A. Whiting,"Electroresponsive Device",published 15 November 1932,issued 17 September 1935
8. "1960: Metal Oxide Semiconductor (MOS) Transistor Demonstrated". The Silicon Engine. Computer History Museum . Retrieved August 31, 2019.
9. "Applying MOSFETs to Today's Power-Switching Designs". Electronic Design . 23 May 2016. Retrieved 10 August 2019.
10. "google.com/patents - Transistor converter power supply system". google.com. Retrieved 21 March 2018.
11. Ken Shirriff (January 2019). "Inside the Apollo Guidance Computer's core memory". righto.com. Retrieved 4 July 2019.
12. µA723 Precision Voltage Regulators, http://www.ti.com/lit/ds/symlink/ua723.pdf data sheet August 1972 revised July 1999
13. "slack.com - Test Equipment and Electronics Information". slack.com. Archived from the original on 2 August 2002. Retrieved 21 March 2018.
14. "7000 Plugin list". www.kahrs.us. Retrieved 21 March 2018.
15. tek.com - 7000 Series oscilloscopes FAQ
16. docmesure.free.fr - TEKSCOPE March 1971 7704 High-Efficiency Power Supply (service manual march-1971 .pdf)
17. Shirriff, Ken (August 2019). "The Quiet Remaking of Computer Power Supplies: A Half Century Ago Better Transistors And Switching Regulators Revolutionized The Design Of Computer Power Supplies". IEEE Spectrum. Retrieved 2019-09-12.
18. Kilbane, Doris (2009-12-07). "Robert Boschert: A Man Of Many Hats Changes The World Of Power Supplies". Electronic Design. Retrieved 2019-09-12.
19. "jacques-laporte.org - The HP-35's Power unit and other vintage HP calculators". citycable.ch. Retrieved 21 March 2018.
20. "Y Combinator's Xerox Alto: restoring the legendary 1970s GUI computer". arstechnica.com. Retrieved 21 March 2018.
21. "HP 3048A". hpmemoryproject.org.
22. "Information about the mild tingling sensation - US". pcsupport.lenovo.com.
23. "Ban Looms for External Transformers". 080224 sound.whsites.net
24. "DC Power Production, Delivery and Utilization, An EPRI White Paper" (PDF). Page 9 080317 mydocs.epri.com
25. Notes on the Troubleshooting and Repair of Small Switchmode Power Supplies: Switching between 115 VAC and 230 VAC input. Search the page for "doubler" for more info. Retrieved March 2013.
26. Foutz, Jerrold. "Switching-Mode Power Supply Design Tutorial Introduction" . Retrieved 2008-10-06.
27. Pressman 1998 , p. 306
28. ON Semiconductor (July 11, 2002). "SWITCHMODE Power Supplies—Reference Manual and Design Guide" (PDF). Retrieved 2011-11-17.
29. "An active power filter implemented with multilevel single-phase NPC converters". 2011. Archived from the original on 2014-11-26. Retrieved 2013-03-15.
30. "DC-DC Converter Basics". Archived from the original on 2005-12-17. 090112 powerdesigners.com
31. "DC-DC CONVERTERS: A PRIMER" (PDF). Archived from the original (PDF) on 2009-04-18. 090112 jaycar.com.au Page 4
32. "Heinz Schmidt-Walter". h-da.de.
33. Irving, Brian T.; Jovanović, Milan M. (March 2002), Analysis and Design of Self-Oscillating Flyback Converter (PDF), Proc. IEEE Applied Power Electronics Conf. (APEC), pp. 897–903, archived from the original (PDF) on 2011-07-09, retrieved 2009-09-30
34. "RDFC topology for linear replacement". Archived from the original on 2008-09-07. 090725 camsemi.com Further information on resonant forward topology for consumer applications
35. "Gain Equalization Improves Flyback Performance Page of". 100517 powerelectronics.com
36. "Error - EDN". EDN. Archived from the original on 2016-05-23.
37. "Bad Capacitors: Information and symptoms". 100211 lowyat.net
38. "Wayback Machine" (PDF). 15 March 2012.

## Related Research Articles

In electrical engineering, the power factor of an AC electrical power system is defined as the ratio of the real power absorbed by the load to the apparent power flowing in the circuit, and is a dimensionless number in the closed interval of −1 to 1. A power factor of less than one indicates the voltage and current are not in phase, reducing the average product of the two. Real power is the instantaneous product of voltage and current and represents the capacity of the electricity for performing work. Apparent power is the product of RMS current and voltage. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power may be greater than the real power. A negative power factor occurs when the device generates power, which then flows back towards the source.

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction.

A diode bridge is an arrangement of four diodes in a bridge circuit configuration that provides the same polarity of output for either polarity of input.

A power supply is an electrical device that supplies electric power to an electrical load. The primary function of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to power the load. As a result, power supplies are sometimes referred to as electric power converters. Some power supplies are separate standalone pieces of equipment, while others are built into the load appliances that they power. Examples of the latter include power supplies found in desktop computers and consumer electronics devices. Other functions that power supplies may perform include limiting the current drawn by the load to safe levels, shutting off the current in the event of an electrical fault, power conditioning to prevent electronic noise or voltage surges on the input from reaching the load, power-factor correction, and storing energy so it can continue to power the load in the event of a temporary interruption in the source power.

A power inverter, or inverter, is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC).

In electronics, a linear regulator is a system used to maintain a steady voltage. The resistance of the regulator varies in accordance with the load resulting in a constant voltage output. The regulating device is made to act like a variable resistor, continuously adjusting a voltage divider network to maintain a constant output voltage and continually dissipating the difference between the input and regulated voltages as waste heat. By contrast, a switching regulator uses an active device that switches on and off to maintain an average value of output. Because the regulated voltage of a linear regulator must always be lower than input voltage, efficiency is limited and the input voltage must be high enough to always allow the active device to drop some voltage.

The Ćuk converter is a type of DC/DC converter that has an output voltage magnitude that is either greater than or less than the input voltage magnitude. It is essentially a boost converter followed by a buck converter with a capacitor to couple the energy.

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 to very high.

A voltage regulator is a system designed to automatically maintain a constant voltage level. A voltage regulator may use a simple feed-forward design or may include negative feedback. It may use an electromechanical mechanism, or electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.

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.

Power electronics is the application of solid-state electronics to the control and conversion of electric power.

A voltage doubler is an electronic circuit which charges capacitors from the input voltage and switches these charges in such a way that, in the ideal case, exactly twice the voltage is produced at the output as at its input.

A low-dropout or LDO regulator is a DC linear voltage regulator that can regulate the output voltage even when the supply voltage is very close to the output voltage.

A boost converter is a DC-to-DC power converter that steps up voltage from its input (supply) to its output (load). It is a class of switched-mode power supply (SMPS) containing at least two semiconductors and at least one energy storage element: a capacitor, inductor, or the two in combination. To reduce voltage ripple, filters made of capacitors are normally added to such a converter's output and input.

A buck converter is a DC-to-DC power converter which steps down voltage from its input (supply) to its output (load). It is a class of switched-mode power supply (SMPS) typically containing at least two semiconductors and at least one energy storage element, a capacitor, inductor, or the two in combination. To reduce voltage ripple, filters made of capacitors are normally added to such a converter's output and input.

A push–pull converter is a type of DC-to-DC converter, a switching converter that uses a transformer to change the voltage of a DC power supply. The distinguishing feature of a push-pull converter is that the transformer primary is supplied with current from the input line by pairs of transistors in a symmetrical push-pull circuit. The transistors are alternately switched on and off, periodically reversing the current in the transformer. Therefore, current is drawn from the line during both halves of the switching cycle. This contrasts with buck-boost converters, in which the input current is supplied by a single transistor which is switched on and off, so current is only drawn from the line during half the switching cycle. During the other half the output power is supplied by energy stored in inductors or capacitors in the power supply. Push–pull converters have steadier input current, create less noise on the input line, and are more efficient in higher power applications.

Ripple in electronics is the residual periodic variation of the DC voltage within a power supply which has been derived from an alternating current (AC) source. This ripple is due to incomplete suppression of the alternating waveform after rectification. Ripple voltage originates as the output of a rectifier or from generation and commutation of DC power.

The flyback converter is used in both AC/DC and DC/DC conversion with galvanic isolation between the input and any outputs. The flyback converter is a buck-boost converter with the inductor split to form a transformer, so that the voltage ratios are multiplied with an additional advantage of isolation. When driving for example a plasma lamp or a voltage multiplier the rectifying diode of the boost converter is left out and the device is called a flyback transformer.

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 the duty cycle of the control switch (S1).

A regulated power supply is an embedded circuit; it converts unregulated AC into a constant DC. With the help of a rectifier it converts AC supply into DC. Its function is to supply a stable voltage, to a circuit or device that must be operated within certain power supply limits. The output from the regulated power supply may be alternating or unidirectional, but is nearly always DC.

## References

• Pressman, Abraham I. (1998), Switching Power Supply Design (2nd ed.), McGraw-Hill, ISBN   0-07-052236-7
• Basso, Christophe (2008), Switch-Mode Power Supplies: SPICE Simulations and Practical Designs, McGraw-Hill, ISBN   0-07-150858-9
• Basso, Christophe (2012), Designing Control Loops for Linear and Switching Power Supplies: A Tutorial Guide, Artech House, ISBN   978-1608075577
• Brown, Marty (2001), Power Supply Cookbook (2nd ed.), Newnes, ISBN   0-7506-7329-X
• Erickson, Robert W.; Maksimović, Dragan (2001), Fundamentals of Power Electronics (Second ed.), ISBN   0-7923-7270-0
• Liu, Mingliang (2006), Demystifying Switched-Capacitor Circuits, Elsevier, ISBN   0-7506-7907-7
• Luo, Fang Lin; Ye, Hong (2004), Advanced DC/DC Converters, CRC Press, ISBN   0-8493-1956-0
• Luo, Fang Lin; Ye, Hong; Rashid, Muhammad H. (2005), Power Digital Power Electronics and Applications, Elsevier, ISBN   0-12-088757-6
• Maniktala, Sanjaya (2004), Switching Power Supply Design and Optimization, McGraw-Hill, ISBN   0-07-143483-6
• Maniktala, Sanjaya (2006), Switching Power Supplies A to Z, Newnes/Elsevier, ISBN   0-7506-7970-0
• Maniktala, Sanjaya (2007), Troubleshooting Switching Power Converters: A Hands-on Guide, Newnes/Elsevier, ISBN   0-7506-8421-6
• Mohan, Ned; Undeland, Tore M.; Robbins, William P. (2002), Power Electronics : Converters, Applications, and Design, Wiley, ISBN   0-471-22693-9
• Nelson, Carl (1986), LT1070 design Manual, AN19, Linear Technology Application Note giving an extensive introduction in Buck, Boost, CUK, Inverter applications. (download as PDF from http://www.linear.com/designtools/app_notes.php)
• Pressman, Abraham I.; Billings, Keith; Morey, Taylor (2009), Switching Power Supply Design (Third ed.), McGraw-Hill, ISBN   0-07-148272-5
• Rashid, Muhammad H. (2003), Power Electronics: Circuits, Devices, and Applications, Prentice Hall, ISBN   0-13-122815-3