Switched-mode Power Supply

A switched-mode power supply, or switching-mode power supply (SMPS), is an electronic power supply unit (PSU) that incorporates a switching regulator. Whereas a linear regulator maintains the desired output voltage by dissipating excess power in a "pass" power transistor, the SMPS rapidly switches a power transistor between saturation (full on) and cutoff (completely off) with a variable duty cycle whose average is the desired output voltage. The resulting rectangular waveform is low-pass filtered with an inductor and capacitor. The main advantage of this method is greater efficiency because the switching transistor dissipates little power in the saturated state and the off state compared to the semiconducting state (active region). Other advantages include smaller size and lighter weight (from the elimination of low-frequency transformers, which have a high weight) and lower heat generation from the higher efficiency. Disadvantages include

• greater complexity
• generation of high amplitude, high-frequency energy that the low-pass filter must block to avoid EMI
• ripple voltage at the switching frequency and the associated harmonic frequencies

Interior view of an ATX switched-mode power supply.
A - bridge rectifier
B - input filter capacitors
C - transformer
D - output filter coil
E - output filter capacitors

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

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

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

SMPS and Linear Power Supply Comparison

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

How an SMPS Works

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

Input rectifier stage

AC, half-wave and full-wave rectified signals

If the SMPS has an AC input, then the first stage is to convert the input to DC. This is called rectification. The rectifier circuit can be configured as a voltage doubler by the addition of a switch operated either manually or automatically. This is a feature of larger supplies to permit operation nominally from 120-volt or 240-volt supplies. The rectifier produces an unregulated DC voltage, which is then sent to a large filter capacitor. The current drawn from the mains supply by this rectifier circuit occurs in short pulses around the AC voltage peaks. These pulses have significant high-frequency energy, which reduces the power factor. Special control techniques can be used to force the average input current to follow the sinusoidal shape of the AC input voltage. Thus, the designer should try correcting the power factor. An SMPS with a DC input does not require this stage. An SMPS designed for AC input can often be run from a DC supply (for 230V AC this would be 330V DC), as the DC passes through the rectifier stage unchanged. Consult the manual, however, before trying this. Most supplies are quite capable of such operation even though nothing is mentioned in the documentation. However, this type of use may be harmful to the rectifier stage as it will only use half of the diodes in the rectifier for the full load. This may result in overheating of these components, and make them fail as short circuits.

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

Inverter stage

The inverter stage converts DC to AC by running it through a power oscillator, whether directly from the input or from the rectifier stage described above. The oscillator's output transformer is very small, with few windings at a frequency of tens or hundreds of kilohertz (kHz). The frequency is usually chosen to be above 20 kHz to make it inaudible to humans. The output voltage is optically coupled to the input and thus is very tightly controlled. The switching is implemented as a multistage MOSFET amplifier (to achieve high gain). MOSFETs are a type of transistor with a low on-resistance and a high current-handling capacity. This section refers to the block marked "Chopper" in the block diagram.

Voltage converter and output rectifier

If the output must 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 "on"-state voltage drops.

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

Simpler, non-isolated power supplies contain an inductor instead of a transformer. This type includes boost converters, buck converters, and the so-called buck-boost converters. These belong to the simplest class of single- input, single-output converters, which utilize one inductor and one active switch (MOSFET). The buck converter reduces the input voltage in direct proportion to the ratio of the active switch "on" time to the total switching period, which is called the duty cycle. For example, an ideal buck converter with a 10V input operating at a 50% duty cycle will produce an average output voltage of 5V. A feedback control loop is used to maintain (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. 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; however, 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. By adding additional active switches, on the other hand, various bridge converters can be realized.

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

Regulation

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

Open-loop regulators do not have a feedback circuit. Instead, they rely on feeding a constant voltage to the input of the transformer or inductor, and assume that the output will be correct. Regulated designs work against the parasitic capacity of the transformer or coil. Monopolar designs also work against 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 standby is added.

Transformer Design

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

There are several differences in the design of transformers for 50 Hz vs 500 kHz. Firstly, a low-frequency transformer usually transfers energy through its core (soft iron), whereas the (usually ferrite) core of a high-frequency transformer limits leakage. Because the waveforms in a SMPS are generally high speed (PWM square waves), the wiring must be capable of supporting high harmonics of the base frequency resulting from the skin effect, which is a major 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 the utility transformers and standard AC electric motors, and may cause stability problems in some applications such as in emergency generator systems or aircraft generators. Harmonics can be removed through the use of filter banks but the filtering is expensive, and the power utility may require a business with a very low power factor to purchase and install the filtering onsite.

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

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

Types

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

Applications

Switched-mode PSUs in domestic products such as personal computers often have universal inputs. This means that they can accept power from most mains supplies throughout the world with rated frequencies from 50 Hz to 60 Hz and voltages from 100V to 240V (although a manual voltage "range" switch may be required). In practice, they will operate from a much wider frequency range and often from a DC supply as well. In 2006, Intel proposed the use of a single 12V supply inside PCs, because of the high efficiency of switch mode supplies directly on the PCB.

Most modern desktop and laptop computers already have a DC-DC converter on the motherboard. The converter's purpose is to step down the voltage from the PSU or the battery to the CPU core voltage -- as low as 0.8 V for a low-voltage CPU to1.2-1.5 V ( typically) for a desktop CPU as of 2007. Most laptop computers also have a DC-AC inverter to step up the voltage from the battery to drive the backlight, typically around 1,000 Vrms.

In certain applications such as in the automobile industry and in some industrial settings, DC supply is chosen to avoid hum and interference and ease the integration of capacitors and batteries used to buffer the voltage. Most small aircraft use 28 V DC; however, larger aircraft often use 120 V AC at 400 Hz, though they often have a DC bus as well. Some submarines such as the Soviet Alfa class submarine used two synchronous generators, thus providing a variable three-phase current, 2 x 1500 kW, 400 V, 400 Hz.

In the case of TV sets, for example, one can test the excellent regulation of the power supply by using a variac. For example, in some models made by Philips, the power supply starts when the voltage reaches around 90 volts. From there, one can change the voltage with the variac and go as low as 40 volts and as high as 260, and the image will show absolutely no alterations.