Switched-mode power supply

A switched-mode power supply, switch-mode power supply, or SMPS, is an electronic power supply unit (PSU) that incorporates a switching regulator — an internal control circuit that switches power transistors (such as MOSFETs) rapidly on and off in order to stabilize the output voltage or current. Switching regulators are used as replacements for the linear regulators when higher efficiency, smaller size or lighter weight are required. They are, however, more complicated and their switching currents can cause noise problems if not carefully suppressed. As with any offline electronic systems employing peak-hold AC-DC conversion, simple SMPS designs may have a poor power factor. The power output to cost crossover point between SMPS and linear regulating alternatives has been falling since the early 1980s as SMPS technology was developed and integrated into dedicated silicon chips. In early 2006 even very low power linear regulators became more expensive than SMPS when the cost of copper and iron used in the transformers increased abruptly on world markets.

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

• AC in, DC out: rectifier, off-line converter
• DC in, DC out: voltage converter, or current converter, 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.

[edit] SMPS and linear power supply comparison

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

• Size and weight — Linear power supplies use a transformer operating at the mains frequency of 50/60 Hz. This low-frequency transformer is several times larger and heavier than a corresponding transformer in an SMPS, which runs at typical frequencies of 50 kHz to 1 MHz.
• Output voltage — Linear power supplies regulate the output by using a higher voltage in the initial stages and then expending some of it as heat to produce a lower, regulated voltage. This voltage drop is necessary and can not be eliminated by improving the design, even in theory. SMPSs can produce output voltages which are lower than the input voltage, higher than the input voltage and even negative to the input voltage, making them versatile and better suited for widely variable input voltages.
• Efficiency, heat, and power dissipation — A linear supply regulates the output voltage or current by expending excess power as heat, which is inefficient. A regulated SMPS will regulate the output using duty cycle control, which draws only the power required by the load. In all SMPS topologies, the transistors are always switched fully on or fully off. Thus, ideally, an SMPS is 100% efficient. The only heat generated is in the non-ideal aspects of the components. Switching losses in the transistors, on-resistance of the switching transistors, equivalent series resistance in the inductor and capacitors, and rectifier voltage drop will lower the SMPS efficiency. However, by optimizing SMPS design, the amount of power loss and heat can be minimized. A good design can have an efficiency of 95%.
• Complexity — A linear regulator ultimately consists of a power transistor, voltage regulating IC and a noise filtering capacitor. An SMPS typically contains a controller IC, one or several power transistors and diodes as well as power transformer, inductor and filter capacitors. Multiple voltages can be generated by one transformer core. For this an SMPS has to use duty cycle control. Both need a careful selection of their transformers. Due to the high operating frequencies in SMPS, the stray inductance and capacitance of the printed circuit board traces become important.
• Radio frequency interference — The current in a SMPS is switched on and off sharply, and contains high frequency spectral components. Long wires between the components may reduce the high frequency filter efficiency provided by the capacitors at the inlet and outlet. This high-frequency current can generate undesirable electromagnetic interference. EMI filters and RF shielding are needed to reduce the disruptive interference. Linear PSUs generally do not produce interference, and are used to supply power where radio interference must not occur.
• Electronic noise at the output terminals — Inexpensive linear PSUs with poor regulation may experience a small AC voltage "riding on" the DC output at twice mains frequency (100/120 Hz). These "ripples" are usually on the order of millivolts, and can be suppressed with larger filter capacitors or better voltage regulators. This small AC voltage can cause problems or interference in some circuits; for example, analog security cameras powered by switching power supplies may have unexpected brightness ripples or other banded distortions in the video they produce or cause mains hum to be audible in audio amplifiers. Quality linear PSUs will suppress ripples much better. SMPS usually do not exhibit ripple at the power-line frequency, but do have generally noisier outputs than linear PSUs. The noise is usually correlated with the SMPS switching frequency.
• Acoustic noise — Linear PSUs typically give off a faint, low frequency hum at mains frequency, but this is seldom audible (vibration of windings in the transformer is responsible). SMPSs, with their much higher operating frequencies, are not usually audible to humans (unless they have a fan, in the case of most computer SMPSs). A malfunctioning or unloaded SMPS may generate high-pitched sounds, since they do in fact generate acoustic noise at the oscillator frequency.
• Power factor — Linear PSUs have low power factors because current is drawn from the mains at the peaks of the voltage sinusoid. The current drawn by simple SMPS is uncorrelated to the the supply's input voltage waveform, so the early SMPS designs have a mediocre power factor as well and their use in personal computers and compact fluorescent lamps present a growing problem for power distribution. An SMPS with power factor correction (PFC) can reduce this problem greatly, and are required by some electric regulation authorities (European in particular).
• Electronic noise at the input terminals — In a similar fashion, very low cost SMPS may couple electrical switching noise back onto the mains power line and may cause interference with A/V equipment connected to the same phase. Linear PSUs rarely do this.
• Risk of electric shock — It is inherent in both linear and switching power supplies. In linear supplies, the chances of getting ventricular fibrillation are limited to having either the full mains voltage or having the secondary terminals in contact with the chest (if the transformer secondary produces a high enough voltage to overcome the body's electrical resistance and passes enough current to stop the heart). Due to regulations concerning EMI and RFI, all modern SMPS's contain inductors and capacitors connected before the bridge diodes for power factor correction and EMI/RFI filtering. Both Live and Neutral connections are connected to Earth via the two filter capacitors. The side effect of this is if the equipment if not earthed via the plug either intentionally (as in the case with many new A/V equipment) or unintentionally (if an earthed equipment develops an earth fault), these two capacitors form an impedance divider that energises the case and common rail of the equipment together with all unearthed equipment connected to it at half the mains voltage unless one of the equipment is earthed. This gives the operator an electric shock ranging from a tingling to a bite or even be fatal should the capacitor fail by shorting internally, putting the full mains voltage across the common rail of the equipment, likely destroying it in the process. Since a tiny amount of current flows through the filtering capacitors, nuisance tripping can be a problem on the most sensitive residual-current devices.
• Risk of equipment destruction — Some SMPS even have capacitors bridging the primary and secondary sides of the equipment. This causes the voltage at the DC connector to float at half the mains voltage, presenting another risk for electric shock (see above). Other than that, the voltage relative to earth is capable of destroying transistors input stages in amplifiers because the base-emitter voltage (assuming that the emitter is earthed) across the transistor forces it into the zener breakdown region, causing the gain to severely drop and noise levels to increase sharply [1].

[edit] How an SMPS works

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

[edit] Input rectifier stage

AC, half-wave and full wave rectified signals

If the SMPS has an AC input, then its first job is to convert the input to DC. This is called rectification. The rectifier circuit can be configured as a voltage doubler by the addition of a switch operated either manually or automatically. This is a feature of larger supplies to permit operation from nominally 120 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 employed by the following SMPS to force the average input current to follow the sinusoidal shape of the AC input voltage thus the designer should try correcting the power factor. A SMPS with a DC input does not require this stage. A SMPS designed for AC input can often be run from a DC supply, as the DC passes through the rectifier stage unchanged. (The user should check the manual before trying this, though most supplies are quite capable of such operation even though no clue is provided in the manual!)

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

[edit] Inverter stage

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

[edit] Voltage converter and output rectifier

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

If a DC output is required, the AC output from the transformer is rectified. For output voltages above ten volts or so, ordinary silicon diodes are commonly used. For lower voltages, Schottky diodes are commonly used as the rectifier elements; they have the advantages of faster recovery times than silicon diodes (allowing low-loss operation at higher frequencies) and a lower voltage drop when conducting. For even lower output voltages, MOSFET transistors 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 utilise 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, called the Duty Ratio. For example an ideal buck converter with a 10V input operating at a duty ratio of 50% will produce an average output voltage of 5V. A feedback control loop is employed to maintain (regulate) the output voltage by varying the duty ratio to compensate for variations in input voltage. The output voltage of a boost converter is always greater than the input voltage and the buck-boost output voltage is inverted but can be greater than, equal to, or less than the magnitude of its input voltage. There are many variations and extensions to this class of converters but these three form the basis of almost all isolated and non-isolated DC to DC converters. By adding a second inductor the Ćuk and SEPIC converters can be implemented or by adding additional active switches various bridge converters can be realised.

Other types of SMPS 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 charge pump.

[edit] Regulation

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

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

[edit] Power factor

Early switched mode power supplies incorporated a simple full wave rectifier connected to a large energy storing capacitor. Such SMPS draws 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 may cause stability problems in some applications such as in emergency generator systems or aircraft generators. 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 75W. 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.

From :Wikipedia.com

Posted by sisilea on Sunday, September 9, 2007 at 6:07 AM | Permalink