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SMPS can also be classified into four types according to the input and output waveforms, as follows.
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 Input rectifier stage 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. 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. MOSFET s 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 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 Diode 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, MOSFET transistors may be used as Synchronous Rectifier s; compared to Schottky diodes, these have even lower "on"-state voltage drops. The rectified output is then smoothed by a filter consisting of Inductor s and Capacitor 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 Converter s'', '' Buck Converter s'', and the so called '' Buck-boost Converter s''. 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 . 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. 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 Transformer s, 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. TYPES Switched-mode power supplies can be classified according to the circuit topology.
APPLICATIONS Switched-mode PSUs in domestic products such as Personal Computer s often have universal inputs, meaning that they can accept power from most mains supplies throughout the world, with rated frequencies from 50 Hz to 60 Hz and voltages from 100 V to 240 V (although a manual voltage "range" switch may be required). In practice they will operate from a much wider frequency range and often from a DC supply as well. In 2006, Intel proposed the use of a single 12 V supply inside PCs, due to 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, to step down the voltage from the PSU or the battery to the CPU Core Voltage -- as low as 0.8V for low voltage CPU to typically 1.2-1.5V for 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 1000 Vrms {Link without Title} . Certain applications, such as in 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 24 volt DC, but larger aircraft often use 120V AC at 400Hz, though they often have a DC bus as well. 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. SEE ALSO EXTERNAL ARTICLES
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