Electricity from the grid is transmitted in the form of poorly regulated AC (which varies worldwide from 100 to 240VAC), while electronic circuits normally require well stabilized low-voltage DC isolated from the mains. That's why virtually every piece of electronic equipment needs some form of power conversion. Power supply unit (PSU), technically speaking, is a device that transfers electric energy from a source to a load and in the process changes its characteristics to meet specific requirements. A typical application of power supplies is to convert a utility's AC into required regulated DC rail(s) and to provide safety isolation from the mains. Depending on the mode of operation of the semiconductors, the devices can be linear or switching (SMPS). SMPS stands for switch mode PSU. In such a supply, power handling electronic components are continuously switching on and off with high frequency in order to provide the transfer of electric energy via energy storage components (inductors and capacitors). By varying duty cycle, frequency or a relative phase of these transitions the average value of output voltage or current is controlled. The frequency range of a commercial SMPS units varies typically from 50 kHz to several MHz. Below is an circuit diagram of a typical off-line SMPS. This tutorial will introduce you to its operation. AC power supplied via an input connector, first passes through fuses and a line filter. Then it is rectified by a full-wave bridge rectifier. The rectified voltage is next applied to the power factor correction (PFC) pre-regulator followed by output DC-DC converter(s). Note that except for ATX computer power supply connectors and Compact PCI systems, output connectors in general are not standardized and are left up to the manufacturers.
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F1 and F2 shown on the left of the circuit diagram are input fuses. A fuse is a safety device designed to physically open the circuit when the current being drawn through it exceeds its rating for a certain period of time. The fusing time depends on the degree of overload. Due to this time delay, fuses will not always protect electronic components from a catastrophic failure caused by some abnormal conditions. Their main purpose is to protect the line from overloading and overheating, prevent tripping of an external circuit breaker, and prevent a fire that may be triggered by components that failed into short circuit. The low-pass EMI filter is designed to reduce high frequency currents getting into the AC line to an acceptable level. This is necessary to prevent interference on the other devices connected to the electrical wiring. There is a number of standards (such as EN55022 for Information Technology equipment) that govern the maximum level of EMI caused by PSU. The filter is followed by the bridge rectifier- a circuit that converts bipolar AC waveforms to unipolar pulsating ones. It has four diodes in a bridge arrangement to provide the same polarity of the output voltage for both polarities of the input. Electronic manufacturers often place these four diodes in one package with 4 leads, which sells as a single part.

PFC regulator

. The rectified input voltage is fed into the next stage, which increases power factor (PF). Note that PF by definition is the ratio between watts and volt-amps. In the process, the PFC pre-regulator usually also boosts the voltage to a regulated 370-400 VDC. There are also designs where "boost" DC-link follows the peak of input AC voltage instead of being fixed, or where a buck is used instead of a boost. Here is how this circuit works. A PFC controller monitors both the voltage across sense resistor and output voltage. While regulating DC output, it controls at the same time input AC current, so that it is in phase with mains AC and its waveform repeats the source waveform. Without this, the current would be delivered to the SMPS in short high peak pulses, which have a high harmonic content. The current harmonics do not supply any real energy to the load, but cause additional heating in the wiring and distribution equipment. They also reduce the maximum amount of electricity that can be taken from a standard wall outlet, since home circuit breakers are rated by current rather than by watts. There are various regulations that limit the input current harmonic content, such as EN61000-3-2 (for equipment connected to public low-voltage distribution systems) or DO-160 (for airborne equipment). To meet these harmonics requirements you can use PF correction techniques: a device with a high PF draws a nearly sinusoidal current from the source (at a sinusoidal input), which automatically results in low harmonic content. Currently there are no mandatory international standards that specifically regulate the PF of an electronic equipment, but there are various national and industry standards as well as voluntary incentive programs. For example, 80 PLUS® and Energy Star® programs require computers to demonstrate PF>0.9 at rated load. PF as well as current harmonics can be measured with commercially available analyzers or special instrumentation grade AC voltage sources. These standards also specify minimum efficiency of certain classes of electronic devices.
The efficiency of a PSU is the ratio between the values of output and input wattage: Efficiency=Pout/Pin. To measure Pin you need a true wattmeter: since any real apparatus has PF<1, you cannot just multiply input volts and amps because Pin=V*A*PF. A typical commercially available power analyzer can display both Pin and PF. To measure Pout for a DC output you will need a voltmeter and an ammeter.

There are two main types of power factor correction circuits- active and passive. Below is a block-diagram of an active PFC stage.


The downstream DC-DC converter runs off PFC output, generates a set of DC busses required for the load, and normally also provides input-to-output isolation. There are a number of topologies utilized in a DC-DC converter. In isolated offline SMPS the most popular are full bridge, half-bridge, forward and flyback. The above block diagram depicts a forward converter. Most low-voltage non-isolated DC-DC converters use buck regulators (single or interleaved multi-phase). There is a large variety of PWM ICs suitable for each of these topologies. The selection of the right power topology and controller depends on specific requirements for the product (including cost and time factors).

Finally, the housekeeping supply provides bias for all control circuitry and may also provide a separate stand-by voltage (SBV) which remains active even when the PS unit is shut down for any reason. In today's computer power supplies a 5VDC SBV is a standard feature.

Note that unlike electric generators, an electronic converter does not have any moving parts. For more information on power conversion units you may read SMPS design reference manual with application notes on the regulators basics. If you want to learn practical PSU design, you may start with Unitrode seminar manuals, where you can find a comprehensive collection of power supply tutorials, references, practical schematic diagrams, and guides.