Power Supply

Electricity from the grid is transmitted in the form of poorly regulated AC, while electronic circuits normally require a stabilized DC. A

power supply
unit (PSU) 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 voltage into regulated DC voltage(s) required for an electronics and to provide safety isolation from the mains. Depending on the mode of operation of power semiconductors, PSU 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 varies typically from 50 kHz to several MHz.

Below is a circuit diagram of a typical off-line SMPS. In such a circuit, AC voltage supplied via an input connector, first passes through fuses and a line filter and is rectified by a full-wave bridge rectifier. The rectified voltage is next applied to the PFC (power factor correction) pre-regulator followed by output DC-DC converter(s). Note that unlike input connectors which in most countries are standard, output power supply connectors and pinouts are standardized primarily only for PCs.

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 circuit components from a catastrophic failure caused by some abnormal conditions.Their main purpose is to protect the input line from overloading and overheating, prevent tripping of an external circuit breaker and prevent a fire inside the PSU that may be triggered by components that failed into short circuit.
The lowpass EMI filter is designed to reduce high frequency currents getting from PSU into the AC line to an acceptable level. This is necessary to prevent the PSU from causing interference on the other devices connected to the input 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- the circuit that converts bipolar AC voltage to unipolar pulsating voltage. It uses four diodes in a bridge arrangement to provide the same polarity of output voltage for both polarities of input voltage.

The rectified input voltage is next applied to the PFC pre-regulator which increases power factor (PF) and in the process usually boosts voltage to 370-400 VDC. There are also designs where boost voltage follows the peak of input AC voltage, or where a buck is used instead of boost. PF in general is the ratio between watts and volt-amps. A PFC pre-regulator controls input AC current so that it is in phase with mains AC voltage and its waveform repeats the input voltage waveform. Without this, the input 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 power 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 PSU with a high PF draws a nearly sinusoidal current from the input (at a sinusoidal input voltage), which results in low harmonic content. Currently there are no mandatory international standards that specifically regulate the power factor of 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 computer PSU to demonstrate PF>0.9 at rated load. PF as well as current harmonics can be measured with commercially available power analyzers or special instrumentation grade AC voltage sources. These standards also specify minumum efficiency of certain devices.
The efficiency of a PSU is the ratio between the values of output and input wattage: Efficiency=Pout/Pin. To measure Pin you would need a true wattmeter: since any real device has PF<1, you cannot just multiply input volts and amps. A typical commercially available power meter can display both Pin and PF. To measure Pout you will need a voltmeter and an ampmeter.

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

The downstream DC-DC converter runs off PFC output, generates a set of DC voltages 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. Most low-voltage non-isolated DC-DC converters use buck regulators (single or interleaved multi-phase). There is a large variety of regulator ICs suitable for each of these topologies.The selection of the right topology and controller depends on specific requirements for the PSU (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 PSU is shut down for any reason. In today's computer power supplies a 5VDC SBV is a standard feature.

For a comprehensive collection of power supply design tutorials and guides see Unitrode seminar manuals.