power supply unit (PSU)
A power supply unit (PSU) is the unit within a device that supplies the electronic circuits with the appropriate voltages and currents. The supply voltages are generated from the mains voltage.
In principle, all PSUs work according to the same scheme: the AC voltage of the mains, in Germany 230 V, is transformed into lower AC vol tages via a transformer circuit. These are then rectified, smoothed, stabilized at the desired voltages via voltage regulators and supplied to the electronic components with the required currents.
Power supplies usually generate several DC voltages with different power and level, with positive or negative polarity. The voltage rails of PC power supplies have +3.3 V, +5 V, +12 V, -5 V and -12 V. The individual voltages are specified in terms of their percentage deviation from the nominal value, their minimum, nominal and maximum voltages, and their ripple. The voltage of +5 V is the usual supply voltage for motherboards, +12 V for fans and drives. As for the supply voltage of the centralprocessingunit, the various microprocessors and graphics processors, this is derived from the +5 V and +12 V supply voltages using Voltage Regulator Modules( VRM), which are DC-DC converters. To shield the electromagnetic radiation( EMI) caused by the thyristors and transformer leakage, power supplies are intrinsically shielded units with fans for cooling.
Efficiencies and power factors of power supplies
The effectiveness of power supplies is characterized by their efficiency, the ratio of power output to power input. The efficiency depends on whether the power supply works with analog or digital power control. The latter, working with digital power, have a higher efficiency compared to the classical power supplies working with linear control. In addition, there are power supplies and switching power supplies that adapt the power to changing power fluctuations, these power supplies work with power factor correction( PFC). Other evaluation criteria of power supplies are the rated power, rated voltage and rated current, voltage stability under load changes and no-load losses. Furthermore, the loudness of the fan plays an important role in some applications.
An important selection criterion for PC power supplies is the form factor. A distinction is made between two formats: the Full Brick format with dimensions of 117 x 61 x 13 mm and the Half Brick format with dimensions of 58.4 x 61 x 13 mm. Despite these standardized specifications, power supply manufacturers still have enough leeway for their own variants.
Mobile devices with an external power supply are equipped with an external AC adapter.