A switched-mode power supply or switch-mode power supply or SMPS, is an electronic power supply unit that incorporates a switching regulator that switches power transistors rapidly on and off (typically 40 to 80 thousand times per second) into the primary of a much smaller, more efficient transformer in order to stabilize the output voltage or current. By varying duty cycle, frequency or a relative phase of these transitions average value of output voltage or current is controlled.
SMPS converts the input AC voltage to the DC voltages needed by the personal computer. In older computers, AT SMPS were used. Currently ATX 12V SMPS are dominant.
SMPS supplies power to all components of a PC located in the CPU viz. motherboard, hard disk, floppy drive, CD ROM, fans etc. Most computer SMPS's have the appearance of a square metal box. They have a fan installed at one end and a large bundle of wires emerge out from the other end having:
Modern desktop SMPS's produce DC voltages of +/-5V, +3.3V and +/-12V outputs. The DC-DC converters on the motherboard step down SMPS voltage to the CPU core voltage and other low voltages needed for other motherboard components.