All important computer hardware need specific amount of el. energy. This means that computer needs constant, stabile and reliable el. energy source with exact defined voltage and power values. With the development of PC computers, power supply unit also has made bigger or minor design changes. First models PC-XT used XT power supply units with 150-200W. Starting with AT286 computers used AT PSU with 200-250W, until first Pentium 2 computers. Today it is mostrly used ATX (for Pentium 2 and Pentium 3 computers) and ATX-P4 with 300-350W. Voltages that request computer hardware components are mostly standardized:



Comparing to standard ATX PSU ATX-P4 has extra 4-pin connector with +12/6+6A
In addition to standard PSU for PCs are used unstandard versions, mostly for server configurations. Those PSU allow so called tandemic work, meaning working two or more PSU at the same time so reliability of power supply could increase and leave redundancy which is of high importance for network servers. Those PSU are usualy allowed replacement with power on (hot plug). Mostly are montated on industrial boxes, rack.

SWPSU - Switching Power Supply Unit



Rin has multiply role:
1) reduces current that flows towards MoIL and SB (supervisor block) and by doing them, it protects them of high voltage.
2) Rin with Cin devides voltage which determins potential of R point (Ur)
3) Rin and Cin makes stabilization time constant τ = Rin * Cin by which is stabilized value of Ur voltage.

This is neccesary, because otherwise, devider of voltage with to resistance do point R to much unstabile which would affect adversly to MoILs work.

Capacitor Cin has two time role:
1. It makes voltage devider with Rin
2. With its capacity affection stabilizes Ur voltage

Diode D1 has also two time role:
1. Prevents flew of D.C. through N3 that would be warmed unneccesary, and also N3 without diode would short circuit Cin so Cin would loose its function.
2. D1 rectify current that inducts in N3. That voltage is proporcional to output voltage, so voltages referent points like Ur depends from Ua voltage, which reprezents state of output voltage(PSU weight).
This is very clever solution because MoIL reacts on on changes on network power, network condition, and also to changes to weight on PSU.

Zener diode protects swithcing transistor from high values of induced EMF (elektro-magnetic force), that appears when closing inductive suppliers (primar of transformer, and also collector weight on T1). Most of switching transistors for proffesional purposes have integrated diode into transistor box, so integrity of transistor can't be examined with classic ohm way.