Conventional power supplies have certain limitations in terms of their efficiency, weight and voltage consistency. Conventional power supplies obtain the various supply voltages required by the motherboard and other components of computers from the AC mains voltage of 50 Hz and 230 V. To do this, they transform the AC voltage, rectify it and stabilize it. Since the transformation operates at 50 Hz, the efficiency for the transformer, rectification and regulation is 30% to 50%.
Switched mode power supplies (SMPS), on the other hand, operate at frequencies that are several powers of ten higher at 50 kHz, 250 kHz and up to 1 MHz. This is immediately noticeable in the efficiency of the transformation, which is 80% to 90% and above for switched-mode power supplies. They also have the advantage that transformers operating at higher frequencies are much smaller and lighter. And the third advantage is better stabilization, which equally compensates for fluctuations in the mains voltage and the supply voltage. This aspect is of particular importance, since electronic components react to the slightest voltage fluctuations.
The function of switching power supplies
In principle, flyback converters or rectifiers with filter circuits are used for switched-mode power supplies, which are equipped with a control circuit. This consists of a switching unit with downstream transformation, a further rectifier with filter circuit and the feedback control mechanism. The control circuit optimizes the efficiency by controlling the pulse width of the clock signal and thus adapting the energy consumption to the energy consumption by means of power factor correction( PFC). At the same time, this ensures that only a small amount of heat is dissipated. The switching frequency is between 25 kHz and 500 kHz and higher, which makes the transformer, which also provides potential isolation in addition to the transformation, relatively small and light. The switching function itself is performed by transistors, MOSFETs or thyristors by means of pulse width modulation.
Switching power supplies operate at high switching frequencies and generate signals with an extremely large number of harmonics due to the steep edges of the switching operations. Since these are not sufficiently suppressed even by filter circuits, the regulated and stabilized supply voltage must also be cleaned of harmonics by inductors in order to keep the ripple as low as possible.
Inpersonal computers, switching power supplies generate the +5 V voltage required for the motherboard and plug-in cards, and +12 V for motors, drives and fans. Other positive or negative voltages are of course possible.