A switching power supply consists of several stages.
A filter for the mains power sits right behind the input, filtering out surges, harmonics and various other undesirable phenomena found in the mains power. It also prevents any EMI noise produced by the PSU to affect nearby devices.
In the second stage, the AC power stream is rectified and screened by one or more bridge rectifiers. At this point, we’re dealing with about 325V (with 230V input), which are fed to the APFC converter. The APFC’s FETs (usually two) separate the intermediate DC voltage into constant pulse sequences. These pulses are smoothed out by the bulk capacitor(s) and are fed to the main switchers. The latter chop the DC signal coming from the smoothing capacitor into pulses, whose amplitude is the magnitude of the input voltage, while the duty cycle is controlled by a switching regulator controller. Thus, the DC signal is converted to an AC rectangular waveform that is fed to the main transformer. The higher the switching frequency of the primary switches the smaller the size of the main transformer and we have gains also in EMI noise, ripple suppression and transient response. On the other hand, lower switching speeds increase efficiency, though a larger transformer is needed and the EMI noise is increased, ripple suppression is affected and the transient response becomes slower.
Eventually, differing voltages of 3.3, 5 and 12V are required, which means that simple PC switching power supplies have either a single output rail with different taps for each voltage or separate rails for each voltage. Top-end PSUs even have separate coils for the voltages (if they don’t use an LLC resonant converter since PSUs with these don’t need coils; even if they do exist, they just play a role in the filtering process), which are then corrected and smoothed a second time following transformation. The most important thing is that these voltages have to stay consistent. Regardless of whether the PC is idling or under full load, voltages may not deviate from their spec by more than five percent according to the ATX spec.