Main Switchers And Transformers
The main switchers operate in two modes only: on (fully conductive) and off (fully non-conductive). They 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 transformer.
The transformer steps down the voltage feeding the secondary rectifiers that generate all DC outputs (+12V, 5V, 3.3V, 5VSB, minus 12V). The transformer also acts as an isolator between the primary side and the secondary side. In some larger-capacity PSUs, there are two main transformers, instead of one, installed in parallel, simply because a single transformer would have very large dimensions and thus wouldn't fit inside the PSU's casing.
When the switchers are in the "on" state, there is zero voltage running across them (theoretically), and when they are in the "off" state, there is zero current running through them. Therefore, V x I = 0. This means there is no power loss on the main switching FETs.
However, this represents an ideal situation. In real life, there are power losses, as no transistor/FET can switch instantly. There is always a small period when a transistor (switch) is between the "on" and "off" states and VxI does not equal zero. That's why all FETs in a PSU are cooled down by heat sinks and, in most cases, by a fan (because there are also passively-cooled PSUs).