But let’s get back to the design and operation of the PSU. After the power factor correction unit (if none is present, then directly from the diode bridge) the rectified voltage arrives to the smoothing capacitors C1 and C2 and then to the switch (it is typically made of two transistors) that controls the power transformer T1. The switch in a computer PSU typically works at a frequency of 60 kHz.
A computer PSU outputs up to six voltages (+12v, +5v, +3.3v, -5v, -12v, and +5v of the standby mode), so six voltage regulators are necessary as an ideal. In practice, however, it is not possible to pack even two independent high-power regulators (say, for the +5v and +3.3v power rails) into the cramped space of a power supply without raising its price to astronomic heights. That’s why all modern PSUs use a single switching regulator (frankly speaking, there’s one more regulator – the source of the +5v standby voltage is an independent low-power regulator; its low power (about 10 watts) allows for an easy implementation, though).
So, all output voltages, save for the +5v of the standby mode, are taken from the same transformer T1 (there are only two voltages shown on the flowchart for the sake of simplicity). Note that in all modern PSUs the switches are controlled through pulse-width modulation (the width of impulses changes, while their frequency is constant) rather than frequency modulation (when the switching frequency changes). The wider the impulse, the more power is pumped up into the transformer each cycle and the higher the output voltage is.
But if you just take the feedback signal from one of the output voltages, the PSU will regulate this signal only. Suppose it is the +5v rail. Then, as the load on this rail becomes higher, the voltage goes down. The pulse-width modulation (PWM) controller will increase the width of the impulses, lifting the voltage up back to the norm – and increasing the rest of the voltages as well. Several solutions are employed to avoid this effect.
First, the feedback signal is taken from the two most loaded output lines (+12v and +5v) through a dividing resistor. Thus, the regulation of each of these voltages independently worsens, but the PSU’s regulator now reacts to changes of the load on both voltages, i.e. the PSU works normally at different distributions of the load between these two power rails.
Second, the third heavy-current rail, +3.3v, has its own auxiliary regulator in the majority of PSUs – the so-called saturated-choke circuit. Regulators of that type feature a relatively high efficiency and a good regulation coefficient, although they are not of the switching variety. The +3.3v voltage comes from the same coil of the transformer as the +5v – the surplus 1.7 volts are damped on the choke. Well, there are PSUs where the manufacturer saves on the cost of the auxiliary regulator and just winds additional coils on the power transformer for the +3.3v voltage. Since the regulator doesn’t have a feedback circuit for this voltage, its stability in such PSUs is rather poor.
Third, the feeble-current rails, i.e. -12v and -5v, are sometimes equipped with ordinary linear regulators. The low efficiency factor of such regulators doesn’t practically affect the overall efficiency of the PSU due to the low load currents on these rails. Well, frankly speaking, the -5v voltage is only regulated this way – it is generated from the -12v with the help of a linear regulator for the sake of economy. And since modern PSUs don’t need this voltage at all, linear regulators have left computer power supplies altogether.