Fourth, the output voltages all pass through different coils of the so-called group regulation choke L1. Suppose the consumption on the +5v line has increased. The PWM controller reacts by making the impulses wider and the +5v voltage returns to the norm, but the other voltages also grow up, although their loads have remained the same. Yes, the above-described regulation methods are employed for them, yet it is the +5v voltage that is paid the most attention to. But the group regulation choke does the following: when the current in one coil goes up, the voltage increase in the other coils due to this current is subtracted from the corresponding output voltages. Thus, in our case, the current increase in the +5v coil leads to negative voltages in the coils corresponding to +12v and +3.3v – and these voltages won’t grow up as much as they would do without a group regulation choke.
Thanks to the above-described methods, the power supply ensures an acceptable regulation of all output voltages in a wide range of loads. Still, this regulation is far from perfect and gives rise to the most common problem of computer PSUs – irregular distribution of the output voltages. If the load of the PSU is unevenly distributed across its power rails (for example, the system consumes a high current on +5v, and a low current on +12v, which is typical for many systems with top-end Athlon XP models), the regulator cannot hold the voltages within the necessary limits. The more loaded rails bottom out, while the less loaded rails have too high voltages. That’s also why it is impossible to control independently the output voltages of the PSU: their ratio is strictly determined by the parameters of the power transformer and the group regulation choke, while the pulse-width modulation can only increase or reduce all the voltages at once.
Recently, an interesting solution has been implemented in expensive PSUs (for example, in models from OCZ Technology or Antec): auxiliary regulators on saturated chokes are installed not only on the +3.3v rail, but also on the +12v and +5v rails. This ensures a very good (as computer power supplies go) regulation coefficient of all output voltages and also allows to adjust each voltage independently from the others by changing the parameters of its own auxiliary regulator. Once again, this design is a prerogative of expensive PSU models while middle-range products have the above-described dependence of their output voltages on the load on each of the power rails.
After the group regulation choke, there are electrolytic high-capacitance capacitors (denoted as C3-C6 in the figure above) and filtering chokes at the output of the PSU: they must smooth out the pulsation of the output voltage at the frequency of the PWM controller and the power transformer. Although there is a group regulation choke, discrete filter chokes are still necessary. Due to their low inductance, they are good at suppressing high-frequency noise, which comes through the group regulation choke that has a rather high inductance.
Thus, the two inherent problems of any computer PSU are the dependence of each output voltage on the load on each of the power rails, not only on its own rail, and the pulsation at the frequency of the PWM regulator (usually 60 kHz) at the output of the PSU.
Of course, the manufacturers of PSUs, especially of low-end products, come up with their own “cost-effective innovations”, which it would take too long to enumerate. First of all, the ratings of the components suffer: for example, instead of the diode assemblages at the power transformer’s output, they can put not only assemblages rated for a smaller current than written on the PSU’s label, but even discrete diodes (high-quality units use only assemblages which consist of two diodes in one casing). This often leads to the PSU’s failing after a few minutes under the full load – the more so as the manufacturers often save on the dimensions of the heatsinks these diodes are mounted on.