Output Rectifiers And Filters
The role of output rectifiers and filters is, as their names imply, to rectify and filter the high-frequency switching waveform created by the switchers (FETs) and fed through the secondary side of the main transformer(s). In this stage, we see two types of rectification designs: passive and synchronous.
In the passive design, Schottky barrier rectifiers (SBRs) are used, and in the synchronous design, FETs take the place of SBRs. In synchronous rectification, efficiency is increased, since we get rid of the forward voltage drops of SBRs.
To make this easier to follow, let's look at an example. A typical SBR has a 0.5V voltage drop, so if we want to conduct 40A, then we have 40A x 0.5V = 20W. If we use a FET instead of an SBR, and assume that the FET's RDS(on) is 3mΩ, then we have 40A x 40A x 0.003Ω = 4.8W. This results in 15.2W less power dissipation and a 24 percent increase in efficiency.
In addition to the passive and synchronous designs, sometimes a hybrid one, called semi-synchronous design, may be employed where FETs and SBRs are used to reduce cost and increase efficiency above the passive design's levels.
The generation of -12V is usually done with a conventional diode, since PSUs don't demand much power from this rail (below 1A, in most cases). However, in some high-end PSUs, there is a dedicated regulation circuit, even for this insignificant rail. For 5VSB, a completely independent circuit with a transformer is usually used, since 5VSB rails are working continuously even when the PSU is in standby mode. For the generation or filtering of the main outputs (+12V, 5V and 3.3V), there are three regulation types: group regulation, independent regulation and DC-DC conversion.
Group regulation is usually used in low-capacity and budget PSUs. An easy way to identify a group regulation design is by checking the number of coils used in the secondary side. If you find only two, then group regulation is present. The bigger coil is used for 12V/5V, and the smaller one is used for 3.3V.
In this regulation type, +12V and 5V are generated together, and both of them feed their output voltage error to the regulator controller. This means that if the load is unbalanced between the rails, then the regulator controller will have a very hard time retaining a proper regulation. For example, if the load at +12V is high and the load at 5V is low, the voltage on the second rail will be raised, because the regulator controller tries to raise the +12V rail's voltage. But because the latter is tied to 5V, both of them are raised. This is why most group-regulated PSUs fail to keep their rails within +/-5 percent tolerance during cross-load tests.
In group regulation, the 3.3V rail is usually regulated by a magnetic-amplifier post regulator from 5V or from 12V. Most group-regulated PSUs fail to meet Intel's Haswell-ready test, where the PSU must keep all of its rails within the ATX spec when the full load is applied on the minor rails (5V and 3.3V) and load on the +12V rail is at a minimum (0.1A).
Independent regulation is used in higher-capacity and performance PSUs, where cost reduction isn't the top priority. In this type of regulation, all main DC outputs are generated by independent circuits, so unbalanced loads do not cause voltage problems. The +12V rail is regulated by the main regulator controller and 5V/3.3V mag-amp post regulators.
You can easily identify a PSU that uses independent regulation by the number of toroidal coils in its secondary side. If you find three of them (one for each rail), then the PSU uses independent regulation, so it won't have any problems when handling unbalanced loads among the rails.
DC-DC Converters/Voltage Regulation Modules
In many PSUs nowadays, the minor rails are generated independently with the use of buck (or step-down) converters (DC-DC converters or voltage regulation modules). In these PSUs, the minor rails (5V and 3.3V) are generated through the +12V rail, and this provides a significant efficiency boost and good performance, even with unbalanced loads. In some high-end platforms, the DC-DC converters are installed on a modular board in order to minimize energy loss due to power transfer.
Before we move on to the next stage, we should stress that the toroidal chokes located after the rectifiers take part not only in the rectification, but also in the filtering process, since they are used for ripple, voltage and current reduction on the DC outputs. However, in PSUs that utilize LLC resonant converters, usually there are no toroidal chokes in the secondary side (for the +12V generation), and if there are any, they are used only for filtering purposes.