As hardware fans, you are probably aware that PC power supplies are used to supply power to other hardware components, but a curious fact is that they do so with different voltage values: 12V, 5V, and 3.3V at least. . So a power supply works with different voltage values, known as rails, but why does it work like this? Can't the whole PC work with the same voltage?
You probably also know that different hardware components need different voltage values to function. Thus, for example, the USB ports work at 5V but the DDR4 RAM needs between 1.2 and 1.35V for correct operation. The objective of the power supply is to be able to supply the energy that each component needs, but if the source enters between 220 and 240 volts through the plug (between 100 and 125 in America), how do you do it then?
The 12V, 5V and 3.3V rails of the source, explained
To begin with, you should know that although 220V enters the power supply through the plug, that is alternating current, but a PC works with the direct current so one of the first components to come into operation is the AC / DC converter, which converts the alternating current in direct. This converter passes the current to 12V, the main voltage with which the source works.
In the diagram above you can see some of the main components that a power supply has inside; In addition to the AC / DC converter, you can see that we also have a + 5V converter and a + 3.3V converter, thus generating the three rails with which a power supply works: 12V, 5V, and 3.3V. In summary, when the energy enters the source, it converts it to direct current at + 12V, and from those 12V it then generates two additional rails, one of 5V and the other of 3.3V, each of which with its own circuitry independent.
Now comes a somewhat complicated concept, because if the source works with these three rails and the RAM, for example, it needs 1.35V to work, isn't the source providing too much? Indeed, it is, but for that, the motherboards have their own regulators and converters, so that they can modify the voltage supplied by the power supply to suit the needs of each hardware component that needs to be powered.
As we have said, each component needs a certain voltage to function, and the goal of the source is to provide you with the voltage closest to what you need so that the motherboard has to work as little as possible, no more, no less. Thus, if 1.35V is needed for the RAM, the 3.3V rail from the source will be used to provide it because it is the closest. However, if a fan runs at 12V then it will use the 12V rail of course.
Why isn't a single voltage used?
Explained this now comes the following question that curls the curl even more: why then does the source not supply the 12V voltage and that it is the motherboard that modifies it accordingly? The answer is as simple as it is simple: because it is much more efficient at converting voltage from a power supply than the motherboard.
Thus, one of the reasons why this is done is because when it comes to converting the voltage, the power supply is better prepared and modifies it at source in a much more efficient way than the motherboard would do, which It would need to integrate much more advanced circuitry than they have today. The work of the motherboard is, so to speak, much finer when it comes to regulating the voltage, while the power supply does it in a more efficient but grosser way.
The current design of power supplies, with rails that have independent circuitry for each one of them, has proven to be the most efficient since in the past, in the first AT and ATX supplies prior to the ATX12V specification, the conversion of voltages was done in a later stage, but they realized that doing it at the source itself increased efficiency considerably and therefore reduced the heat generated as well.
Imagine that the source provided only 12V to the motherboard, and this was in charge of regulating this voltage depending on the component that it must feed. This being the case, a lot of heat would be generated in addition to extra work on the motherboard that would involve integrating much more complicated and, above all, large circuits in it. Why else can the source do it directly and more efficiently?
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