Articles: Cases/PSU
 

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Thus, the cross-load characteristic tells us that the given PSU outputs quite stable voltages and can yield the declared wattage, but it would be preferable for modern systems where the CPU and the graphics card are supplied from the +12v power rail, since this PSU behaves better when there’s a high load on the +12v rather than on the +5v.

For the comparison’s sake, here’s the CLC diagram of a cheaper PSU, the L&C LC-B300ATX model with a declared wattage of 300W. The diagram is again built for the +12v power rail only:

This diagram obviously differs from the previous one. The bottom line of the CLC is not horizontal anymore, but is going up from left to right. The red color of the diagram there indicates that this is not because the +5v voltage is out of the normal range (it often happens when there’s a high load on the +12v), but because the +12v power rail bottoms out. Then, there’s no horizontal “ceiling” on top of the diagram; its top point corresponds to a load of about 150 watts on the +5v rail, which means you cannot get the promised 180 watts from this power supply in practice at any combination of the loads. Then, although the declared wattage on the +5v and +3.3v rails is higher with this PSU than with the MP-360AR (180 watts against 130 watts), you can see that the slanting line in the top left corner of the MP-360AR’s diagram starts at a load power of more than 80 watts on the +5v rail. With the LC-B300, however, this slanting line starts at abut 50 watts. This means that although the LC-B300 boasts a higher declared wattage on the +5v rail, you can more often get a much higher real wattage on this rail from the Macropower unit.

Attentive readers may have noted that if we put the two diagrams in one, in the same scale, the CLC of the Macropower unit will be more stretched along the +12v axis than the CLC of the unit from L&C. This comes as these two units comply with two different versions of the ATX/ATX12V Power Supply standard which described different preferable distributions of the loads among the PSU’s power rails. The following figure shows you the CLCs which PSUs of different production years should have had, according to Intel (who is the originator of all the family of ATX standards):

You can see that the ATX standard originally supposed a big consumption from the +5v and 3.3v rails, and the whole stuffing of the computer did feed on these two rails, while the +12v was only loaded by the mechanics of the optical and hard disk drives.

Things were changing, however. As central processors became power-hungrier, their supply from the +5v poised a whole lot of problems before mainboard developers: firstly, it was already clear then that the growth of the power consumption of CPUs would continue further and would raise the problem of transferring such high currents to the mainboard – the standard connectors might not hold. Secondly, the mainboard’s power connector would have to be either crammed in somewhere near the CPU’s voltage regulator module or placed farther, but it would be difficult to wire a high-current bus from this connector to the VRM.

Taking these things into consideration, Intel proposed the ATX12V standard, according to which the CPU should be supplied by the +12v rail. Evidently, at the same consumption, it means a current 2.4 times smaller. But as the main ATX connector had only one +12v wire, they had to introduce an additional 4-pin ATX12V connector, which solved two problems at once: 1) the connector’s contacts wouldn’t burn because of high load currents and 2) the mainboard’s PCB design was simplified as it was easier to find a place for a small 4-pin connector near the VRM than for a big 20-pin one.

 
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