Although the PC power supply acts as the source of several voltages, the main of which are +12V, +5V, and +3.3V, there is a common regulator for the former two voltages in many PSU models. This regulator is oriented at the arithmetic mean between the two controlled voltages, this design being known as joint voltage regulation.
The pros and cons of it are obvious. On one hand, the cost of the PSU is reduced. On the other hand, the voltages depend on each other. For example, if the load on the +12V rail is increased, the appropriate voltage goes down and the regulator tries to pull it up again to the previous level. But controlling both voltages simultaneously, the regulator increases the +5V voltage as well. The regulator considers the situation normal when the average deflection of both voltages from the nominal value is zero, but it means that the +12V voltage is slightly below the nominal, and the +5V voltage is slightly above the nominal value. If the former voltage is increased higher, the latter will increase as well. If the second voltage is reduced, the first one will lower, too.
PSU developers are trying to find ways to solve the problem. Their attempts can be evaluated by means of cross-load diagrams.
Example of a cross-load diagram
The X-axis of the diagram shows the load on the PSU’s +12V rail (the combined load on all of its +12V lines if there are several of them in the given PSU). The Y-axis shows the combined load on the +5V and +3.3V rails. Thus, each point of the diagram corresponds to a certain load distribution among these power rails. For better readability we paint the diagram different colors denoting the deflection of the voltages from the nominal values, from green (a deflection below 1%) to red (a deflection of 4-5%). A deflection of higher than 5% is considered as unacceptable.
For example, the cross-load diagram above shows that the tested PSU keeps the +12V voltage rather stable, most of the diagram being green. It is only when the load distribution is misbalanced towards the +5V and +3.3V rails that the +12V voltage becomes red.
Moreover, the diagram is limited from the left, bottom and right with the minimum and maximum allowable load on the PSU, but the uneven top border is due to the voltages exceeding the 5% deflection. According to the industry standard, the PSU should not be used at such loads.
Typical workload area on the cross-load diagram
It is also important in what exactly area the voltages deflect the most. The hatched area in the diagram above denotes the power consumption typical of modern PCs: today, all high-power components (graphics cards, CPUs) are fed from the +12V line, which can be under a very high load. The +5V and +3.3V rails, on the contrary, are only responsible for hard disks and mainboard components now. Their load cannot be higher than a few dozen watts even in a top-end PC system.
Comparing these two diagrams built for two PSUs, you can see that the first PSU goes red in the area that is unimportant for modern PCs whereas the red zone of the second PSU is located differently. So, even though the two PSUs have similar results considering the whole range of loads, the first one is going to be preferable for practical applications.
We are tracking the three main power rails of the PSU during this test: +12V, +5V and +3.3V. The cross-load characteristics are presented in our articles as an animated three-frame image, each frame corresponding to one of the mentioned rails.
There have recently been more and more PSUs with dedicated regulation of the output voltages in which the classic design is complemented with additional saturated-core regulators. Such PSUs show much weaker interdependence between the output voltages. Their cross-load diagrams are mostly green.