The first and foremost test for each power supply is the construction of the so-called cross-load characteristic. As I told you in the theoretical part of this article, each output voltage of the PSU depends on the load on the corresponding power rail, but also on the loads on the rest of the rails.
The ATX standard describes the maximum possible deviations of the output voltages from their defaults: 5 percent for all positive voltages (+12v, +5v and +3.3v) and 10 percent for all negative voltages (-5v and -12v; modern PSUs have only the latter of them, though). The cross-load characteristic (CLC) of a PSU is the set of possible combinations of the loads when none of the output voltages goes out of the permissible range.
The CLC is represented on a plane where the X axis shows the load on the +12v power rail, and the Y axis – the total load on the +5v and +3.3v rails. When constructing the CLC graph, the testbed is automatically changing the load on these rails stepping 5 watts, and if the output voltages of the PSU all fit into the required ranges, a dot is placed on the plane. The color of the dot – from green to red – reflects the deflection of the voltages from the standard. Our testbed controls three main output voltages, so three diagrams are produced for each PSU, in which the same area is colored with different colors that reflect the stability of each of the voltages. Below you see the cross-load characteristics of a Macropower MP-360AR Ver.2 PSU, colored for the +12v voltage (we will publish animated pictures in our reviews that will show all three voltages one by one; the currently displayed voltage will be indicated in the top right corner of the diagram, above the color scale).
Each dot of this diagram corresponds to one measurement step. For the sake of convenience the dots where the voltages are out of the acceptable limits are colored gray and have a smaller size – it helps the tester to follow the progress of the test in real time. After the measurements are made, the data are processed with bilinear interpolation. Thus, instead of discrete dots we get a colored area with sharp edges for better reading:
So what does this diagram say? The tested PSU excellently handles the load on the +12v rail – it is capable of outputting the necessary voltages when there’s the maximum load on this rail and only 5 watts on the +5v rail (5 watts is a typical starting value in our measurements; for high-power PSUs, which are unstable at so small loads, we start with 15 or 25 watts).
The straight vertical borderline in the bottom right part of the diagram says that the PSU has got to the power limit of the +12v rail (for the given PSU, it is 300 watts) and the testbed didn’t increase the load current further to avoid damaging the PSU. The vertical line changes into a slanting one in the top right corner of the diagram – this is where the testbed reached the maximum allowable power of the PSU (340 watts with this model) and had to reduce the load on the +12v to increase the load on the +5v, while still keeping the PSU out of danger.
Then, in the top part of the diagram, the slanting line becomes perfectly flat. This is where the testbed reached the maximum allowable load on the +5v rail and didn’t proceed further for safety reasons, although the voltages the PSU yielded were within the norm.
And lastly, in the top left corner of the diagram we see an irregular slanting line which is evidently not a power limit, since the load on the +12v is too low for this area. But this line is explained by the red color of the diagram: when the load was high on the +5v, but low on the +12v rail, the +12v voltage deviated more than 5 percent out of the norm, thus setting up the border of the CLC diagram.