Articles: Cases/PSU

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Generally speaking, it is quite hard to meet Intel’s strict requirements to the cross-load characteristic. There are few units available that can boast a full compliance with the standard, but the egregious violation of the recommendation the LC-B300 is an example of happens rarely, too.

As for the coloring of the CLC diagram, it would be all green in an ideal world. In our reality, it is normal when each voltage, except the relatively stable +3.3v, goes through the entire range from green or yellow-green at one end of the diagram to red at the other end. Sometimes, there’s no green color in the CLC diagram at all – it means the voltage is originally higher than necessary. The worst case, however, is when a voltage goes twice through the entire color range, from red at one end, through green in the middle, and to red at the other end (see the diagram of the LC-B300 above). It means that the voltage sags low at one end of the CLC (clearly, when there’s a small load on the +5v and a high load on the +12v, the only thing the latter rail can do is to sag), but rises high at the other end; in other words, this voltage lacks stability.

To end the CLC-related section of this article, I want to offer you an example of an ideal power supply. I mentioned PSUs from Antec and OCZ with dedicated voltage regulators on each of the primary rails, so here’s the experimentally measured CLC of a PowerStream OCZ-470ADJ unit from OCZ Technology (this is the comprehensive picture with all the three voltages – the animated GIF is changing each 5 seconds):

You can see that the whole CLC area is limited by the maximum allowable load of this PSU. Moreover, none of the voltages comes close to a 5-percent deviation. Alas, such power supplies are rather expensive as yet…

Of course, we don’t end our tests with building the cross-load characteristic. We also check out the stability of each PSU under a constant load from zero to the maximum, stepping 75 watts. Thus, we can see if the PSU can sustain such a load at all.

Then, we measure the temperature of the diode assemblages of the PSU and the rotational speed of its fan under different loads (the fan speed depends on the temperature one way or another in nearly all modern PSUs).

The temperature measurements should be regarded with some skepticism, though. The design of the heatsinks and the placement of the diode assemblages vary between different PSU models, so the temperature measurements are not very accurate. On the other hand, the showings of the thermometer may be quite interesting in critical situations when the PSU is close to dying from overheat (this sometimes happens to cheapest models). I have seen PSUs whose heatsinks were above 100°C hot under full load.

The measurements of the fan speed produce more curious results. Although all manufacturers claim the fan speed to be temperature-dependent, the practical realization of the fan control system varies greatly. As a rule, the starting fan speed of low-end PSUs is about 2000-2200rpm and only grows up by 10-15 percent as the unit heats up. With high-quality models, the starting speed may be just 1000-1400rpm and may double under full load. So, the PSU will always be noisy in the first case, but in the second case owners of average system configurations may hope for noiselessness.

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