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1. Output Power
We check the ability of the PSU to deliver DC power from 40W all the way up to full rated power. At each power level, the balance of loads on each voltage line is kept proportional to the maximum ratings of each of the lines at full power. Both the voltages and the current in every line are manually measured to ensure accuracy.
2. Low Load
We check the power consumption of the power supply on standby (unit power switch
on, plugged into AC). The results are of interest to anyone who cares about energy efficiency. We also check power demand with the unit turned on without any load, a condition that can cause problems for some high efficiency power supplies.
3. Efficiency
The AC/DC conversion efficiency is the ratio of DC output power to AC input power, expressed in percentage, with 100% being perfect. It is calculated for each power point; typically it's lowest at very low load, best around 50~80% of rated capacity, and a bit lower at maximum load. If a PSU requires an input of 400W in AC to deliver 300W in DC voltages, then it has an efficiency of 75%, at this point, and 25% of the power is lost as heat within the power supply.
4. Power Factor
Power factor is the ratio of real power to apparent power. It is a difficult concept to summarize because power factor touches on many complex concepts such as alternating current, phase, reactive and resistive loads, etc. There are many detailed explanations of PF on the web, some of which will be linked in the reference pages of this article. PF has nothing to do with the DC output, and everything to do with the way the power supply handles the AC input. The higher the PF, the "easier" it is for the utility to deliver "real" power" demanded by the power supply. The best PF is 1.0. Power factor can be corrected passively, for typical values of 0.7~0.8. Active PF correction usually leads to near-perfect PF, typically >0.95. Better PF does mean lower energy consumption from the point of view of the utility, but not high AC/DC efficiency, which is an entirely different matter. We measure PF at every power output level.
5. Voltage Regulation
It is the ability of the power supply to hold each voltage rail (or line) at the required 12V, 5V or 3.3V under a wide variety of conditions. Standard requirements are ±5%, but many quality PSUs do much better. VR is checked at every power test we perform, from 40W all the way to full power, during crossload tests, and during low VAC input tests.
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