SPCR Power Supply Test Rig, v.4 (and v.4.1)

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THE TESTS: ELECTRICAL PERFORMANCE

These tests are centered around the DBS-2100 load tester. (Check the reference equipment list, pages 5 and 6.) The equipmment needed to run the tests include the DBS-2100 PSU load tester, Extech 380803 Power Analyzer / Data logger, Digital / Analog thermometers, USB Instruments DS1M12 digital oscilloscope, Extech 560 Digital Multimeter, and the Ling Bridge TDFC 2J-3 0~140VAC Variac.

1. Power Output
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. 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.

3. 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.

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. 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.

6. Ripple (new for V4 PSU test system)
It is the amount of alternating current (AC) that appears in the DC output lines. A switch-mode converter, the type used in all PC power supplies, tends to generate a significant amount of ripple voltage. Many PSU specs express ripple as a percentage; typically it is <1%. On the 12V line, this means <120mV; on 5V, it means <50mV. Very low ripple usually indicates a higher quality power supply. Higher than normal ripple can cause instability with some components. Ripple is measured and expressed in mV on all the voltage lines at every load. Screen capture images of the waveform may also be shown.

7. Low AC Input Voltage (new for V4 PSU test system)
Blackouts
may be relatively rare in most areas of the developed world, but brownouts are much more common. Brownouts are periods of low voltage in utility lines that can cause lights to dim and equipment to fail. Also known as voltage sags, this is the most common AC power problem, accounting for up to 87% of all power disturbances. The severity, frequency and duration of voltage sags vary. PC power supplies are designed to continue working with some variance in AC voltage input, but just how well they perform under low VAC conditions is not well known. Certainly, intermittent, frequent voltage sags could affect PC stability; a PSU that does not handle low VAC well could be the source of mysterious computer instabilities that some of users face.

We now test PSUs with 110, 100, and 90 VAC input at 75% of rated power. The 75% load is about the highest power load that a PSU is likely to be see, as most builders factor in some overload headroom. It is a tough test, especially at <100VAC input, when the AC current will increase proportionately to maintain the same DC output power. A PSU that can handle 75% load well with low VAC will certainly do fine at lower loads. We monitor all AC and DC parameters as the VAC is lowered. A 0~140 VAC variac rated for 20A is used to control AC input voltage to the PSU in these tests.

8. Crossloading (new for V4 PSU test system)
Our power load testing follows standard industry protocol used by most organizations such as Intel, the 80 Plus program and the power supply manufacturers themselves. It is a proportionate load testing. In other words, the load placed on the various voltage lines at 90W or 200W total DC output is proportionate to the maximum ratings of each of the lines at full power. Crossloading describes a condition when the load is not proportionate. The most common occurrence of crossloading today occurs in a high power gaming rig where the 5V and 3.3V lines are at minimal (typically under 3A each, or 15W and 10W), but dual video cards and a power hungry CPU draw great amounts of power on the 12V line. The 12V load in an extreme gaming system could be as high as 350~400W at peaks, or 30~33A, while the power draw on the 5V and 3.3V lines amounts to well under 30W, less than 10% of the total DC output. We will examine the voltage regulation (stability) and AC ripple under crossload conditions.



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