PSU reviews have become bread-and-butter articles at SPCR. There are very few serious PSU reviews on the web; in this context, ours have stood out. However, in the last 18 months, PC power supplies have undergone tremendous changes. As a result, weaknesses in our testing setup have become evident:
* The accuracy of our AC/DC conversion efficiency results; and
* The adequacy of the test system to provide high enough load on the 12V line.
This article documents the many changes we’ve made to address these issues in an improved, updated power suppy testing system. It’s our PSU Test Platform, Version 3.0. ;D
Sept 19, 2005 by Mike Chin
PSU reviews have become bread-and-butter articles at SPCR over the years. This is partly because of our close attention to detail, fundamentally sound test procedures, and decent test gear. It is also because there are very few serious PSU reviews done by hardware web sites. In this context, SPCR PSU reviews really stand out.
The old saying about not resting on your laurels holds true for PSU testing, however. In the 18 months since the last refinement to SPCR’s PSU testing system, PC power supplies have undergone tremendous changes. As a result, weaknesses in our testing setup have become evident. This article documents those weaknesses and the changes we’ve made to address them. Our goal is greater accuracy and precision for future PSU reviews.
There are two main issues, both related to the need to test increasing more powerful power supplies, increasingly with much of the output power concentrated in the 12V line:
- The accuracy of our AC/DC conversion efficiency results; and
- The adequacy of the test system to provide high enough load on the 12V line.
Before we dive into these topics, let’s take a brief look at SPCR’s existing PSU testing system.
PSU TEST SETUP, Q1 2004
As documented in the SPCR’s Revised PSU Testing System article, the test platform has been refined in steps over the years, sometimes with useful critical input from readers. To summarize the article, our PSU test system seeks to characterize:
- The quality and level of noise emitted at various standard output levels, all the way to maximum rated output.
- The AC-to-DC conversion efficiency at each output level.
- The voltage regulation at each output level.
- The rise in temperature as air flows through the PSU.
A primary feature of the test system is a simulated mid-tower case with modest airflow. The heat generated in the load tester by the output of the tested PSU is forced into the simulated mid-tower case. This means that the higher the output power level, the greater the amount of heat there is in the test box, which replicates the thermal conditions faced by a PSU in a real PC. It is a close simulation of actual-use conditions for a PSU in a typical quiet mid-tower case. A modest airflow fan performs the same role as a back panel exhaust in a mid-tower case. This fan is decouple-mounted in foam to minimize noise and is voltage limited to provide just 10~12CFM of measured airflow.
 Photo shows wooden case used for thermal simulation of quiet, low-airflow mid tower case. Note exhaust fan decouple-mounted in foam. A thermal sensor is placed at the exhaust of the PSU, and its fan lead tapped to monitor voltage. A Kill-a-Watt meter used to monitor AC power input to the PSU can be seen at the bottom of the photo. The DBS-2100 PSU load tester pressed up against the wooden box actually feeds its internal heatinto the box via four slow 80mm fans. |
 The four cooling fans of the DBS-2100 PSU load tester feeds the heat generated by the loading resistors into the thermal simulation case. NOTE: The wooden box as shown above was in an earlier incarnation before the back exhaust fan was installed and top of the PSU mounting area was cut away. |
Here’s a list of all the gear used currently for PSU testing:
- Digital readout thermometer. There are several in the lab that are used. (Like DigiDoc.) They measure within ~1°C of each other, which is good enough for our purposes. Powered by the PSU being tested. (<1W power draw.)
- Digital readout multimeters. A couple of ordinary DMMs are used to measure DC voltage.
- DBS-2100 PSU load tester. Made specifically for load-testing computer power supplies, it consists of a large bank of high power precision resistors along with an extensive selection of electronic switching on the front panel calibrated in Amps (current) and grouped into 6 voltage lines: +5, +12, -12V, +3.3, -5, +5SB. Leads from the PSU plug into the front panel.
Front panel of DBS-2100
PSU load tester.
- Kill-a-Watt or Seasonic Power Angel AC power meter is used to monitor the AC power input to the tested PSU. These AC meters also measure Power Factor for the PSU.
- B&K model 1613 sound level meter. This professional caliber SLM is >20 years old, weighs over 10 pounds, and is completely analog in design. It has a dynamic range that spans 140 dB. The unit can measure accurately down to about 16~18 dBA. A quiet environment is a prerequisite to low noise testing; the lab has been measured down to ~17 dBA at night, and a <16 dBA dBA adjacent room is also available for any PSUs that are quieter.
NEW CHALLENGES FOR PSU TESTING
The PSU testing rig as described above has been in use since Q1 2004, nearly a year and a half. In that time, we’ve reviewed about two dozen power supplies. In general, we’ve been reasonably satisfied about the accuracy of our testing. However, there have been a couple of growing issues.
1. CHANGES IN PSU OUTPUT POWER DISTRIBUTION
For some two decades, Intel has had the role of setting and driving technical standards for other companies to follow. It is part of the reason the PC industry grew as big and as quickly as it did. Without broad acceptance for literally hundreds of standards to define everything from the mechanical fit of components to guidelines for firmware, the industry would be very different than it is today. Intel’s Form Factors web site and the various specifications authored by special teams at Intel are a visible sign of this standards-setting role. The Power Supply Design Guides for various form factors are just a few of many such standards.
The ATX12V Power Supply Design Guide grew out of the previous ATX PS Design Guide. It is not a mandatory standard, per se, but everyone knows that conformance with the current version of the Guide ensures unquestioned acceptance from other hardware makers and low risk of incompatibility.
In the time before ATX12V, much of the electrical power for the CPU was drawn by the motherboard from the 5V line. The first generation of Intel P4 systems changed this so that all the power for the CPU came from the 12V line, via the then-new AUX12V (2x12V) connector, which was defined and specified in the first iteration of the ATX12V PS Design Guide. For a while only P4 motherboards made use of the 2x12V connection, but in the last two years, AMD followed suit with its Athlon 64 (and related Sempron) processors, so that all 754, 939 and 940 motherboards draw power for the CPU from AUX12V. Today, the other major power hungry component in the PC, the VGA card, also relies almost entirely on 12V lines, whether through the motherboard or through direct connection to the PSU. PCI Express also draws mostly on the 12V line. The 5V line, once heavily used to power the CPU, now is called to supply power mostly to hard drives. The 3.3V line seems to be used mostly for RAM, PCI and AGP.
Each subsequent version of the ATX12V PSU Design Guide — 1.1, 1.2, 1.3, 2.0, 2.1, 2.2 — has increased the recommended 12V current. Version 2.0 introduced the concept of two 12V lines for higher power PSUs, with each 12V line not to deliver more than 18A. The +12V2 line is marked specifically to power only the AUX12V connector for the CPU.
What does all this have to do with SPCR’s PSU testing system?
The DRAM DBS-2100 PSU load tester has the capability to provide a load of up to 23A on the 12V line. This was perfectly adequate with up to 400~450W PSUs conforming to v1.3 or earlier versions of Intel’s ATX12V Power Supply Design Guide, but with the introduction of version 2.0 in mid-2004, 23A become inadequate for >400W power levels.
The ATX12V Guide provides typical power distribution recommendations for PSUs of different power output capability. In version 1.3, the most powerful model for which power distribution recommendations were provided was a 300W PSU:
+12V: 18A, +5V: 26A, +3.3V: 27A, -12V: 0.8A, +5VSB: 2A
Let’s compare this to the current v2.2 recommendation for a 300W model and for a 450W model (the most powerful model profiled in the ATX12V Guide v2.2). Also noted are the maximum loads that the DBS-2100 PSU Load Tester can provide.
ATX12V Guide
Power Distribution Recommendations
ATX12V Guide
+12V1
+12V2
+5V
+3.3V
-12V
+5VSB
v1.3
(300W)
18A
26A
27A
0.8A
2A
v2.2
(300W)
8A
13A
12A
18A
0.3A
2.5A
v2.2
(450W)
14A
16A
15A
22A
0.3A
2.5A
DBS-2100 PSU Load Tester
Load Capability
23A
39A
31A
1.5A
3.1A
For the electrically challenged, power (Watts) is obtained by multiplying the voltage (V) and current (Amps)
for each line. The DBS-2100 is capable of loading a PSU to 276W on the 12V line (12Vx23A=276W)
.
Looking first at the ATX12V Guide data, you can see that the 12V line is more highly stressed now than ever before, while at the same time, the reliance on the +5V and +3.3V lines has dropped dramatically. Compared to v1.3, the current recomendation is for less than half the power on the +5V line and about 2/3 on the +3.3V line. Converting the current (A) numbers to power (W) illustrates it more dramatically:
- For a 300W model, Version 1.3 called for 216W on the 12V line, and a combined maximum of 195W on the +5V and +3.3V lines. (If all lines are drawn upon simultaneously, the total power would be limited to 300W.) A peak current capability of 19.5A on the 12V line (234W) for 17 seconds was also stipulated.
- For a 300W model, Version 2.2 calls for a total of 252W on the 12V lines, and a combined maximum of just 120W on the +5V and +3.3V lines. (If all lines are drawn upon simultaneously, the total power would be limited to 300W.) A peak current capability of 25.5A on the 12V line (306W) for 17 seconds was also stipulated.
- For a 450W model, Version 2.2 calls for 360W on the 12V lines, and a combined maximum of just 130W on the +5V and +3.3V lines. A peak current capability of 34A on the 12V lines (408W) for 17 seconds is also stipulated.
We recently published the article, Power Distribution in Six PCs, which confirms the high reliance on +12V in current PCs, up to 90% at full load in high power CPU systems, and the concomittant load reduction on the +5V and +3.3V lines. These were typically under 5A maximum.
The current capacities available on the various lines of the DBS-2100 PSU loader shows its age. It was built pre-ATX12V v1.3; you can see the enormous capacity available on the 5V and 3.3V lines, 195W and 102.5W, respectively. It is not capable of presenting more than a 23A load on the 12V line. This limitation could be worked around with v1.3 compliant PSU models, even those rated for much higher than 400W output power, because it was possible to max out the 12V line, then add more loading as needed on the +5V and +3.3V lines.
With the most recent v2.2 compliant PSU models, even testing at 400W is a bit of a challenge, and we’ve had to resort to maxing out the -12V and +5VSB lines to avoid overloading the +5V and +3.3V lines on the PSU. With some recent >500W PSU models, there was no choice but to overload all the non-12V lines on the PSU by a small amount in order to reach maximum load. This may have led to errors and unaccounted misbehavior.
TECH TIP: Even though there are two 12V lines in a >400W PSU that conforms to ATX12V v2.0 or higher, it is safe to wire these outputs in parallel. This is what’s is done in the DBS-2100: All the 12V lines are electrically joined together. The reason this can be done is because the “independent” 12V lines are independent only in the sense that each line has a limiter which keeps the current to <20A. All the 12V lines actually originate from the same 120VAC:12VDC transformer and rectifier.
2. ERRORS IN EFFICIENCY RESULTS
There has been a real and dramatic increase in PSU efficiency since SPCR first started measuring it in 2002. It was a pleasant surprise back then when a PSU managed to reach >70% efficiency at any load. The across-the-board >70% efficiency reached by the Seasonic Super Silencer 400 in mid-2003 was something of revelation, and no other PSU came even close to its maximum 78% efficiency until almost a year later.
Since the Enermax Noisetaker 475 broke the 80% mark in the spring of 2004, there have been at least 10 other PSU models that have reached 80% on the SPCR test bench. Almost all of these have been compliant with ATX12V v2.0 or higher, with much greater power on the 12V lines than on the others. This in itself was probably the source of a bit of the general rise in efficiency. The conversion from 120VAC to 12VDC is a more efficient process than the futher step down to 5V or 3.3V. One PSU engineer confided that a simple way to increase efficiency without making any substantial changes in a PSU is to rate the +12V line for slightly higher current and to derate the +5V and +3.3V lines for correspondingly lower current. The end result will be an efficiency rating that could be 2~3 percentage points higher.
The increases in efficiency also coincides with another aspect of the ATX12V Guide’s evolution. Until version 1.3, the Guide included only the most cursory note about efficiency; it was required to be at least 68%. In v1.3, the required minimum was raised to 70%. In version 2.0, the minimum efficiency was recommended to be 75% at full load, 80% at typical load and 68% at light load. This was far more specific and demanding about efficiency than Intel had been before.
In version 2.2, the required minimum remains at 70%, but the recommended guidance is even higher: 77% at full load, 80% at typical load and 75% at light load. Typical and light loads are now defined in the same way that 80 Plus defines it: 50% and 20% of full rated power. In fact, Intel has adopted the entire PSU load testing protocol used by the 80 Plus program. A PDF copy of this document, Internal Power Supply Test Protocol Rev. 4.0, can be downloaded from the web site www.efficientpowersupplies.org.
With Intel setting higher recommendations, 80 Plus urging >80% efficiency and SPCR extolling the potential cool and quiet virtues of a high efficiency PSU, perhaps it is no surprise that manufacturers have responded by making more efficient PSUs. Still, there were questions about the several PSUs we’ve tested recently that reached >85% efficiency. If 80% is such an easily reached efficiency target, why aren’t there more 80 Plus approved models?
Our AC/DC conversion efficiency results have almost always tended to be a bit higher than the specifications published by the manufacturers of the tested PSUs. It seemed only a small discrepancy at first, but as the rated maximum power and 12V current capacity of tested units increased, so did the discrepancy.
The Seasonic SS-400HT Active PFC F3, which we tested in August, was one that had passed the 80 Plus requirements for >80% efficiency. 80 Plus had tested a sample of this model, and their result showed a high of 85%, compared to 90% in our results. This was a real cause for concern, as several months of previous dialogue with the 80 Plus team had convinced me of their careful, professional approach to PSU testing. Why were we getting such different results?
To allay or confirm our growing suspicion that something was amiss, we conducted a complete examination of all the factors and procedures that go into obtaining efficiency in our test system. This included:
A. AC Power meters. All the various Kill-a-Watt and Seasonic Power Angel AC power meters were checked against each other and against readings off a couple of multimeters. As far as we could tell, the AC power meters appear to provide very good accuracy for power, typically within 1% of each other.
B. The Question of PSU Voltage Drop. We had noted but never accounted for the drops in voltage that occurs at >50% load with most power supplies. The worst case is usually at maximum power, where the 12V line might drop to 11.80V, for example, which is still well within the 5% tolerance specified by the ATX12V Guide. However, at high load, a slight drop on the 12V line can mean more than a couple of watts. Looking at the data collected for the Seasonic SS-400HT, however, we found the 12V to be dead on at full load. There were small drops on the +5V and +3.3V lines; these were examined and found to cause a total drop of ~5W at 400W output. This means that instead of 400W, the unit was actually delivering ~396W. It led to a 1% drop in efficiency, down to 89%. That was not enough to explain the discrepancy between the 80 Plus test results (83.2% at 393W load) and ours.
C. Accuracy of the DBS-2100 Load Tester. This was an issue we dreaded facing. Was the PSU Loader giving us loads that were lower than indicated by the markings on the front panels switches? If a switch marked 8A on the 12V line was acutally only forcing the PSU to draw 7A, for example, this would be 12W less power output than expected. A few miscalibrated switches like this would certainly cause our efficiency results to be inflated erroneously.
There was only one way to test this: Measure the DC current drawn on each output connector, systematically going through each of the relevant 27 switches across the DBS-2100 control panel. We used a reasonably high quality Fluke 36 clamp ammeter for this job, and also measured the voltage at the output terminals with a digital multimeter for each and every test. The current meter was checked for accuracy against another more expensive meter; accuracy seemed good, as the readings were generally within 0.1A of the other meter. Three different power supplies were used to check on whether interaction between the PSU and the load tester affected the output current.
 A Fluke 36 Clamp Meter was used to measure the current for every load setting, including innumerable combinations. It is shown above measuring a current of 4.3A flowing across the +12V line (+12V1) of a Molex output connector. |
This exhaustive testing took over two days. At the end of the testing, we had some sobering answers to several questions about the DBS-2100, which did turn out to be the main source of our efficiency error:
1. Does the PSU deliver the current as marked on the PSU loader switches? Not always. In many cases, the delivered current is lower than marked. It becomes worse at higher loads. It is not a linear across all the power levels, the error varies with total power load and voltage lines.
2. How bad is the error? At lower loads, there is no error, or it’s just 1~2% low. As the load is increased, it can get as bad as 6% low, at the highest loads with certain combinations of switches on certain voltage lines. Going back to our example of the Seasonic SS-400HT, it turns out that the output power was not 400W as we thought, but just 378W if we assume all the voltage lines were precisely at 12V, 5V and 3.3V, which they were not. Adjusting for the ~4W reduction due to the slight voltage drops on the +5V and +3.3V lines, the actual power delivered was 374W. Recalculating efficiency (374 ÷ 443 x 100%) we get 84.4%, which is very close to the 83.2% at 393W output reported by 80 Plus on their sample of the same PSU model.
3. Is the error consistent with different power supplies? Generally, yes, although there are variances. At our standard test load distribution settings for 400W, for example, we obtained 367W, 374W and 375W, respectively, from the three PSUs.
THE SOLUTIONS
Our findings call for a complete revision of our testing procedure for efficiency and loading. It does not change the validity of the other tested PSU parameters such as noise, fan controller behavior, voltage regulation, or thermal performance in any significant way. The details learned about the DBS-2100 suggests we should be monitoring the current individually for every setting and voltage line, and factoring in any sags in PSU output voltage into the efficiency calculation. Or perhaps it’s time for a better more accurate PSU testing device.
Pursuing the latter option first, I contacted some PC power supply engineers at Antec, Intel, Seasonic and Fortron. I found that the most widely used PSU testing rigs are those made by Chroma ATE. Inc. The reason for the popularity of Chroma PSU testing systems is that for their level of accuracy and testing automation capability, they are cost-effective. Unfortunately, cost effective for a PSU manufacturer is not the same as cost effective for SPCR. Even used, a suitable Chroma testing rig would set us back at least US$5,000. The unit would most likely require updating and changes if Intel changed its ATX12V Guide substantially.
A complication of this option is that there is no easy way to feed the heat of the load back into the PSU thermal simulation box or to rig up something similar in function. Furthermore, the Chroma and other advanced PSU testing equipment all have substantial fan cooling, which cannot not be disengaged. This means noise testing would suffer; noise testing of the PSUs requires low ambient noise levels, which can be obtained with our current test rig. The sheer cost and the various drawbacks associated with the Chroma option made it impossible for us to consider seriously at this time.
So… We went back to our existing test system and considered the changes that would make it viable for more accurate testing results in the context of today’s 12V-heavy power distribution. The objectives we identifed:
1. The load distribution on the voltage lines at the various power output levels should reflect the proportional ratings of each line of the tested PSU. This is the methodology used by the 80 Plus program, and also by Intel’s PSU design / test engineers. A PDF copy of the protocol, Internal Power Supply Test Protocol Rev. 4.0, can be downloaded from the web site www.efficientpowersupplies.org. It’s a matter of a bit more math for each PSU load test, rather than using fixed load settings for each line based on power output level as we have done up to now. This change will make our results more directly comparable to those obtained by 80 Plus and other organizations.
2. Increase the 12V load capacity. An additional 20A load capacity for the 12V line would be very useful.
3. Measure the actual current and voltage for each line individually for each power level rather than trust the current value marked for each switch on the DBS-2100 loader, and use these measured numbers to determine actual DC power output. Use this data to calculate efficiency. This means a longer, more tedious process of data collection when running PSU tests, but it will ensure that the inaccuracies of the DBS-2100 loader never enters the data stream.
GOAL #1 is achieved simply by following the referenced documentation closely. However, rather than test at 20%, 50% and 100% loads, we have decided to retain our previous power output levels of 40W, 65W, 90W, 150W, 200W, 250W and 300W. This is the critical range where most systems operate 95% of the time. For PSUs rated for higher power, we’ll go up beyond 300W by 100W or 150W increments to full power.
GOAL #2, increasing the 12V load capacity, required more attention and more work. There was much discussion about various tools and options. In the end, my choice was dictated by availability, cost and ease of implementation.
SWITCHABLE RESISTOR BANKS
I fabricated a bank of 12 20W resistors into a series / parallel network to provide five individually switchable loads of approximately 1.7A, 1.7A. 3.2A. 6.4A and 6.4A at 12V, and any combination thereof, up to a total of 19.4A. The network was wired to a 4-pin 2x12V and an 8-pin 4x12V connector so that either the AUX12V or EPS12V output could be used.
As the resistors would have to dissipate up to 230W, I organized the network into two banks. Each bank of six resistors are clamped between a pair of heavy aluminum plates that act as heatsinks. There is enough space between the resistors to allow airflow between them. The photos and captions below will explain better than words alone.
 One of the resistor banks, placed on bottom aluminum plate. The wiring was recycled from old, dead PSUs. |
 Thicker, heavier top plate atop the resistors. |
 The resistors are clamped between the aluminum plates, with screws raising the structure for airflow below. To ensure good thermal conduction, a small thermal interface pad was placed between the resistors and the plates. This material acts much like TIM goop for CPUs and heatsinks, filling gaps and evening the contact. |
 |
| The finished resistor banks are at the bottom of the PSU simulation box, in the flow of air from the fans in the DBS-2100 load tester. There is an inch of space behind each bank so the airflow can pass through between the resistors. All wiring connections are soldered. Note 5-switch “front panel” with 2x12V and 4x12V connectors. Unfortunately, an error was made with the latter ° it’s a “male” 4x12V plug that was laboriously soldered in place, but what’s required is a “female”, which I have not been able to locate since discovering this error. Ah well… at least it does no harm. |
How the DBS-2100 and PSU thermal simulation box go together.
The end result is that the PSU can now be loaded up to ~23A on +12V1 using the DBS-2100, and up to ~19A on +12V2 using this new additional resistor bank, for a total of about 42A, or over 500W. All this on just the 12V lines. The load banks are entirely independent, so the current and power to each can be reported separately.
GOAL #3: Having tackled goals #1 and #2, it was time to look at the goal of measuring the actual current and voltage for each line individually for each power level. Voltage measurement is no problem, we’ve been doing it from day one, manually with multimeters. Given the voltage drops that can occur through multiple contact points, our approach has always been to insert the probe pins into the actual output leads from the PSU for highest accuracy, and we will continue this. To measure current, we had planned to use the clamp meter, but several days of trials showed us just how inconvenient, slow, and most importantly, prone to manual errors this would be. Another solution had to be found.
SHUNT RESISTORS
In the end, the idea for our solution came from a web denizen known as jtr1962 in the forums of Storage Review. This SR forum member had built a hard drive power consumption measurement tool for Storage Review, which he said had been calibrated to be accurate to “within 1% under nearly all conditions, and in most cases better than 0.5%.” After reading through his comments, I decided jtr1962 might be able to help. Perhaps the same method he used for the power tool for SR could be applied here. Joe (which the “j” in jtr1962 stands for) turned out to be very helpful indeed. Here is the pertinent part of his email reply:
“Now as for measuring current more accurately, I might suggest just inserting a low-ohm resistor into each line and measuring the voltage drop across it. There are very low value resistors with 1% tolerance which would be suitable. A 0.01 ohm, 25 watt resistor is what I think would be most suitable here. You can read the voltage drop on the 200mV scale of your multimeter and then convert directly to amps just by dividing the voltage drop in millivolts by 10. The 25 watt maximum power dissipation (if heatsinked properly) means that you can deal with currents up to 50 amps, provided of course that the rest of your resistor load can handle it.”
Of course! I had no idea that such low value resistors even existed. An accuracy of 1% for a 0.01 ohm part is fantastic; this seems not much more than the resistance of a piece of wire! It’s mindboggling to think about how such an item could even be manufactured to such tolerance. Never mind, it’s a mystery I don’t need to solve. Such resistors do exist, and they were duly purchased online.
Serious looking devices, these resistors. The 4-pin Molex plug is for visual scale.
Four of these 0.01 ohm resistors were used for the four resistive load banks in the PSU loading system: One each for the two 12V lines, the 5V line and the 3.3V line. A resistor was hardwired in series for each bank of load resistors (for each voltage line). Because the resistance is so low, it has no effect on the load seen by the PSU being tested. This is the first benefit of the 0.01 ohm value.
By measuring the voltage drop across the resistor, the current can be easily calculated.
Ohm’s Law states:
I (current) = V (voltage) ÷ R (resistance)
So if we measure a voltage drop of 20 millivolts across the resistor, this means the current in the circuit is 0.02V ÷ 0.01 ohm or 2A. If we get a reading of 80 millivolts, this means the current is 0.08V ÷ 0.01 ohm or 8A. Note the multiplier factor to convert from Volts to Amps: It is a very convenient 100. This is the second benefit of using the 0.01 ohm value. (Thank you, Joe!)
Now, we measure the voltage aross the terminals of the PSU output connector. Multiply the measured output voltage by the current obtained above and we get the actual delivered power to within ~1% accuracy. It’s all pretty straightforward. (It is, in fact, the same approach we took with our own hard drive power consumption measurement system.)
|
NOTE: Of course, there is nothing new under the sun. A web search on the term “shunt resistor” brings up many examples of 0.01 ohm resistors used as shunts to measure current. In other words, it’s a common practice. Here are a couple of examples:
http://www.tpub.com/content/neets/14175/css/14175_33.htm |
DIGITAL CURRENT METERS
One deviation I made from Joe’s advice: Rather than manually measuring the voltage drop across each resistor with a multimeter, I chose to hardwire digital LCD panel meters instead. It would be much more convenient and time-saving in the long run. The extra cost was minimal, and accuracy would be the same or even better due to less variabiliy in contact resistance (which can occur at the probe points for a multimeter). Installing and wiring the measuring resistor and meter into the +12V2 loader was not difficult, but modifying the DBS-2100 PSU Loader was truly a pain in the you know what. The internal wiring is a nightmare, with extremely stiff large gauge wiring soldered everywhere, going everywhere. All in all, the work took several days to complete. Here are photos of the finished results.
The 12V2 current meter: The toggle switch turns the 9V battery for the LCD panel meter on/off.
The meter only draws 1 mA, and a typical 9V battery can do 500 mA, so with a bit of care, a single battery could last a couple years.
We took some time to test the new current meters with numerous power supplies at varied loads against multimeters and the old clamp meter, with excellent consistency between all the measurement tools. Amongst them, the clamp meter exhibited greatest variance, mostly due to manual errors. As the clamp meter had indicated during the discovery phase of our research, the actual load on the PSU was accurate or very slightly lower than indicated by the DBS-2100 switch markings at low loads, but the error became progressive worse as the load was increased. This is no longer a concern, as the markings on the PSU load switches are used just to bring us into the ballpark, then the current and voltage readings are taken manually to obtain the precise load on each line, and adjusted as needed.
Note that the -12V and +5VSB lines are not measured for current or voltage. We only put a few watts load on these lines together; even if it was 10% low, the error would rarely exceed a single watt, which is not significant.
We tried to duplicate the testing done by 80 Plus and www.efficientpowersupplies.org on a couple of PSUs that we also had samples of. We are relieved to report that our efficiency findings and theirs are virtually identical, within one percentage point. We are now confident that any PSU load we report is within 1% accuracy.
 |
|
The new rig, with 12V1 current meter reading 3.89A and 12V2 showing 11.15A. The total AC input is 248W, as shown by the Seasonic Power Angel on the right. The new meter on the DBS-2100 displays measured current on 12V, 5V or 3.3V line depending on the position of the rotary switch. |
NEW MEASURED DATA REPORTS
We’ve been able to present all the measured data in our PSU reviews in a single table thus far. This is no longer practical, given the additional data we wish to include. So changes have been made. The new test data for the Seasonic SS-400HT PSU which we’ve been referring to all through this article will be used to guide you through the new presentations. For many readers, it will be self-explanatory.
1) Output & Efficiency
This table lays out the AC input required to achieve a given target DC Output, and the Voltage and Current measured for each output line on the PSU.
DC Output Voltage (V) + Current (A)
Total DC Output
AC Input
Calculated Efficiency
12.10
0.99
12.10
1.67
5.00
1.01
3.31
0.99
0.1A
0.1A
42.1W
55W
76.6%
12.10
1.92
12.10
1.67
5.00
2.10
3.31
2.11
0.1A
0.3A
63.6W
78W
81.5%
12.10
1.92
12.10
3.22
5.00
2.85
3.31
2.84
0.2A
0.4A
90.2W
109W
82.8%
12.10
3.88
12.10
4.96
4.96
4.78
3.25
4.69
0.3A
0.8A
153.5W
180W
85.3%
12.10
5.81
12.10
4.96
5.00
7.21
3.21
7.40
0.3A
0.8A
197.2W
231W
85.3%
12.05
7.81
12.05
6.69
4.92
7.21
3.19
8.78
0.5A
1.4A
251.2W
298W
84.3%
12.05
8.68
12.05
8.00
4.92
9.80
3.19
10.50
0.6A
1.6A
297.9W
355W
83.9%
12.00
11.60
12.00
11.10
4.90
13.70
3.19
14.40
0.6A
1.7A
401.3W
486W
82.6%
NOTE: The current and voltage for -12V and +5VSB lines is not measured but based on switch settings of the DBS-2100 PS Loader. It is a tiny portion of the total, and potential errors arising from inaccuracies on these lines is <1W.
Voltage at specific Current loads are reported for each line. Note that the required tolerance for voltage is °5% on the 12V1, 12V2, 5V and 3.3V lines. The -12V and 5VSB lines are much less critical and only need °10%.
DC Output is obtained by multiplying the measured voltage and current to obtain the power for each line, then adding them all up. Our standard power output targets are 40W, 65W, 90W, 150W, 200W, 250W, 300W. Above that range, the targets will go up by increments of 100W or 150W, depending on the rating of the PSU. The actual DC output will usually not be right on target because of the effect of voltage variances and the limited steppings in our PSU loading system. This is unavoidable, but we will strive to get within 5~10W of the target output.
AC Input is taken directly from the Seasonic Power Angel or Kill-a-Watt AC power meter that goes between the wall AC outlet and the PSU AC plug. These AC power meters are probably made by the same manufacturer, judging by the features and overall look and feel. The accuracy is typically 0.5%.
Efficiency is DC Output as a proportion of AC Input: DC Output ° AC Input x 100%. The difference between input and output power is the heat generated within the PSU during the AC:DC conversion process. It’s useful to think about efficiency inversely. The difference between a 70% efficient PSU and one that’s 80% doesn’t seem big, but 30% of AC input being converted to heat compared to 20% is very substantial. For a system that draws 200W, a 70% efficient PSU will generate 85W of heat. An 80% efficent PSU will only generate 50W of heat. That 35W of heat is nothing to scoff at. Assuming efficiency is the only difference, the 80% PSU will run cooler with the same airflow. Alternately, less airflow will be required to keep it at the same temperature as the first, which means lower noise ° and this acoustic result is the primary reason for our interest in high efficiency. For fanless PSUs, high efficiency is mandatory.
2) Other Data Summary
This table summarizes all the other data measured at each power level. The procedure for collection of this data remains unchanged from before.
Fan Voltage SPL (dBA@1m) NOTE: The ambient room temperature during testing
varies a few degrees from review to review. Please take this into account
when comparing PSU test data.Intake Temp is the air temperature measured at a point 1″ below and 1″ behind the PSU in its mounted position in the thermal simulation box. If the ambient temperature is kept the same, this temperature should be affected primarily by the output power of the PSU. The heat is rising up from the PSU resistive banks that are being loaded. If 200W is the load, the same amount of heat is in the thermal simulation box regardless of PSU. The intake temp reading will be slightly affected by the airflow of the test PSU fan, and by the airflow created by the exhaust fan on the “back panel” of the thermal simulation box. If the airflow generated by the PSU is higher or lower than usual, this will show up as a correspondingly lower or higher intake temp.
Here’s is an interesting observation: Looking back at the PSU review data over the past 2 years, if the temp data is “normalized” to the typical ambient temp of 21°C, the intake temp is 30~31°C at 150W load for nearly all fan-cooled PSUs we considered to be quiet. Naturally these had lower airflow. The noisier PSUs have lower intake temps at this load, as low as 26°C. The noise level varies from a low of 20 dBA@1m to a high of over 32 dBA@1m.
Exhaust Temp is the air temperature measured directly at the hottest point of the back exhaust panel of the PSU. This temp is affected by all the factors that affect Intake Temp, plus the amount of heat produced within the PSU (the inverse of efficiency) and the efficacy of the PSU’s cooling system. The latter includes the size, shape and design of the heatsinsk as well as the fan and fan controller.
Temp Rise (°C) is the difference between Intake and Exhaust temps. It tells us in a single number the aggregate effects of efficiency and the PSU cooling system. Lower is better, especially in combination with low SPL.
Fan Voltage is the voltage measured across the PSU fan(s) teminals. It usually varies with temperature and load. It is a very good indicator of noise at a given power output load, and tells us how important forced airflow is for cooling in a given PSU design.
SPL is the sound pressure level measured in decibels (A-weighted) from a distance of 1 meter, at a slight angle behind the back panel of the PSU. This number is never complete by itself. It should be considered together with the text about the quality of the noise and the noise recordings made at specific levels.
Power Factor is taken directly from the Seasonic Power Angel or Kill-a-Watt AC power meter the test PSU is plugged into. Note that power factor has nothing to do with efficiency. For details on PF, please read page 5 of the article, Power Supply Fundamentals & Recommendations.
The ambient noise and temperature during testing are always noted separately in the accompanying text, along with AC voltage. We are lucky that the AC electricity supply here in Vancouver is extremely stable. I have never seen it go higher than 121V or lower than 118V.
UPDATING EXISTING PSU REVIEWS
Some of the questions that will be on readers minds… and answers to those questions.
1) What is the real efficiency of the PSUs you have already tested?
In
general, most of the efficiency figures for power levels up to about 250W are only very slightly too high. It’s the reported efficiency for 300W and higher that are more seriously affected.2) Can reported efficiency for future PSU reviews be fairly compared to that from existing reviews?
Not for power levels above ~250W, as noted above. We are tackling this issue by running new efficiency tests on all the PSUs we have on hand that are still in production and on the recommended PSU lists. The revised data will be reported in a separate article, and links to this new article will be posted in all the older PSU reviews.3) Are any of the other results affected, in particular the analysis and data regarding noise? How about their relative rankings for noise or efficiency?
No and No. The noise profile for each tested PSU remains unchanged. The rankings in the Recommended PSU lists are unchanged, not for efficiency nor for noise. The main issue is that the power load was slightly lower than what we reported it to be at the various test points, which is why the reported efficiency numbers were affected. As we’ve mentioned many times in the PSU reviews, noise levels are more directly tied to PSU intake temperature than to power output; the relationship between intake temperature and noise remains unaffected by our new findings.It’s pertinent to note that the biggest errors in efficiency measurements occurred at very high output power levels that will rarely be reached by the vast majority of desktop systems.
CONCLUSION
Testing equipment and procedures require routine maintenance and calibration, especially when errors are noted. They also require updating when the primary target specifications of the tested products change significantly. This article described major changes made to the equipment and procedures used in the SPCR power supply testing system. The work was done in the spirit of continuous self-improvement in order to provide accurate and realistic assessment of PC products for noise-conscious consumers. I expect it to be but one of many steps in our ongoing quest.
* * *
