Cooler Master Real Power 550

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TEST RESULTS

For a fuller understanding of ATX power supplies, please read our article Power Supply Fundamentals & Recommended Units. Those who seek source materials can find Intel's various PSU design guides, closely followed by PSU manufacturers, at Form Factors.

For a complete rundown of testing equipment and procedures, please refer to the article SPCR's Revised PSU Testing System. It is a close simulation of a moderate airflow mid-tower PC optimized for low noise.

In the test rig, the ambient temperature of the PSU varies proportionately with its output load, which is exactly the way it is in a real PC environment. But there is the added benefit of a precise high power load tester which allows incremental load testing all the way to full power for any non-industrial PC power supply. Both fan noise and voltage are measured at various standard loads. It is, in general, a very demanding test, as the operating ambient temperature of the PSU often reaches >40°C at full power. This is impossible to achieve with an open test bench setup.

Great effort has been made to devise as realistic an operating environment for the PSU as possible, but the thermal and noise results obtained here still cannot be considered absolute. There are far too many variables in PCs and far too many possible combinations of components for any single test environment to provide infallible results. And there is always the bugaboo of sample variance. These results are akin to a resume, a few detailed photographs, and some short sound bites of someone you've never met. You'll probably get a reasonable overall representation of that person, but it is not quite the same as an extended meeting in person.

REAL SYSTEM POWER NEEDS: One very important point is that the while our testing loads the PSU to full output (even >600W!) in order to verify the manufacturer's claims, real desktop PCs simply do not require anywhere near this level of power. The most pertinent range of DC output power is between about 65W and 250W, because it is the power range where most systems will be working most of the time. To illustrate this point, we recently conducted system tests to measure the maximum power draw that an actual system can draw under worst-case conditions. Our most powerful P4-3.2 Gaming rig drew ~180W DC from the power supply under full load — well within the capabilities of any modern power supply. Please follow the link provided above to see the details. It is true that very elaborate systems with SLI could draw as much as another 150W, but the total still remains well under 400W in extrapolations of our real world measurements.

SPCR's high fidelity sound recording system was used to create MP3 sound files of this PSU. As with the setup for recording fans, the position of the mic was 3" from the exhaust vent at a 45° angle, outside the airflow turbulence area. The photo below shows the setup (a different PSU is being recorded). All other noise sources in the room were turned off while making the sound recordings.

Ambient conditions during testing were 23°C and 20 dBA, with input of 120 VAC / 60 Hz measured at the AC outlet.

COOLER MASTER REAL POWER 550 RS-550-ACLY TEST RESULTS
DC Output (W)
65
90
150
200
300
400
550
AC Input (W)
89
120
195
248
368
485
670
Efficiency
73%
75%
77%
81%
82%
82%
82%
Intake Temp (°C)
25
25
28
29
31
33
36
PSU Exhaust (°C)
28
30
35
38
41
44
51
Fan Voltage
5.5
5.5
5.9
7.0
10.8
12.1
12.3
Noise (dBA/1m)
29
29
32
37
45
46
47
Power Factor
0.96
0.98
0.99
0.99
0.99
0.99
0.99

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.

ANALYSIS

1. VOLTAGE REGULATION was within ±3% throughout testing except for the full load test, where the +3.3V line was just over 3% low. For a desktop machine, this is not a problem; ATX12V allows up to 5% variance within its specification. However, EPS12V specifies that the +3.3V line can sag by a maximum of only 3%, which was missed by 0.01V at full load. This variance is so small that it is within the bounds of error for our test system, so it's impossible to say for certain whether the power supply was out of spec — only that it got very close. Keep in mind that this variance occurred at a load of 550W, which is near impossible to achieve.

  • +12V: 12.07 to 12.26
  • +5V: 4.82 to 5.03
  • +3.3V: 3.19 to 3.34

2. EFFICIENCY was low-to-average in the important <150W range, although it poked up above 80% across the higher wattages. This performance is typical of a high capacity unit. Those looking to buy for a low power system should look elsewhere, but a high-end gaming rig or workstation will be well served by this power supply.

3. HUMAN COMPUTER INTERFACE

While the 450W version of this power supply appeared to display the AC power draw, this model did not appear to be the same. The displayed amount of power was ~50W lower than the DC load throughout testing. Obviously, this is not a precise piece of equipment, but it appears to be quite poorly calibrated. At best, it is useful for checking the relative load on the power supply and for showing off to friends, but it is not suitable for collecting objective data.

4. POWER FACTOR

The active power factor correction in the Real Power 550 kept the power factor consistently close to the ideal 1.0 value. This is much better than most ATX12V power supplies, which often lack PFC of any kind, at least in North America where power factor correction is not required by law. EPS12V requires active power correction, so the Real Power is no better than any other EPS12V power supplies in this regard.

5. FAN, FAN CONTROLLER and NOISE: The test environment is live, so readings are higher than would be obtained in an anechoic chamber readings, due to reflections and reinforcement of sound waves off the walls, ceiling and floor.

The high speed dual ball bearing fan shows its colors as soon as the unit is powered up: Even at the minimum voltage of 5.5V it's a noisy beast. The noise character is rough and clattery with a pure tone that increases in pitch and volume when the fan speed increases. Bearing noise could be heard whenever the power supply was on.

The starting voltage of 5.5V is a little higher than most other power supplies, which also contributes to the loudness of the unit under low load. Even with the high airflow provided by the fan, however, it still began to increase the voltage around the 150W level. There is a reasonable amount of hysteresis built into the fan controller, so changes in fan speed are smooth and fairly slow.

Cooler Master has been very conservative with the cooling of this power supply. The high speed fan, and the higher than average fan voltages guarantee that internal cooling will not be a problem for this power supply. This is reflected in the high MTBF rating of 400,000 hours. Cooler Master is proud of this rating; they link the engineer's test report from the product page.

MP3 Sound Recordings of Cooler Master Real Power 550

Cooler Master Real Power 550 RS-550-ACLY @ <90W (29 dBA/1m)

Cooler Master Real Power 550 RS-550-ACLY @ 150W (32 dBA/1m)

There was no need to make recordings at higher power levels; it's simply too loud.

Sound Recordings of PSU Comparatives

Seasonic Tornado 400 @ 65W (19 dBA/1m)

Seasonic S12-430 @ 150W (19 dBA/1m)

Enermax Noisetaker 600W (2.0) @ 150W (27 dBA/1m)

Raidmax RX-520XPW @ 200W (30 dBA/1m)

Nexus 92mm case fan @ 5V (17 dBA/1m) Reference

HOW TO LISTEN & COMPARE

These recordings were made with a high resolution studio quality digital recording system. The microphone was 3" from the edge of the fan frame at a 45° angle, facing the intake side of the fan to avoid direct wind noise. The ambient noise during all recordings was 18 dBA or lower.

To set the volume to a realistic level (similar to the original), try playing the Nexus 92 fan reference recording and setting the volume so that it is barely audible. Then don't reset the volume and play the other sound files. Of course, tone controls or other effects should all be turned off or set to neutral. For full details on how to calibrate your sound system to get the most valid listening comparison, please see the yellow text box entitled Listen to the Fans on page four of the article SPCR's Test / Sound Lab: A Short Tour.

CONCLUSIONS

There is much to like about the Real Power 550. Cooler Master's decision to rate it according to its continuous output spec is commendable. The added support for EPS12V also speaks well of the design, as it has more stringent requirements than ATX12V spec. The three +12V rails should make for an exceptionally stable power supply.

Confusion could arise from the similarity between the Workstation and PCIe connectors. Some documentation, ideally a warning sticker on the connector itself would be a good idea to prevent newbies from just plugging in whatever fits.

The 400,000 hour MTBF is also an impressive (some would say unrealistically high) number. For those of you who are counting, 400,000 hours = ~45 years. This seems a stretch. Most other manufacturers rate their power supplies for 80,000~100,000 hours. Personally, I would have preferred Cooler Master to sacrifice some reliability for a slower, quieter fan. The stock fan makes the Real Power 550 a model to avoid if noise is any concern at all. While the 450W version was hardly whisper quiet, it was much, much better than the 550W model.

The Real Power 550 is a well-built product, but Cooler Master has not paid enough attention to aural ergonomics to meet SPCR standards. If performance is all you care about, then you should be well satisfied by this power supply, but if you are at all concerned with the noise in your system, there are plenty of quieter alternatives out there.

* * *

Much thanks to Lucy at Cooler Master for the opportunity to examine this power supply.

POSTCRIPT: Efficiency Correction
October 22, 2005

Recently, we discovered that our power supply testing equipment and methodology were providing erroneously high efficiency results. In general, the biggest errors occurred at higher output load points above 300W. At lower output levels, the efficiency error was often no more than one or two percentage points. No other tested parameters were significantly affected.

Through a fairly arduous process of discovery, analysis and old fashioned problem solving, we modified our testing equipment and methodology to improve the accuracy of the efficiency results and described it all in the article SPCR's PSU Test Platform V.3. As part of this revision, we re-tested most of the power supplies on our Recommended PSU List. In most cases, the same sample was used in the second test.

The corrected and original efficiency results for all the re-tested PSUs are shown in in the article, Corrected Efficiency Results for Recommended Power Supplies. The relative efficiency of the tested power supplies has not changed. If the tested PSUs are ranked by efficiency, the rankings remain the same whether we use the original results or the new results.

This data is also being added to relevant reviews as postscripts like this one.

CORRECTED EFFICIENCY: Cooler Master Real Power 550
Target Output
65W
90W
150W
200W
300W
400W
550W
Actual Output
67.6W
91.6W
152.0W
201.6W
300.3W
400.1W
551.7W‡
Efficiency
Corrected
76.9%
79.0%
81.3%
81.3%
80.1%
77.5%
74.7%
Original
73%
75%
77%
81%
82%
82%
82%

In this case, our original efficiency calculations were 3~4% too low through to about 200W output. Above that, the original results were too high, and the error kept increasing with rising output power till it reached over 7 percentage points off at maximum load. The new figures show much better efficiency at the more important <200W level, which suggests a quieter, slower fan or smarter fan controller would work well in the PSU for typical systems.

Discuss this article in the SPCR Forums.



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