M12D-850W: Seasonic joins the Power Race

TESTING

For a fuller understanding of ATX power supplies, please read the reference article Power Supply Fundamentals. Those who seek source materials can find Intel's various PSU design guides at Form Factors.

For a complete rundown of testing equipment and procedures, please refer to SPCR's PSU Test Platform V4.1. The testing system is a close simulation of a moderate airflow mid-tower PC optimized for low noise.

Acoustic measurements are now performed in our anechoic chamber with ambient level of 11 dBA or lower, with a PC-based spectrum analyzer comprised of SpectraPLUS software with ACO Pacific microphone and M-Audio digital audio interfaces.

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 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.

The 120mm fan responsible for "case airflow" is deliberately run at a steady low level (6~7V) when the system is run at "low" loads. When the test loads become greater, the 120mm fan is turned up to a higher speed, but one that doesn't affect the noise level of the overall system. Anyone who is running a system that draws 400W or more would definitely want more than 20CFM of airflow through their case, and at this point, the noise level of the exhaust fan is typically not the greatest concern.

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 too many variables in PCs and 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 pretty good overall representation, but it is not quite the same as an extended meeting in person.

REAL SYSTEM POWER NEEDS: While we test the PSU to full output 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 40W and 300W, because it is the power range where most systems will be working most of the time. To illustrate this point, we conducted system tests to measure the power draw of several actual systems under idle and worst-case conditions. Our most power-hungry overclocked 130W TDP processor rig with an ATI Radeon X1950XTX-512 graphics card drew ~256W DC peak 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 the most power hungry dual video cards today might draw as much as another 150~200W, but the total should remain under 500W in extrapolations of our real world measurements.

INTERPRETING TEMPERATURE DATA

It important to keep in mind that PSU fan speed varies with temperature, not output load. A power supply generates more heat as output increases, but is not the only the only factor that affects fan speed. Ambient temperature and case airflow have almost as much effect. Our test rig represents a challenging thermal situation for a power supply: A large portion of the heat generated inside the case must be exhausted through the power supply, which causes a corresponding increase in fan speed.

When examining thermal data, the most important indicator of cooling efficiency is the difference between intake and exhaust. Because the heat generated in the PSU loader by the output of the PSU is always the same for a given power level, the intake temperature should be roughly the same between different tests. The only external variable is the ambient room temperature. The temperature of the exhaust air from the PSU is affected by several factors:

The thermal rise in the power supply is really the only indicator we have about all of the above. This is why the intake temperature is important: It represents the ambient temperature around the power supply itself. Subtracting the intake temperature from the exhaust temperature gives a reasonable gauge of the effectiveness of the power supply's cooling system. This is the only temperature number that is comparable between different reviews, as it is unaffected by the ambient temperature.

TEST RESULTS

The ambient temperature was 21~22°, and the ambient noise level was 11 dBA.

1. EFFICIENCY — This is a measure of AC-to-DC conversion efficiency. The ATX12V Power Supply Design Guide recommends 80% efficiency or better at all output power loads. 80% efficiency means that to deliver 80W DC output, a PSU draws 100W AC input, and 20W is lost as heat within the PSU. Higher efficiency is preferred for reduced energy consumption and cooler operation. It allows reduced cooling airflow, which translates to lower noise.

At 20W load, efficiency was quite good at 62%. 20W load it's a harsh efficiency test for a PSU rated at 850W . Efficiency rose quickly as the load was increased. 80% efficiency was reached around the 80W mark, and it kept climbing to a plateau about 87% starting just below 200W to 400W, where the peak of 87.9% was reached. Efficiency began sliding above this point, dropping below 85% at 650W and down gradually to 81.3% at full power.

These are excellent results, expected of a PSU that's certified Silver by 80 Plus. In our test, the sample did not make 85% efficiency at full load, but this is hardly a miss because of the thermal severity of our test. The 80 Plus testing is done at typical room temperature (18~28°C) while our test conditions feed the heat of the PSU output back into its operating ambient. There is an ideal working temperature for electronics to reach maximum efficiency, but exceed this range even by a bit and efficiency drops off rapidly; like so many other things, it is a bell curve with steep slopes on either side. For a PSU to reach 81.3% efficiency at 850W output with an intake air temperature of 47°C is excellent. Note that this measurement is in keeping with Seasonic's scrupulous disclosure about full power output at 40°C.

2. VOLTAGE REGULATION refers to how stable the output voltages are under various load conditions. The ATX12V Power Supply Design Guide calls for the +12, +5V and +3.3V lines to be maintain within ±5%.

At all load levels, the critical 12V lines were with 0.21V (1.75%) of 12V, and even at the highest loads, the voltages never dropped below 12V. This is excellent performance. The 3.3V regulation was equally good, while the 5V line was slightly farther off. But a worst case of a 0.14V drop is still very good; it's just 2.8% down.

3. AC RIPPLE refers to unwanted "noise" artifacts in the DC output of a switching power supply. It's usually very high in frequency (in the order of 100s of kHz). The peak-to-peak value is measured. The ATX12V Guide allows up to 120mV (peak-to-peak) of AC ripple on the +12V line and 50mV on the +5V and +3.3V lines. Ripple on all the lines was excellent at all power levels, generally staying under 10mV through much of the bottom half of the power range. Even at maximum power, the 12V ripple stayed in the 10~20mV range, with only an occasionally sally into the low 30mVs. It's close to the best ever measured on any PSU test sample.

4. POWER FACTOR is ideal when it measures 1.0. In the most practical sense, PF is a measure of how "difficult" it is for the electric utility to deliver the AC power into your power supply. High PF reduces the AC current draw, which reduces stress on the electric wiring in your home (and elsewhere up the line). It also means you can do with a smaller, cheaper UPS backup; they are priced according to their VA (volt-ampere) rating. Power factor was very good for this model, running no lower than 0.9 at any point during testing.

5. LOW LOAD TESTING revealed no problems starting at very low loads. Our sample had no issue starting up with no load, either, and the power draw was much lower than normal.

6. LOW & 240 VAC PERFORMANCE

The power supply was set to 500W load with 120VAC through the hefty variac in the lab. The variac was then dialed 10V lower every 5 minutes. This is to check the stability of the PSU under brownout conditions where the AC line voltage drops from the 110~120V norm. The M12D 850W is rated for operation 100VAC ~ 240VAC 50/60 Hz Most power supplies achieve higher efficiency with higher AC input voltage. SPCR's lab is equipped with a 240VAC line, which was used to check power supply efficiency for the benefit of those who live in 240VAC mains regions.

Efficiency improved around 2.7% with 245VAC input at this load. The sample passed the 100VAC minimum input without any issues. Neither voltage regulation nor ripple changed appreciably during the test.

7. TEMPERATURE & COOLING

Cooling was good, staying at no higher than 10°C until maximum power was approached. At 650W and above, the low airflow design of our test box was not enough to evacuate the massive heat buildup within. So even with the fan in the PSU running at full tilt, the temperature rise reached nearly 20°C. The hot conditions are the man reason why the unit did not reach 85% efficiency at full power. Nonlinearities in the temperature rise curve are directly related to changes in cooling as the PSU fan began ramping up in speed.

It's possible that cooling at high power loads could have been better with thinner fins on the heatsinks, as in the S12-600W of yesteryear. Admittedly, this is probably the kind of conjecture that gets engineers riled; there's no way for us to verify one way or another.

An Aside: Suitability of the Current PSU Test Box for Very High Power PSUs — Is it reasonable to expect a single low speed 120mm fan to cool a PC that's drawing 850W from the PSU? Not really. Loads higher than ~650W push the thermal limits of our test box. Gaming systems that might demand such high power levels (even if only during momentary spikes) are usually built in big cases that have multiple fans, both intake and exhaust, often much larger than 120mm diameter. This means much higher airflow and cooling capability than the current PSU test box, which was designed to replicate the thermal conditions of a quiet, low airflow, modestly powered PC. It may be necessary for us to put together a testing rig specifically for high power PSUs. Such high power PSUs don't belong in ultra-quiet low power PCs. If we continue to review high power PSUs, then we need to accept that such reviews are catering to gamers, and adjust our testing accordingly to obtain information and insights of value to this audience. More on this in the future.

Help support this site, buy from one of our affiliate retailers!

Search: All ProductsComputersPhotographyElectronicsSoftwareVideo GamesMoviesMusicBooksToysOfficeJewelry & WatchesApparelHealth & BeautyHome & GardenBabies & KidsFlowers & GourmetSporting GoodsMusical Instruments