M12D-850W: Seasonic joins the Power Race

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Dec 19, 2008 by Mike Chin

M12D 850W 850W computer power supply
Sample Supplier
Seasonic USA
Seasonic Electronics
Suggested Price

Seasonic, one of the most popular PSU brands for quiet computing, has been slow to join the power race that's been raging for the last couple of years. While competing brands, both old and new, began releasing kilowatt PSUs as far back as two years ago, Seasonic has soldiered on with lower rated models, for its own brand as well as for its OEM lines, such those for Corsair and Antec. Prior to the newly announced M12D series, the most powerful model in Seasonic's stable was the M12-700W, released in late 2006.

This doesn't mean the company has been lagging. Its conservative engineering department appears to prefer assured performance over rapid deployment, and a longer development / beta-testing phase is employed than many of its competitors. The new M12D series models are very high efficiency 80 Plus Silver rated, with assured 85% minimum efficiency at all power levels, and in October, Seasonic became one of the first to achieve the Gold 80 Plus award (87% minimum efficiency) — for its upcoming 550LT, 650KM and 750KM models. Seasonic-made PSUs have dominated the top ranks of SPCR's Recommended PSU list, and still do, with a couple of exceptional entries from Enermax and Zalman.

Most silent PC enthusiasts shrug and roll their eyes at 4-digit rated power supplies; no PC demanding that much power will be able to run quietly. Besides, SPCR's real-world power testing shows that it's difficult, even with two high power graphics cards, to put together a gaming PC that really demands that much power. (For example, in the recent Antec Fusion Remote Max case review, a system with an AMD Phenom 9600 quad-core CPU with two ATI Radeon 4780 graphics cards in Crossfire mode drew a maximum peak of 387W AC, which is just ~330W in DC output to the components.) Still, with overclocking now such a mainstream interest, gamers have increasingly discovered the fun and challenge of making their PCs both fast and quiet, so there is more interest in higher rated power supplies that are still quiet.

Seasonic still hasn't given in to the kilowatt temptation. The new modular-cable M12D series comprises just two models, an 850W and a 750W. The length of the M12D features list is a hint to the high technology that goes into advanced PSUs these days, Yet, a PSU still becomes invisible as soon as it is installed in a system. Most users, enthusiasts or not, don't usually point with pride to their PSU. The case, the CPU, the video cards.... but not usually the PSU. Makers and marketers of PSUs have been working hard to change this, of course, and Seasonic is no exception. This is especially important when the asking price reaches a lofty $299, the MSRP of the M12D 850W. Admittedly, it's the same MSRP as the Antec Signature 850W, a bit cheaper than the $330 of the PC Power & Cooling PPCT860, a touch more than the $279 Cooler Master RS900, and similar to the anticipated MSRP of the Enermax Revolution 85+ 850.

With the M12D-850W review sample, Seasonic included a small auxiliary PCB which contains the DC-to-DC converter — this is the actual circuit used in the PSU — and a USB key containing various reviewer-oriented information and data.


The colorful cardboard retail box is bigger than others from Seasonic in the past.

The contents include a fabric pouch for the detachable cables, paper manual and spec sheet, a hefty 16-gauge AC cord, mounting screws, and the PSU itself.

Seasonic M12D-850W FEATURE HIGHLIGHTS (from the web product page)
FEATURE & BRIEF Our comment
Supports Latest ATX12V and EPS12V standards The former is the desktop standard; the latter is more for workstations but also provides guidelines on power for dual-graphics cards.
DC to DC Converter Design
Superior dynamic response & greater system stability.
DC-to-DC conversion from the main 12VDC line eliminates the usual additional transformers. This approach is starting to be used on some other high end PSUs as well, such as the Antec Signature series.
San Ace Silent Fan [Sanyo Denki]
Premium quality fan for silent operation.
It's a brand well respected by engineers but not known to consumers, and have not made low noise fans before. This new quiet fan model is said to be exclusive to Seasonic.
High Reliable 105°C Japanese Capacitors
Exceptionally reliable components extend product life.
Many high end PSUs are flaunting this feature.
Solid Caps on 12Vs for Extreme Stress Operation Conditions
Great stability at higher operating temperatures, frequencies and currents.
These advanced caps are also used for critical circuits on motherboard these days.
Super High Efficiency [up to 90%]
Green solution for lowering energy consumption, noise & heat.
90% is probably only with >220VAC input. 80 Plus Silver status requires efficiency greater than 85% at 20% and 100% load, and 88% at 50% load.
Dual Sided PCB Layout
Better utilization of PCB space to enhance quality and performance.
Tight Voltage Regulation
Improved load regulation to reduce voltage variations.
±3% claimed for 3.3V, 5V and 12V lines instead of the ±5% recommended by the Intel ATX12V design guide.
Practical Dual +12V Rails
Ample 12v output with OCP for extreme utilization.
Safety: UL/CUL, TUV, CE, CB, CCC, FCC Class B, C-Tick Very good.
Protection from short circuits (SCP), over voltages (OVP), over power (OPP), over temperature (OTP), and over current (OCP) The more the merrier.
Universal Input, Active PFC Like just about every PSU on the market... but Seasonic was the pioneer with these features on computer PSUs.
MTBF: Over 150,000 hours at 25 Deg C. excluding the DC Fan.
Very good.
Seasonic 5 year warranty Very good!
Net Weight: 5.82 lbs
Gross Weight: 10.3 lbs
Size: W150 x L160 x H86 mm
Slightly longer than normal.


The label on the PSU usually tells us most of what we need to know, but not all.

The manual provides information about both the 850W and the 750W models, as well as an important detail about the output power rating:

Operating Temperature: 0 to 50°C (The rated power will reduce linearly from 100% to 80% from 40°C to 50°C)

The italicized text above states that full rated power cannot be delivered when the environmental temperature exceeds 40°C; it is derated to 80% at 50°C — this means 680W for the 850W model. This is exactly the kind of detail that the conservative engineers at Seasonic would want specified, probably against the wishes of the marketing department. Our standard testing procedures will show whether it is true, because at >600W load, the temperature inside our test box at the intake area of the PSU routinely approaches 50°C; it will impact the efficiency results we obtain at high loads.

Keep in mind that the testing for 80 Plus approval is done at normal ambient room temperature. The 80 Plus Testing Guidelines (PDF) specify only that "ambient temperature shall be maintained at 23°C ± 5°C throughout the test." In contrast, the ambient temperature of our test system is directly proportionate to the load. At low load, the air intake is at or just above room temperature, but as load increases, the temperature rises steadily. Typically, with a fan-cooled PSU, the intake temperature in our test system reaches 45~50°C at >600W loads. This is an extremely tough test condition, as PSU efficiency naturally drops off at high and low loads; combine high load with high temperature and it's essentially a torture chamber. It does replicate reasonably well the conditions that prevail in a typical tower PC. For full details, please refer to SPCR's PSU Test Platform V4.1.

Two 12V lines: Note that two 12V lines are indicated. This means there are two separate paths for the 12VDC from the transformer/rectifier to the outputs. Each of these paths has a limiter on it to keep the current from exceeding the stated maximum of 40A per line. A user who seeks the maximum current from this PSU would want to know which outputs correspond to which 12V lines; this would allow the 12V load to be distributed evenly. This information is not provided in any of the standard retail documentation.

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