Few things have influenced product politics as much as 80 Plus Gold. Seasonic was the first manufacturer to list power supplies on its homepage meeting the EPRI (Electrical Power Research Institute) guidelines, and they presented their first certified models at CeBIT 2009. With the recently launched X-series, Seasonic aims to prove the marketability of 80 Plus Gold PSUs. We have the 750W model for review today, so let's see how it measures up. Is it a great new PSU, or is 80 Plus Gold simply a new way to increase prices?

Beyond the remarkable efficiency and high-quality technology, the power supply contains a fully modular cable system and makes a good impression with its simple elegance. The X-Series also includes "hybrid silent fan control", which regulates the three speed settings of the fan. The power supply runs completely silent up to 20% load, at which point the fan begins to rotate slowly; at higher loads fan speed increases again to enable sufficient cooling. Seasonic uses a PWM fan to allow graceful ramping up of fan speeds, which we will cover later in this review. Beyond the various certifications and features, this ATX 2.3 power supply offers a 5-year manufacturer warranty and uses active PFC to allow its use on all the major power grid standards.

Enermax delivered good results in our previous test with the latest Pro87+/Modu87+ series, and it will likely be Seasonic's main competition. Of particular note is that the noise levels of the Pro87+ and Modu87+ are extremely quiet throughout the load range. Enermax also delivers products catering to the midrange 500W market, but we will have to wait until summer for Seasonic's answer. When the other models are ready, Seasonic let us know that they will have PSUs rated below 500W for those that don't run a high-power system. We should also see additional manufacturers soon with their own 80 Plus Gold offerings, so it remains to be seen who will have the best product. Apart from the marketing hype, we'll need to see new features in order to surpass the competition.

The X-Series has an MSRP of $200 for the 750W unit we're reviewing, but you can find it online starting at $180. So let's see if this expensive power supply is worth the money and if Seasonic can achieve new efficiency levels. We will also look at the construction, voltage output, and the unusual topology; high efficiency isn't the only point of interest with the X-Series.

Packaging

The contents of the package are what you'd expect. You get the required four screws and power cord, naturally, along with a Seasonic sticker, a user manual with product data and safety references, and a cable bag for storage of the modular cables. The power supply is well protected by packaging and comes in its own bag.

Seasonic prefers a large single-rail 12V design, rated at 62A (744W). As we've seen in other recent power supplies, the reason for the high rating of the 12V rail, which nearly corresponds to the total rated output, lies with the DC-to-DC conversion. All the smaller rails come off the +12V rail. The small rails are rated at 25A each with a combined output of 125W; that's comparatively weak compared to some older PSUs, but since modern PCs usually don't need much from the low voltage rails, this will hardly be a problem.

Appearance

Seasonic likes to keep their power supplies very simple and eschews any fancy aesthetics like LED fans or custom paint jobs. The X-series looks elegant and subdued, though there are some aesthetic improvements compared to the old M12D series like the honeycombed holes instead of the classical fan grill. Only a simple X-Series sticker and the modular sockets at the front interrupt the black color. The length of the casing is just 160mm, making the X750 much smaller than comparable products.

Fan

As mentioned, Seasonic favors a fan with PWM control. They chose a 120mm Sanyo Denki 9S1212P4M61 with ball bearings and seven fan blades. The blades are rounded off in contrast to many other brands. The power consumption of the fan amounts to only 0.13A and a plastic guard blocks part of the intake area to help direct airflow. We spoke with Seasonic at CeBIT about how important the fan is as it's the only mechanical component in a power supply. When we asked how much the fan costs, sales manager Walter Sun simply answered wide-eyed and laughing, "It's very expensive."

X750 - Connectors and Cable Lengths

Since the X750 is equipped with a fully modular connector panel; even the main plugs are detachable. That can be useful for those who don't want to deal with fixed cables during installation, and it potentially allows custom cables (i.e. shorter harnesses) in the future. A sticker above the sockets shows where to attach each type of cable harness. On the left side are two connectors—one 16-pin and one 10-pin—that combine to form the main 24-pin cable. There are two PCIe connectors with two 6/8-pin PCIe connections per harness. 4-pin ATX12V and 8-pin EPS12V are available, along with five sockets for SATA/Molex harnesses. All of the cables are sheathed with a soft mesh fabric, through which you can still see the cable colors.

The harnesses are reasonably long but you'll want to measure on larger cases just to be safe. The 24-pin, ATX12V, and EPS12V are 55cm, as are the four 6/8-pin PCIe harnesses. The peripheral cables are up to 80cm long, with an additional adapter that lets you connect two floppy plugs (15cm). Using all of the harnesses, you get eight SATA and Molex connections, so there's enough for a good size RAID setup with a few optical drives.

X750 - Resonant Topology

Steep square-wave impulses are the standard output of integrated circuit voltages, steered by a PWM-chip. While the DC-to-DC converter improves efficiency on the DC side, there are still some switching losses on the AC side. Depending on various influences like operating time during a cyclic duration, switching frequencies, voltage, and current the level of loss varies. In order to prevent this loss effect, you can use resonant switching discharge with a resonant circuit. In order to understand the procedure, we have to clarify the significance of the term resonance first. Resonance means an increase in amplitude of a signal induced by an external influence that has a frequency close to the natural frequency of a system. To negate resonance, the current is forced to run in the opposite direction and clamped to a low voltage level during transfer.

Doing this requires two switches; the first one produces one half of an oscillation while the second creates the other half in reverse, with the resulting output being very close to a sine oscillation. The moment voltage crosses its zero point, where the following current still affects within the minus range, transistors are switching. The procedure can be described with ZVS (Zero Voltage Switching) and also exists in a reverse variant with Zero Current Switching (ZCS). Since the two physical values in the formula of the switching losses at the transistor play a role, the power dissipation is reduced by their minimization. One can operate almost free of loss. Another advantage is that the smooth oscillations (softer transitions) also lower electromagnetic interference; this was a problem with classical topologies, commonly seen in the first 80 Plus Gold PSUs.

Seasonic uses a traditional PWM controller during low load (which results in higher ripple and noise) since the resonance converter isn't easy to run at a high efficiency level at such loads. The integration of a special companion chip is crucial for this assembly. However, Enermax also works with frequency modulation to get higher efficiencies on larger loads. Enermax calls their design a DHT, Dynamic Hybrid Transformer. Dynamic might be overstating things a bit, as they still use fixed frequencies in several stages; a constantly varying modulation would be fully dynamic but it may not be necessary or ideal.

Manufacturers can often reach 80 Plus Gold level with a DC-to-DC converter, and that's exactly what Seasonic and other manufacturers do. However, there is still the possibility of selecting faster switches with an H-bridge design to make improvements, which brings us back to the EMI problem caused by the hard switching. It is much simpler to realize this solution than to implement a completely new technology. As usual, there's a balancing act and manufacturers try to keep the specific details of their designs secret; how they achieve Gold certification can bring disadvantages in some cases.

Regardless, we should note that getting one of these "green", high efficiency PSUs isn't going to save the environment. There are lots of wastes and pollutants created during production, and if you're replacing an old PSU you're still contributing to the land fill. Additionally, it's difficult to save money in the long term as the purchase price is often very high. The true benefit of power supplies like the X-Series is found in the deeper details. For example, higher efficiency means lower PSU temperatures, which affects the ambient temperature and may result in a system that lasts longer with fewer problems.

X750 - Internal Design

Seasonic created a very unique layout with the X-Series. Even though the basic idea is similar to other designs (e.g. Revolution85+ series) in placing both VRMs on the PCB for cable management, it is an unusual structure together with SMDs (Surface Mounted Devices). The layout looks particularly open because all the rectifiers in the secondary circuit are on the back of the plate and only small dissipaters pass through the PCB.

We can see that the X-series wants to meet high-end requirements as opposed to the Pro/Modu87+ series by looking at the line filtering stage (from Yunpen) behind the AC-inlet, which is more expensive and improved compared to using individual filtering components. With a pair of Y-capacitors and a current-compensated choke it is suitable against common mode interferences, contains above all another X-capacitor. Also a MOV was integrated into the filtering, which continues on the motherboard together with some more devices.

Taking a look at the Power Factor pre-regulator, you can see a well stabilized choke and two Nippon Chemi-Con main capacitors. Both are specified at 400V, 360 microfarads, and 105 °C. The circuit with a resonance converter and champion controller (CM6901) follows, as already mentioned. The last component is physically located behind the fan control in the bottom left corner, as you can see on the following picture.

The X750 uses many solid-state capacitors, likewise from Nippon Chemi-Con, and small upright-coiled chokes shape the look of the secondary circuit. The PS223 safety device chip from Silicon Touch provides necessary safety features like OPP. There is still no OCP, but we have never encountered any problems in practice. Current flows over a small number of cables to the DC-to-DC converter, which is equipped with an Anpec controller. The smaller +3.3V and +5V are generated there connect to the connection panel and modular cables, along with +12V.

X750 - Voltage Regulation and Quality

Looking at the graphs, the +5V rail could be regulated much better for an expensive power supply. In fact, it drops very close to the tolerance limit at full load. However, ripple is much more important and a 5% deviation is still ok with a high load—something you will never reach with a normal system. All the other voltages perform fine and are within specification. The all-important +12V rail is still right at 12.00V even at 100% load.

When Seasonic switches to the resonant topology on higher loads, ripple and noise get much lower and overall they can keep up with Enermax. +12V shows a maximum 24.4mV ripple and the smaller rails are always below 20mV. Clean voltage helps all PC components last longer, so these results are very impressive.

Efficiency and PFC

Even at 75W (10% load) Seasonic delivers 83.43% efficiency. 115VAC reaches 89% maximum efficiency from 20% to 50% load (150W to 375W), while 230VAC delivers slightly better results as expected. Overall the X750 is as good as the Enermax Pro87+ and Modu87+ with only minor differences. If you want to save energy then the choice between Enermax and Seasonic won't generally affect the result. Instead, you'll want to look at pricing and other features.

At low input voltage, PFC starts at over 0.971 and reaches 0.993 under full load. With 230VAC the trend is lower (again as expected), starting at 0.800 at 10% load. This is still okay for the European power grid.

Cooling

Seasonic has a lot of confidence in their design and so the fan doesn't rotate at all until you hit 20% load. Thus, from 0 to 20% (150W) we have a result of 0 dB(A) on the graphs. From 20% to 50% load the X750 remains silent, but once load goes beyond 50% (375W) the fan RPMs increase rapidly. At high loads the result is more noise than the Enermax Pro/Modu97+, with a maximum noise level of over 30 dB(A). The small heatsinks and open design come into play here, as they have to compensate for the lack of surface area by pushing more air through the PSU. You can also hear the ball bearings inside the fan at maximum RPM.

X750: Potent for the Price

With the X-series, Seasonic delivers an outstanding performance. It's not only the high efficiency but also the good voltage quality and EMI friendly topology that make the X750 one of the most interesting power supplies on the market. With its double-sided circuit board, Seasonic sets the bar quite high and the semi-passive cooling will be great for users that frequently run at sub-50% loads. While the Enermax Pro87+ and Modu87+ are quieter at high loads, they're noisier at lower loads and still audible at sub-20%; with the fan shut off below 20%, the X750 is truly silent at low loads. Furthermore, the Sanyo Denki fan is a high quality design and should last a long time; the classical ball bearing is out of style and Enermax chose its Twister models wisely.

The selected line filtering stage from Yunpen is an excellent choice for interference suppression during relatively low loads; many manufacturers try to save money here in order to cut down resistance, which prevents optimizing the efficiency. Even though they are not necessary for a good thermal solution, it's good to see the continuing usage of Japanese capacitors. The 105 °C variants last a very long time and there are plenty of capacitors on the secondary circuit to provide good voltage filtering. While the idea of integrating a DC-to-DC converter with the cable management PCB may not have been their own, in practice it works well and their implementation is very good. Overall, the processing and soldering quality are very clean and the build quality is top notch.

In terms of connectors, the X750 is well equipped. Users get four 6/8-pin PCIe connectors and eight SATA/Molex connections, allowing for a wide spectrum of HDD/DVD/fan configurations. Only triple-SLI computers would need more connectors, and we wouldn't recommend running such a system off a 750W PSU—and we wouldn't generally recommend 3-way SLI at all. For modern GPUs like GF100/Fermi (GTX 470/480) and Cypress (HD 5850/5870), the X750 will work well, though SLI with Fermi will need a larger PSU. The cable sleeves are well done and help keep everything looking neat and organized, as does the fully modular cable system. What would help now would be some aftermarket cable packages for users that need/want longer or shorter cables, but even without those the modular cables will make this PSU simple to install in a case. The dimensions of the X750 are also small considering the rated output.

The DC-to-DC converter and high voltage output on the +12V rail makes the X750 a great fit for users running quad-core processors with SLI or CrossFire GPUs. However, if you have an older system you won't need to worry as 125W on +3.3V and +5V are still more than adequate. On 230VAC grids, the Seasonic X750 reaches up to 91% efficiency, but even on 115VAC the 89% efficiency result is nothing to sneeze at. [Gesundheit!] The relatively large voltage drop on the +5V rail is a minor point of criticism, but the 5V rail isn't particularly important for a modern system. The small amount of ripple is very welcome, and the resonant topology reduces ripple at high load.

Seasonic started out building PSUs for a lot of other companies, but these days they're focusing more and more on end users. With improved build quality and aesthetics, a 5-year warranty, and a very competitive price they have a lot on their side. The X-Series is everything you would expect from a modern 80 Plus Gold certified power supply. It will work great with anything from entry level PCs up though high-end SLI/CrossFire systems. The only thing missing from the X-Series are products targeting moderate users (i.e. sub-500W PSUs) as well as 1000W+ behemoths suitable for running GTX 480 SLI.

The Seasonic X750 is available starting at $170 (150€), so as usual quality doesn't come cheap, but the current price is $50 less than the competing Enermax Modu87+ 700W and still less than the non-modular Pro87+ 700W. With similar qualities and performance (we'd give Enermax a slight edge in overall performance), the price gap makes the X750 is an easy recommendation if you have to decide between the two. However, if you're planning on running at higher loads, Enermax can do so without generating as much noise.