Gold and Bronze: Seasonic Power Supply Units Roundup

We are going to talk about four power supply units from Seasonic, three of which continue the legendary S12 series, while the fourth is a completely new product, the first flagship model in the X family.

by Oleg Artamonov
07/15/2010 | 12:34 PM

Seasonic is a well-known manufacturer of power supply units, its S12 series having been an etalon of a silent PSU. Today I am going to tell you about four products from Seasonic, three of which continue the legendary S12 series: S12-II Bronze (two models with different wattage ratings) and M12-II Bronze. The fourth PSU to be discussed is the first model of the flagship X series called X-750 Gold.

Testing Methodology


Click the following link for a description of our testing methodology and equipment and a brief explanation of what the specified and tested parameters of power supplies mean: X-bit Labs Presents: Power Supply Units Testing Methodology In-Depth. If you feel overwhelmed with the numbers and terms this review abounds in, refer to that article.

You can also go to our Cases/PSU section to check out reviews of all other PSU models we have tested in our labs.

We will mark the actual power consumption of three system configurations (discussed in our article PC Power Consumption: How Many Watts Do We Need?) in the cross-load diagrams. This will help you see if the tested PSU can meet the requirements of a real-life PC.

Testing Participants

Seasonic S12-II Bronze (SS-430GB and SS-520GB), 430 and 520 W

I will start out with the S12-II Bronze series which comes in two wattage ratings. Currently, these are low-end models in Seasonic’s product line-up.

These PSUs come in medium-sized boxes painted black and blue. The name of the series and the wattage rating of the specific product are indicated on the box.

Exterior Design

These two PSUs both have the same black case. There is nothing extraordinary in their visual appearance. The case is a standard 140 millimeters long and will suit into any computer case designed for ATX power supplies.

The S12 series does not have detachable cables.

Interior Design

Seasonic S12-II 430 W

Seasonic S12-II 520 W

The 430 and 520-watt models differ internally not only in the color and shape of the heatsinks but also in the component layout but turn to have the same circuit design.

As opposed to many modern PSUs, the S12-II series has joint voltage regulation as is indicated by the number of toroidal chokes at the output. PSUs with dedicated voltage regulation have three such chokes (one choke for each of the key voltages: +5, +3.3 and +12 V) but we see only two here. One choke is for the +3.3V regulator and another is shared by the +5V and +12V regulators.

Interestingly, the Seasonic S12 series used to have dedicated voltage regulation but the newer version is simplified in this respect. You will see shortly how this has affected the real characteristics of the product.

KZE series electrolytic capacitors from United Chemi-Con are installed at the PSU’s output. There are also solid-state capacitors there, one for each output line. These capacitors enjoy a good reputation, so you shouldn’t worry about their service life.


Seasonic S12-II 430 W

Seasonic S12-II 520 W

The specified output power of these PSUs agrees with their model names as well as with their effective output power, i.e. if you take into account that modern computers mostly consume from the +12V rail while the combined load on the other rails is but seldom higher than 50 watts. The +12V rail is split into two “virtual” output lines in the S12-II.

Cables and Connectors

The 430W model is equipped with the following cables and connectors:

The 520-watt model has a similar selection of connectors (it has a 4+4-pin CPU connector instead of a 4-pin one, but it will only be necessary for server mainboards which need two CPU power cables to be plugged in):

The two models differ in terms of wire sections. The lower-wattage model has rather thin PATA and SATA cables (20AWG) while the cables of the higher-wattage model are all the same thickness (18 AWG).

Included with both PSUs are adapters from PATA power connectors to floppy-drive connectors (about 10 cm long).

These connectors are sufficient for building a modern gaming machine with one top-end graphics card but I can note again that the second CPU cable is only necessary for owners of server mainboards. I also think that the first CPU cable shouldn’t be shorter than 60 centimeters. Otherwise it may turn to be too short in popular system cases with a bottom PSU compartment and a space behind the mainboard reserved for hiding the cables. You can use extension cables then, but they are not very reliable, especially for computer configurations with top-end CPUs.

Compatibility with UPSes

Oddly enough, these very similar PSUs behave differently in this test: the S12-II 420 worked with my APC SmartUPS SC 620 at loads up to 350 watts but I could not make the S12-II 520 stable as it would shut down even at a load of 300 watts when the UPS switched to its batteries.

I didn’t examine the circuit design of these PSUs closely, so I cannot tell you the reason for this discrepancy. It may be due to a difference in some component ratings or in the implementation of the active PFC devices in these two models.

Output Voltage Stability

Both PSUs are very good for models with joint voltage regulation. Their voltages deflect no more than 3% in the typical load range of modern computers. It is only when the load is greatly misbalanced towards the +5V and +3.3V rails that the output voltages deflect more than the acceptable 5%. On the other hand, the S12-II series are of course inferior to PSUs with dedicated voltage regulation.

Output Voltage Ripple

The output voltage ripple is rather low at full load. The PSUs meet the industry requirements in this respect.


Each model is cooled by a 120x120x25mm fan from ADDA that has a max speed of 2050 RPM.

Both PSUs are very quiet at loads up to 200 watts, especially the 520W model. Alas, the fans accelerate quickly at higher loads, making both PSUs noisy at 300 watts. The fans produce a powerful din at the maximum speed.

Efficiency and Power Factor

Being over 85% efficient through most of the load range, these PSUs deservedly sport the 80+Plus Bronze certification.

Standby Source

The standby source copes with the full load of 2.5 amperes easily in each PSU.


The Seasonic S12-II Bronze series includes mainstream products. Moreover, the S12-II has lost the dedicated voltage regulation which used to be implemented in the Seasonic S12 we tested about four years ago. It has also become louder at medium and high loads. So, if you’ve got an S12, you shouldn’t try to upgrade it with the newer version because you won’t get any benefits other than higher efficiency. The downside of the newer PSUs is that the top-wattage model was unstable with the UPS.

Anyway, I would recommend this series for mainstream computers, including gaming configurations with one single-processor graphics card, as high-quality and quiet power supplies.

Seasonic M12-II Bronze (SS-620GM), 620 watts

This model comes from the M12-II Bronze series but, judging by its description and specs, its modular cables should be the only difference from the S12-II series.

The PSU comes in a black-and yellow box.

Exterior Design

This PSU has a rather compact case but it is longer than the S12-II because the latter doesn’t have to accommodate the connectors for detachable cables. There is nothing remarkable about the exterior of the M12-II. It is an ordinary power supply painted a matte black paint.

The connectors for detachable cables can be found on the back panel: two for graphics cards and five for peripherals. Despite the same color and the lack of any labels, you cannot mismatch the connectors because they are shaped differently.

Interior Design

The interior design of this PSU is similar to the above-discussed S12-II 520 except that the case is longer and the extra space near the back panel accommodates a card with connectors.

Like the S12-II series models, the M12-II has joint voltage regulation. This is not typical of the M12 series in general. The original M12 used to have dedicated voltage regulation based on chokes, and the M12D model had such regulation based on DC-DC converters.

KZE series capacitors from United Chemi-Con are installed on the PSU’s output.


The PSU is rated for 620 watts and can yield 576 watts via its +12V rail which is split into two virtual output lines.

Cables and Connectors

The PSU is equipped with the following cables and connectors:

Included with the PSU are:

The selection of cables is standard enough. It is good that there are a lot of SATA connectors but I don’t like the 52cm CPU power cable which is going to be too short in many system cases with a bottom PSU compartment. I would request all PSU manufacturers to provide a 60cm or longer CPU cable even at the expense of the second CPU cable which is necessary but for a few users.

Compatibility with UPSes

Like the S12-II 520, this PSU refused to work normally with my UPS. It was stable at loads up to 350 watts when powered by the mains but did not work after switching to the batteries.

Thus, I can suppose that the high-wattage S12-II and M12-II series models have a different active PFC design than the low-wattage models and cannot work normally with UPSes that have non-sinusoid output voltage. The S12-II 430, having a somewhat different interior design than the S12-II 520 and M12-II 620, had no problems working with my UPS.

Output Voltage Stability

The M12-II cannot match PSUs with dedicated voltage regulation in terms of output voltage stability, but it is good enough for its product class. The voltages deflect no more than 3% in the typical load range.

Output Voltage Ripple

The output voltage ripple is low and can only be seen clear enough on the +3.3V rail. There are but occasional spikes of voltage on the other rails.


This PSU is cooled by an ADDA AD1212HB-A71GL fan which is rated for 2200 RPM. This is about 10% faster than in the above-discussed PSUs of lower wattages.

However, the fan starts out at a speed of only 500 RPM and keeps it for a while at higher loads. Then, it accelerates to 1000 RPM at 300 watts (this is the top limit of silent operation; even an undemanding user will hear the fan at a higher speed) and becomes downright noisy at 400 watts. The PSU produces a powerful din at full load.

Thus, like the above-discussed S12-II series models, the M12-II is very quiet at low loads but loud at 300 watts and higher. The only advantage over the lower-wattage S12-II 520 is that the latter’s fan begins to accelerate sooner.

Efficiency and Power Factor

The PSU is 85% and more efficient through the largest part of the load range.

Standby Source

The standby source copes with its job just fine.


The M12-II differs from the S12-II series in having modular cables and a slightly longer case. It is a well-made, highly efficient PSU which is also very quiet at low loads. The downside is that its output voltages are not absolutely stable, its noise quickly grows up at loads above 300 watts, and it is incompatible with UPSes with non-sinusoid output voltage.

Thus, there is no reason to prefer a higher-wattage model in the S12-II or M12-II series if you want to have a quieter PSU. All of them are quiet at loads up to 300 watts, which is less than 50% of their max output power. So, if your computer has an average power draw of 200 watts or lower (this is a rather typical gaming computer with a dual-core processor and a Radeon HD 5850 graphics card), each of these three models will be a good choice for it. But if you’ve got a more advanced configuration, you may want to consider other PSUs in which the fan does not accelerate as fast as in the S12-II and M12-II.

Seasonic X-750 (SS-750KM), 750 W

The star of this review is the new 80+Plus Gold certified Seasonic X-750. Apart from that certification, the X-750 is interesting for having been developed from scratch. Thus, it is going to be completely different from the various modifications of the 12th series (S12, M12, M12-II and M12D).

Exterior Design

The PSU has in a black case with a punched fan grid. The rest of the Seasonic models have a wire fan grid.

Every cable is detachable although I don’t think anybody would ever want to unplug the mainboard cable. The PSU has as many as 11 connectors and the mainboard cable is plugged into two of them at once. The connectors differ in size, so you can’t mismatch them, but it is not quite easy to connect cables because the connectors are too close to each other.

Interior Design

The X-750 is dismantled in an unusual way:

Instead of a standard U-shaped cover, you remove the top panel only.

After that, you can take off the front and side panels by unfastening a few screws.

It is easy to see in the last two photos that the X-750 has little in common with traditional computer PSUs. It has only one large heatsink instead of two or three, and even that heatsink stands aside (this must be the reason why it is so large; it just doesn’t stand in the way of the air flow).

The heatsink carries the power components of the high-voltage section of the PSU.

The largest heatsink in most PSUs is meant for the low-voltage components which heat up a lot due to the high output currents of modern models. The X-750 is different. The output rectifier is only cooled by two slim aluminum plates which can be seen in the photo above.

The explanation is simple. This PSU employs a high-efficiency synchronous rectifier. Such rectifiers use field-effect transistors instead of diodes. When open, the transistors have a resistance of only a few thousandths of an ohm, so there is a very small loss of power on them. The rectifier’s transistors are placed on the reverse side of the PCB and are additionally cooled by the bottom panel of the case through heat-conducting pads.

There is only one rectifier on the main PCB. It is for 12 volts. The Seasonic X-750 features dedicated DC-DC converters which generate the necessary voltages out of 12 volts. Each DC-DC converter is in fact a small switching power supply with a dedicated PWM controller, switches, a choke and a rectifier. These converters, for +5 and +3.3 volts, are placed on a small separate card on the back panel of the PSU.

By the way, it is due to this design that the mainboard cable has to be attached via two connectors: one connector supplies only 12 volts and is located on the main PCB and the other connector, with +5 and +3.3 volt pins, resides on the additional card. This solution is not handy when it comes to assembling a computer, but simplifies the design of the PSU and lowers the loss of power provoked by extra cables and connectors.

Another interesting feature of this PSU is that it uses a resonance converter, like the recently reviewed Enermax MODU87+.

You can see two greatly simplified schematics of an ordinary flyback converter and of a resonant converter. In the first schematic, when the transistor Q1, managed by a PWM controller, opens, the double-wound choke TR1 begins to accumulate energy and the current in its primary winding rises up quickly, reaching some maximum value. Then, the transistor turns off and the TR1 discharges across the diode into the load (the smoothing capacitor C1 and the load proper). This design is good from every aspect save for one: the rapid growth of electric current when the transistor turns on and the subsequent turning off of the transistor at the maximum level of current increase the loss of power due to the switching and the high-frequency noise in the circuit.

The resonant converter circuit adds two components: a choke LR and a capacitor CR whose ratings are low, so they are rather small and cheap (compared to the main choke TR1). This addition improves the circuit dramatically. First, the choke LR limits the speed at which the current grows up when the transistor opens. Second, the inductances LR and TR1 and the capacitance CR are selected in such a way that the current drops back to zero by the moment the transistor turns off (you can read more about that on page 217 of Power Semiconductor Applications: Switched Mode Power Supplies).

Thus, only two extra components help reduce the loss of power on the high-voltage converter and suppress high-frequency noise. Although developing and setting up resonant converters is somewhat more complex than that of ordinary PWM converters, this design seems to be ready to occupy the market of high-efficiency PSUs.

One peculiarity of resonant converters is that they are controlled through pulse-frequency modulation rather than pulse-width one. As the resonant frequency of the LC circuit is constant and such that the current through the switching transistor rises up and falls back to zero during the transistor's open period, the duration of that period cannot be changed. Thus, PWM cannot be used to control the switch. As a result, it is the frequency rather than duration of pulses that is adjusted in resonant converters (however, there exist combined converters that switch into PWM mode at low load when the power loss on the transistor is going to be low anyway).

I guess I’ve given you enough theory. After all, end-users are more interested in how good the particular PSU is rather than how it is designed.


The PSU can yield 744 out of its maximum 750 watts across the +12V power rail. This rail is monolithic, i.e. not split up into multiple "virtual" rails.

Cables and Connectors

The following cables are included with the PSU:

This is a good selection of cables. There are as many as three different CPU power cables. And there are peripheral cables of different length.

Compatibility with UPSes

Working with my APC SmartUPS SC 620, this power supply was stable at loads up to 403 watts when powered by the mains and up to 375 watts when powered by the batteries. The pair switched the batteries normally; the UPS was stable.

Output Voltage Stability

Like any other model with dedicated DC-DC converters, the X-750 boasts near ideal output voltages. The voltage on the +5V rail is the only one to deflect more than 3% from the nominal value, but only at near-maximum loads.

Output Voltage Ripple

The output voltage ripple on the 12V rail can hardly be seen at all although its load is almost as high as 700 watts! The ripple on the +5V and +3.3V rails is only one half or one third of the allowable maximum.


Contrary to the current trend to use large fans, this PSU is cooled by a San Ace 120. As the name suggests, it is a 120mm model. The quality is high as the fan is manufactured by Sanyo Denki. Take note that it uses a 4-pin connection.

The fan did not rotate at all at loads up to 150 watts. And then it started up at a speed of only 200 RPM. The PSU was silent at loads up to 450 watts (900 RPM) and comfortable at 550 watts. At a load of 600 watts the fan accelerates to 1500 RPM and the noise of the air becomes rather loud. However, that was a uniform and not irritating sound, probably because the roomy interior of the PSU poses but a minimum of aerodynamic obstacles.

Efficiency and Power Factor

As promised by the manufacturer, the efficiency is very high, reaching a maximum of 92%. The PSU is more than 80% efficient even at a load of 50 watts or 7% of its max output power!

Standby Source

The standby source of the X-750 is rated for a current up to 3.0 amperes and copes with its job well.


Seasonic has come up with a very appealing power supply unit. While the M12D with DC-DC converters used to represent but minor modernization of a well-known series, the X-750 is a completely new model that differs in its interior design not only from other Seasonic products but also from other makers’ models. Featuring new technologies and a new component layout, this is a modern, high-quality, stable, efficient and quiet PSU.

And it looks like Enermax with its Revolution 85+ and MODU87+ has got a serious competitor. Seasonic won’t stop after releasing one or two premium-class models but wants to expand its X series with PSUs of different wattage ratings, ranging from 400 to 1200 watts and including fan-less models.

The X-750 is going to be a good choice for configurations with a quad-core processor and one top-end graphics card, especially if you want to have a quiet and stable computer. According to our measurements, even GeForce GTX 480 and Radeon HD 5970 cards consume less than 500 watts together with an overclocked Core i7-920 under more or less realistic load. The X-750 Gold is going to be one of the quietest components in such an advanced gaming machine!

In conclusion we would like to award Seasonic X-750 with our Ultimate Innovation title: