Seasonic M12D 750 W Power Supply Review
By Gabriel Torres on June 19, 2009
Seasonic is a traditional OEM manufacturer, being the real manufacturer behind units from brands like Antec, Corsair and Arctic Cooling (not all power supplies from these brands are manufactured by Seasonic, though). They also sell power supplies using their own brand and today we are going to review their M12D 750 W unit (also known as SS-750EM), which uses a modular cabling system and high-end components. Let’s see whether this is a good unit or not.
One annoying thing about Seasonic is that they have three websites, http://www.seasonic.com, http://www.seasonic.com.tw and http://www.seasonicusa.com, but retail products like the reviewed power supply can only be found on the later. In our opinion this makes no sense and only makes it difficult to consumers to find out more about their products.
M12D 750 W isn’t a long power supply, being 6 1/2” (16.5 cm) deep, using a 120 mm fan on its bottom and featuring active PFC, of course. It comes with a half modular cabling system, as some peripheral cables come from inside the power supply: two auxiliary video card power cables with one six/eight pin connector each and one SATA power cable, with three SATA connectors, besides the main motherboard cable (which uses a 20/24-pin connector), an ATX12V cable and an EPS12V cable. All these cables have a nylon protection that comes from inside the unit.
The modular system has six connectors and this power supply comes with nine cables:
All cables are relatively long, with 20 1/2” (52 cm) between the power supply housing and the first connector on the cable. On the cables with more than one connector, there is 5 29/32” (15 cm) between the connectors.
Peripheral and SATA power cables use 18 AWG wires and the main motherboard cable and video card auxiliary power cables use thicker 16 AWG wires, which is great to see.
The number of cables is terrific for the high-end user, however if you want to install more than two very high-end video cards you will need to use adapters, since each very high-end card takes two auxiliary power connectors each and this power supply comes with “only” four of them.
Now let’s take an in-depth look inside this power supply.
We decided to disassemble this power supply to see what it looks like inside, how it is designed, and what components are used. Please read our Anatomy of Switching Power Supplies tutorial to understand how a power supply works and to compare this power supply to others.
This page will be an overview, and then in the following pages we will discuss in detail the quality and ratings of the components used. The first thing that caught our attention was that all capacitors used are Japanese from Chemi-Con and the secondary filtering stage uses some solid capacitors.
As we have mentioned in other articles and reviews, the first place we look when opening a power supply for a hint about its quality, is its filtering stage. The recommended components for this stage are two ferrite coils, two ceramic capacitors (Y capacitors, usually blue), one metalized polyester capacitor (X capacitor), and one MOV (Metal-Oxide Varistor). Very low-end power supplies use fewer components, usually removing the MOV and the first coil.
On this power supply this stage is flawless. It has two extra ferrite coils, one extra X capacitor and two extra Y capacitors.
One thing that we noticed here was that the active PFC circuit had actually two circuits working in parallel (see the two coils in Figure 8), and this was the first power supply we’ve seen using such design. We will talk more about this in the next page.
In the next page we will have a more detailed discussion about the components used in the Seasonic M12D 750 W.
On this page we will take an in-depth look at the primary stage of Seasonic M12D 750 W. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.
This power supply uses two GBU806 rectifying bridges in its primary, which one feeding a separated active PFC circuit (see the two PFC coils in Figure 8). This is the first time we’ve seen a power supply using this design. Each bridge supports up to 8 A at 100° C, so in theory, you would be able to pull up to 1,840 W from the power grid; assuming 80% efficiency, the bridges would allow this unit to deliver up to 1,472 W without burning them. Of course, we are only talking about these components, and the real limit will depend on all the other components in this power supply.
As mentioned, there are two active PFC circuits, each one using two FDP18N50 power MOSFET transistors, so we have a total of four MOSFETs on the active PFC stage. Each MOSFET is capable of delivering up to 18 A at 25° C or 10.8 A at 100° C in continuous mode (note the difference temperature makes) or 72 A in pulse mode at 25° C.
This power supply uses two electrolytic capacitors to filter the output from the active PFC circuit. The use of more than one capacitor here has absolute nothing to do with the “quality” of the power supply, as laypersons may assume (including people without the proper background in electronics doing power supply reviews around the web). Instead of using one big capacitor, manufacturers may choose to use two or more smaller components that will give the same total capacitance, in order to better accommodate space on the printed circuit board, as two or more capacitors with small capacitance are physically smaller than one capacitor with the same total capacitance. M12D 750 W uses one 330 µF x 400 V and one 390 µF x 400 V capacitor connected in parallel; this is equivalent of one 720 µF x 400 V capacitor.
These capacitors are Japanese, from Chemi-Con and are labeled at 105° C. This is good for two reasons, first, Japanese capacitors do not leak; and second, usually manufacturers use 85° C capacitors here, so it is good to see a manufacturer using a capacitor with a higher temperature rating.
In the switching section, two SPP24N60C3 power MOSFET transistors are used on the traditional two-transistor forward configuration. Each transistor supports up to 24.3 A at 25° C or 15.4 A at 100° C (note the difference temperature makes) or 72.9 A in pulse mode at 25° C.
This power supply uses a CM6802 active PFC/PWM combo controller.
This power supply uses eight SBR40S45CT Schottky rectifiers on its secondary and each one is capable of handling up to 40 A (20 A per internal diode at 110° C). All rectifiers are in charge of producing the +12 V output, with +5 V and +3.3 V being generated from the +12 V output using a DC-DC converter (i.e., a small switching power supply) located on a small printed circuit board. This is not the first time we’ve seen such design: Antec Signature 650 W also uses this concept and at least with this model from Antec this design improved efficiency a lot.
Three of the rectifiers are in charge of the direct rectification, while the remaining five are in charge of the “freewheeling” part of the rectification process (i.e., discharging the coil).
The maximum theoretical current each line can deliver is given by the formula I / (1 - D), where D is the duty cycle used and I is the maximum current supported by the rectifying diode. Just as an exercise, we can assume a typical duty cycle of 30%.
For our math we need to assume the path that has the lower limit, which is the direct rectification path. This would give us a maximum theoretical current of 171 A (40 A x 3 / 0.70). This maximum theoretical current limit is for the whole secondary, since +5 V and +3.3 V are also produced from the +12 V output. The practical limit will depend on other factors, mainly on the coils used and on the design from the small DC-DC converter used to generate the +5 V and +3.3 V outputs. If this 171 A was solely pulled from the +12 V outputs, this would give us 2,052 W.
The DC-DC converter uses solid aluminum caps and two APW7073 controllers, one for each output, with seven APM2556N MOSFETs.
This power supply uses a PS223 monitoring integrated circuit, which is in charge of the power supply protections, like OCP (over current protection), over voltage protection (OVP), under voltage protection (UVP) and over temperature protection (OTP, not implemented on this unit).
Electrolytic capacitors from the secondary are also Japanese, from Chemi-Con and labeled at 105° C. We could find some capacitors installed on the modular cabling system, which is great (and unusual).
In Figure 18, you can see the power supply label containing all the power specs.
This power supply has two virtual rails (which is unusual for a power supply from this power range), distributed like this:
There is not much you can do with only two virtual rails and lots of cables. One suggestion we have for Seasonic is to label outside the power supply the rails each cable is connected to, this would certainly help advanced users.Now let’s see if this power supply can really deliver 750 W.
We conducted several tests with this power supply, as described in the article Hardware Secrets Power Supply Test Methodology.
First we tested this power supply with five different load patterns, trying to pull around 20%, 40%, 60%, 80%, and 100% of its labeled maximum capacity (actual percentage used listed under “% Max Load”), watching how the reviewed unit behaved under each load. In the table below we list the load patterns we used and the results for each load.
If you add all the power listed for each test, you may find a different value than what is posted under “Total” below. Since each output can vary slightly (e.g., the +5 V output working at +5.10 V), the actual total amount of power being delivered is slightly different than the calculated value. On the “Total” row we are using the real amount of power being delivered, as measured by our load tester.
The +12V1 and +12V2 inputs listed below are the two +12 V independent inputs from our load tester. During this test +12V1 input was connected to the power supply +12V1 rail and the +12V2 input was connected to the power supply +12V2 rail.
5 A (60 W)
11 A (132 W)
16 A (192 W)
22 A (264 W)
27 A (324 W)
5 A (60 W)
10 A (120 W)
16 A (192 W)
21 A (252 W)
27 A (324 W)
2 A (10 W)
4 A (20 W)
6 A (30 W)
8 A (40 W)
10 A (50 W)
2 A (6.6 W)
4 A (13.2 W)
6 A (19.8 W)
8 A (26.4 W)
10 A (33 W)
1 A (5 W)
1 A (5 W)
1.5 A (7.5 W)
2 A (10 W)
2.5 A (12.5 W)
0.5 A (6 W)
0.5 A (6 W)
0.5 A (6 W)
0.5 A (6 W)
0.5 A (6 W)
% Max Load
Ripple and Noise
Seasonic M12D 750 W achieved very high efficiency, staying above 90% if you pull up to 60% from its labeled power (450 W). At 80% load (600 W) it presented 88.8% efficiency and at full load (750 W) it could still show efficiency above 85%, which is terrific.
Voltage stability was another highlight from Seasonic M12D 750 W, with all voltages inside 3% of their nominal values(i.e., voltages were closer to their nominal value than needed, as ATX spec allows voltages to be up to 5% from their nominal values, 10% for -12 V). This includes the -12 V output, which usually doesn’t like to stay within this tight tolerance.
And finally we have noise and ripple, which were low all the time: noise level at +12 V was below 20% of the maximum allowed. Below you can see the results for test number five. As we always point out, the limits are 120 mV for +12 V and 50 mV for +5 V and +3.3 V and all numbers are peak-to-peak figures.
Before overloading power supplies we always test first if the over current protection (OCP) circuit is active and at what level it is configured.
Here we were limited by our load tester, which can pull “only” up to 33 A from each one of its +12 V inputs. Pulling 33 A from each input wouldn’t shut the power supply down, which makes sense, as the label says that each rail has a 38 A limit (not forgetting that usually manufacturers configure OCP at a level above from what is written on the label).
The idea behind of overload tests is to see if the power supply will burn/explode and see if the protections from the power supply are working correctly. This power supply didn’t burn or explode.
Below you can see the maximum we could pull from this power supply with it still working within specs. Even under this overloading efficiency was above 80%.
29 A (348 W)
29 A (348 W)
17 A (85 W)
17 A (56.1 W)
3 A (15 W)
0.5 A (6 W)
% Max Load
Seasonic M12D 750 W power supply specs include:
* Researched at Newegg.com on the day we published this review.
Seasonic M12D 750 W is an impressive power supply, being to this date the power supply with the highest efficiency that we’ve ever tested, beating (by just a little bit, let’s be honest) the previous champ Antec Signature 650 W. We could see efficiency above 90% when we pulled up to 60% from the power supply labeled power (i.e., 450 W), at practically 89% when we pulled 80% load (600 W) and 86.5% when we pulled its full 750 W.
The reason why Seasonic M12D 750 W and Antec Signature 650 W achieved efficiency above 90% lies on the chosen design. Instead of having separated rectifiers for each output, these two power supplies produce mainly only one output: +12 V. From this +12 V output a second smaller power supply produces the +5 V and +3.3 V outputs. This is what the manufacturer calls “DC-DC design,” although technically the use of this name itself doesn’t make any sense, as all switching power supplies are DC-DC converters (as they increase and convert the wall voltage into DC before sending to the switching transistors).
Not only this design proved to be superior, but Seasonic decided to use only high-end components inside this unit, which features only Japanese capacitors and solid caps on the DC-DC converter in charge of the + 5 V and +3.3 V outputs.
Voltage stability was another highlight, with all voltages (including -12 V) within 3% from their nominal values, i.e., we saw voltages closer to their nominal values than what allowed by the ATX standard, which specifies a 5% tolerance (10% for -12 V).
We could also pull up to 850 W at 47° C from this unit.
The number of cables is more than enough, but if you plan to build a three-way SLI system you will need to use adapters, as this power supply comes with “only” four auxiliary video card power connectors (and each very high-end video card uses two of them).
Of course all these superior specs come with a price. Seasonic M12D 750 W costs USD 210 at Newegg.com, which is expensive for a 750 W product, however we are talking about a premium product for users that really want “the best” and are really worrried about saving energy.
If you don’t think paying USD 210 on a power supply makes any sense, there are several good alternatives on the market – but with lower efficiency – Zalman ZM-750HP (USD 155), SilverStone Decathlon 700 W (USD 180), Topower PowerBird 900 W (USD 180) and Antec CP-850 (USD 130, but unfortunately only fits a few cases from Antec), just to name a few. If you really want a premium product, you may be satisfied with Antec Signature 650, which costs USD 170.