[nextpage title=”Introduction”]
Seasonic M12D 750 W (SS-750EM) was one of the best-performing power supplies we’ve reviewed to date, presenting only high-end components and a very high efficiency thanks to the DC-DC converter used on its secondary. Seasonic released now a more affordable version from this power supply, called S12D (SS-750HT), which the main difference is not coming with a modular cabling system. Let’s see if this is the only difference between them.
As you may be aware, 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). One annoying thing about Seasonic is that they have three websites, https://www.seasonic.com, https://www.seasonic.com.tw and https://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.
Figure 1: Seasonic S12D 750 W power supply.
Figure 2: Seasonic S12D 750 W power supply.
S12D 750 W isn’t a long power supply, being 6 19/64” (160 mm) deep, using a 120 mm fan on its bottom and featuring active PFC, of course. As mentioned S12D doesn’t have a modular cabling, and all cables have a nylon protection that comes from inside the unit.
This power supply comes with the following cables:
- Main motherboard cable with a 20/24-pin connector.
- One cable with two ATX12V connectors that together form an EPS12V connector.
- One cable with one EPS12V connector.
- Two cables with one six-pin auxiliary power connector for video cards each.
- Two cables with one six/eight-pin auxiliary power connector for video cards each.
- Three cables with three SATA power connectors each.
- One cable with three standard peripheral power connectors and one floppy disk drive power connector.
- One cable with three standard peripheral power connectors.
- One adapter for converting one standard peripheral power connector into two floppy disk drive power connectors.
Each cable has a different length. The main motherboard cable is 20 ½” (52 cm) long, the ATX12V/EPS12V cables are 22” (56 cm) long, the video card cables are 22.5” (57 cm) long and the SATA and peripheral cables have 16 1/8” (41 cm) between the power supply housing and the first connector on the cable, and then 5 7/8” (15 cm) between the connectors, except on the peripheral cable with the floppy disk drive connector, where the distance between connectors is of 4 3/4”(12 cm).
All cables use 18 AWG wires. There is a small difference between S12D and M12D here: M12D use thicker 16 AWG wires on the main motherboard cable and on the video card auxiliary power cables.
Figure 3: Cables.
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.[nextpage title=”A Look Inside The S12D 750 W”]
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.
Figure 4: Overall look.
Figure 5: Overall look.
Figure 6: Overall look.
[nextpage title=”Transient Filtering Stage”]
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.
Figure 7: Transient filtering stage (part 1).
Figure 8: Transient filtering stage (part 2).
In the next page we will have a more detailed discussion about the components used in the Seasonic S12D 750 W.[nextpage title=”Primary Analysis”]
On this page we will take an in-depth look at the primary stage of Seasonic S12D 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, each one feeding a separated active PFC circuit. Each bridge supports up to 8 A at 100° C, so i
n theory, you would be able to pull up to 1,840 W from a 115 V 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.
Figure 9: Rectifying bridges.
As mentioned, there are two active PFC circuits, each one using two FQP13N50C power MOSFET transistors, so we have a total of four MOSFETs on the active PFC stage. Each MOSFET is capable of delivering up to 13 A at 25° C or 8 A at 100° C in continuous mode (note the difference temperature makes) or 52 A in pulse mode at 25° C. These transistors present a 480 mΩ resistance when turned on, a characteristic called RDS(on). The lower this number the better, meaning that the transistors will waste less power and the power supply will achieve a higher efficiency. Interesting enough Seasonic M12D uses more powerful transistors here (18 A at 25° C or 10.8 A at 100° C).
Figure 10: Active PFC transistors.
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. S12D 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, presenting an RDS(on) of 160 mΩ. These are exactly the same transistors used on M12D.
Figure 11: Switching transistors and active PFC diodes.
This power supply uses a CM6802 active PFC/PWM combo controller.
Figure 12: Active PFC/PWM combo controller.
Now let’s take a look at the secondary of this power supply.[nextpage title=”Secondary Analysis”]
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, maximum voltage drop of 0.55 V). 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 design is the current trend among high-efficiency power supplies. This is exactly the same configuration from M12D.
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, but 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.
Figure 13: Rectifiers.
The DC-DC converter uses solid aluminum caps and two APW7073 controllers, one for each output, with seven APM2556N MOSFETs, which present a maximum RDS(on) of only 7.2 mΩ.
Figure 14: DC-DC converter in charge of generating +5 V and +3.3 V outputs from the +12 V output.
Figure 15: DC-DC converter in charge of generating +5 V and +3.3 V outputs from the +12 V output.
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).
Figure 16: Monitoring circuit.
Electrolytic capacitors from the secondary are also Japanese, from Chemi-Con and labeled at 105° C.
[nextpage title=”Power Distribution”]
In Figure 17, you can see the power supply label containing all the power specs.
Figure 17: Power supply label.
This power supply has two virtual rails (which is unusual for a power supply from this power range), distributed like this:
- +12V1 (solid yellow wire): Main motherboard cable, SATA power cables, peripheral power cables, one of the six-pin video card power connectors and one of the six/eight-pin video card power connectors.
- +12V2 (yellow with black stripe wire): ATX12V, EPS12V, one of the six-pin video card power connectors and one of the six/eight-pin video card power connectors.
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.[nextpage title=”Load Tests”]
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.
| Input | Test 1 | Test 2 | Test 3 | Test 4 | Test 5 |
| +12V1 | 5 A (60 W) | 11 A (132 W) | 16 A (192 W) | 22 A (264 W) | 27 A (324 W) |
| +12V2 | 5 A (60 W) | 10 A (120 W) | 16 A (192 W) | 21 A (252 W) | 27 A (324 W) |
| +5V | 2 A (10 W) | 4 A (20 W) | 6 A (30 W) | 8 A (40 W) | 10 A (50 W) |
| +3.3 V | 2 A (6.6 W) | 4 A (13.2 W) | 6 A (19.8 W) | 8 A (26.4 W) | 10 A (33 W) |
| +5VSB | 1 A (5 W) | 1 A (5 W) | 1.5 A (7.5 W) | 2 A (10 W) | 2.5 A (12.5 W) |
| -12 V | 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) |
| Total | 150.0 W | 302.8 W | 454.6 W | 605.3 W | 755.8 W |
| % Max Load | 20.0% | 40.4% | 60.6% | 80.7% | 100.8% |
| Room Temp. | 46.7° C | 45.2° C | 46.7° C | 46.6° C | 46.9° C |
| PSU Temp. | 48.1° C | 49.3° C | 52.8° C | 55.8° C | 57.4° C |
| Voltage Stability | Pass | Pass | Pass | Pass | Pass |
| Ripple and Noise | Pass | Pass | Pass | Pass | Pass |
| AC Power | 174.7 W | 346.1 W | 524.6 W | 706.0 W | 900.0 W |
| Efficiency | 85.9% | 87.5% | 86.7% | 85.7% | 84.0% |
| AC Voltage | 110.5 V | 108.1 V | 107.7 V | 104.5 V | 102.5 V |
| Power Factor | 0.969 | 0.975 | 0.979 | 0.985 | 0.987 |
| Final Result | Pass | Pass | Pass | Pass | Pass |
Seasonic S12D 750 W, like M12D, achieved very high efficiency, staying above 84% all times, including when you pull its maximum labeled power. If you pull between 40% and 60% from its labeled capacity (i.e., between 300 W and 450 W) you will get efficiency around 87%.
One important note: our Seasonic M12D 750 W review isn’t updated yet; the results from this other review were collected with our old power meter, which isn’t so precise and that is why you will see higher efficiency numbers there.
Voltage stability was another highlight from Seasonic S12D 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 a tolerance this tight.
And finally we have noise and ripple, which were low all the time: noise level at +12 V was below 30% 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.
Figure 18: +12V1 input from load tester at 755.8 W (31.6 mV).
Figure 19: +12V2 input from load tester at 755.8 W (30.2 mV).
Figure 20: +5V rail with power supply delivering 755.8 W (14.0 mV).
Figure 21: +3.3 V rail with power supply delivering 755.8 W (15.8 mV).
Now let’s see if we could pull more than 750 W from this unit.
[nextpage title=”Overload Tests”]
Below you can see the maximum we could pull from this power supply with it still working within specs. 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. Even under this overloading efficiency was above 80%.
Input |
Maximum |
| +12V1 | 30 A (360 W) |
| +12V2 | 30 A (360 W) |
| +5V | 20 A (100 W) |
| +3.3 V | 20 A (66 W) |
| +5VSB | 3 A (15 W) |
| -12 V | 0.5 A (6 W) |
| Total | 905.8 W |
| % Max Load | 120.8% |
| Room Temp. | 47.1° C |
| PSU Temp. | 58.2° C |
| AC Power | 1,126 W |
| Efficiency | 80.4% |
| AC Voltage | 98.2 V |
| Power Factor | 0.992 |
[nextpage title=”Main Specifications”]
Seasonic S12D 750 W power supply specs include:
- ATX12V 2.3
- EPS12V 2.92
- Nominal labeled power: 750 W.
- Measured maximum power: 905.8 W at 47.1° C.
- Labeled efficiency: Up to 90%, 80 Plus Silver certified (85% minimum at 20% and 100% loads; 88% minimum at 50% load).
- Measured efficiency: Between 84.0% and 87.5% at 115 V.
- Active PFC: Yes.
- Modular Cabling System: No.
- Motherboard Power Connectors: One 24-pin connector, one EPS12V connector and two ATX12V connectors that together form another EPS12V connector.
- Video Card Power Connectors: Two six-pin connectors and two six/eight-pin connectors.
- SATA Power Connectors: Nine in three cables.
- Peripheral Power Connectors: Six in two cables.
- Floppy Disk Drive Power Connectors: One. Two more if the included adapter that is connected to a peripheral power plug is used.
- Protections: Over voltage (OVP, not tested), over power (OPP, not tested) and short-circuit (SCP, tested and working) protections.
- Warranty: Five years.
- More Information: https://www.seasonicusa.com
- Average price in the US*: USD 160.00.
* Researched at Newegg.com on the day we published this review.[nextpage title=”Conclusions”]
Seasonic S12D 750 W achieved the same performance from M12D, with the terrific advantage of costing 20% less! The basic difference between the two is the presence of a half modular cabling system on M12D. Although S12D uses less powerful transistors on its active PFC circuit this didn’t reflect a bit on the capacity of this power supply in delivering its labeled power.
The main highlight from S12D is its high efficiency and the use of a DC-DC converter on the secondary (i.e., producing +5 V and +3.3 V outputs from the +12 V output) is proving to be a winner for manufacturing highly efficient power supplies. At a load between 300 W and 450 W you should see efficiency around 87%. Remembering that the 80 Plus organization certified this unit as 80 Plus Silver (88% efficiency under typical load), but they test power supplies at only 23 C, while we test them at least double this temperature (efficiency drops with temperature).
Voltage regulation was one of the another highlight from S12D, 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).
Noise and ripple were also very low. When we were pulling 750 W from this unit noise and ripple at +12 V outputs were below 30% the maximum allowed.
Plus we could pull up to 905 W at 47° C from this unit.
The number of cables are 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).
Seasonic M12D and S12D are excellent power supplies, with S12D bringing the price tag users were looking for!
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