Corsair HX750W Power Supply Review
By Gabriel Torres on July 29, 2009
It is very important to notice that Corsair HX750W, HX850W and HX1000W have nothing to do with other models from this same series. They use a complete different (and better, by the way) design using a DC-DC converter to generate their +5 V and +3.3 V outputs, like several other high-efficiency power supplies (PC Power & Cooling Silencer 910, Antec Signature, Antec TruePower New, Seasonic M12D and Cooler Master UCP series). These HX models are manufactured by CWT, while other models from HX series are manufactured by Seasonic. Why Corsair kept the same name is a mystery. In our opinion they should have used a different name so consumers would know they are facing a different product class, targeted to users looking a power supply with high-end parts and very high efficiency.
Something interesting happened with this power supply 80 Plus certification. According to 80 Plus this unit, together with HX850W, is Gold certified – minimum efficiency of 90% at typical load (50% load, i.e., 375 W) and a minimum efficiency of 87% under light (20% load, i.e., 150 W) and full (750 W) loads –, however since the results achieved by 80 Plus were so close to these minimum requirements Corsair decided to downgrade these two units to Silver (88% under typical load and 85% under light and full loads) by themselves. On a market that usually manufacturers like to exaggerate about product characteristics it is really nice to see a manufacturer doing exactly the opposite: reducing the numbers to protect users. Kudos to Corsair.
Corsair HX1000W uses two transformers inside, while HX850W and HX750W use only one. Since we have already reviewed HX850W, on this review we will spot the internal differences between the 750 W and the 850 W models.
HX750W has the same size as HX850W – 7 3/32” (180 mm) deep –, being long units. Both have a 140 mm fan on its bottom, feature active PFC, have a single-rail design and a modular cabling system.
On Corsair HX750W the main motherboard cable (20/24-pin) and the ATX12V/EPS12V cable (two ATX12V connectors that together form an EPS12V one) are permanently attached to the unit. These cables are protected with nylon sleevings that come from inside the power supply housing. The 850 W model has two more cables coming directly from inside the power supply (two auxiliary power cables for video cards with one six/eight-pin connector each), feature not available on the 750W version.
Like HX850W, the modular cabling system from HX750W has ten connectors and the reviewed power supply comes with nine cables (one less than HX850W; the cable that is missing is one extra peripheral power cable) plus two adapters to convert standard peripheral power plugs into floppy disk drive power plugs. The cables included are:
The number of cables and connectors available is perfect for a 750 W product, allowing you to have up to 12 SATA peripherals and up to two high-end video cards. Of course you can have more than two video cards, but in this case you will need to convert standard peripheral power connectors into video card power connectors.
The main motherboard cable, the ATX12V/EPS12V cable and the video card cables are long, measuring 23 5/8” (60 cm), so you probably won’t have any trouble using this power supply on a big full tower case. Peripheral and SATA power cables have a distance of 17 ½” (44 cm) between the end that goes on the power supply and the very first connector on the cable. The distance between each connector on these cables is of 4” (10 cm).
The main motherboard cable use 16 AWG wires, which are thicker, while all other wires are 18 AWG, which is the correct gauge to be used.
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.
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.
This power supply has all required components plus one extra X capacitor and two extra Y capacitors. The MOV can be seen behind the fuse in Figure 8, using a rubber protection.
In the next page we will have a more detailed discussion about the components used in the Corsair HX750W.
On this page we will take an in-depth look at the primary stage of Corsair HX750W. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.
This power supply uses one GBU1506 rectifying bridge in its primary (the same one used on Corsair HX850W), supporting up to 15 A at 100° C if a heatsink is used (which is the case), so in theory, you would be able to pull up to 1,725 W from the power grid; assuming 80% efficiency, the bridge would allow this unit to deliver up to 1,380 W without burning itself out. Of course we are only talking about this component and the real limit will depend on all other components from the power supply.
Two SPW20N60C3 power MOSFETs are used on the active PFC circuit, each one capable of delivering up to 20.7 A at 25° C or 13.1 A at 100° C in continuous mode (note the difference temperature makes) or up to 62.1 A at 25° C in pulse mode. These transistors present a maximum resistance of 190 mΩ when turned on, a characteristic called RDS(on). This number indicates the amount of power that is wasted, so the lower this number the better, as less power will be wasted thus increasing efficiency. The 850 W version uses more powerful transistors here (34.6 A at 25° C or 21.9 A at 100° C in continuous mode, 103.8 A in pulse mode).
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. Corsair HX750W uses two 330 µF x 420 V connected in parallel; this is equivalent of one 660 µF x 420 V capacitor (HX850W uses two 390 µF x 400 V capacitors).
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 IRFP460A power MOSFET transistors are used on the traditional two-transistor forward configuration. Each transistor is capable of delivering up to 20 A at 25° C or 13 A at 100° C in continuous mode (note the difference temperature makes), or up to 80 A in pulse mode at 25° C, with a 270 mΩ RDS(on). Corsair HX850W uses different transistors here but with similar specs.
This power supply uses a CM6802 active PFC/PWM combo controller.
Like Corsair HX850W and HX1000W, this power supply uses a synchronous design, where the rectifiers are replaced with transistors. Also this power supply basically produces only the +12 V output. +5 V and +3.3 V outputs are generated from the +12 V output by two little power supplies located on two small printed circuit boards inside the unit. This design is called DC-DC converter and is also used by several other power supplies on the market, as mentioned on the introduction.
Five IPP037N08N are used, each one capable of delivering up to 100 A at 100° C in continuous mode, or up to 400 A at 25° C in pulse mode, with a maximum RDS(on) of 3.5 mΩ, which is impressively low. Three of them are in charge of the direct rectification, while the remaining two are in charge of the “freewheeling” part of the rectification process (i.e., discharging the coil). Corsair HX850W also uses five transistors, but models with a higher current limit (120 A).
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%.
This would give us a maximum theoretical current of 428.5 A (100 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 converters used to generate the +5 V and +3.3 V outputs. If this 428.5 A was solely pulled from the +12 V outputs, this would give us 5,142 W. Talk about overspecification!
This power supply has two separated DC-DC converters, one for +5 V and another for +3.3 V. Each one uses one APW7073 controller and three APM2556N MOSFETs (which present a maximum RDS(on) of only 7.2 mΩ), using solid capacitors.
This power supply uses a PS229 monitoring integrated circuit, which is in charge of the power supply protections. Unfortunately there is no information about this circuit on the manufacturer’s website.
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.
In Figure 16, you can see the power supply label containing all the power specs.
This power supply uses a single-rail design, so there is nothing to talk about here. Keep in mind that the difference between a single-rail design and a multiple-rail design is how the over current protection (OCP) circuit is connected. On single-rail design there is only one OCP circuit monitoring all outputs, while on multiple-rail design there are several OCP circuits, each one monitoring a group of wires called “rails.”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 and during all test both were connected to the single +12 V rail present on the power supply.
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)
1.5 A (7.5 W)
2 A (10 W)
2.5 A (12.5 W)
3 A (15 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
Failed on +5VSB
Failed on +5VSB
Failed on +5VSB
What efficiency! What efficiency! Corsair HX750W achieved a spectacular efficiency between 85.5% and 89.8% in our tests. Usually power supplies achieve a low efficiency when delivering their labeled power, which isn’t the case with the reviewed unit.
It is always important to remember that the 80 Plus organization is very generous on their tests: they test power supplies at a room temperature of only 23° C, which is too low. We test power supplies with a room temperature of at least the double, which we consider more realistic. Since at higher temperatures efficiency drops, this explains why we achieve lower efficiency numbers than those provided by this organization.
Even though noise and ripple levels for the main voltages were very low as we will show below, the standby (+5VSB) output had an enormous ripple level during all load patterns but tests four and five. Interesting enough we saw the same thing happening on Corsair HX850W, showing us that the problem was not with the sample we got but with the internal project. During test one ripple was at 103.4 mV, during test two it was at 134.4 mV and during test three it was at 103.8 mV. The maximum allowed is 50 mV (all values are peak-to-peak).
After this review was posted, Corsair tested this power supply using the same load patterns presented on the table above and, with a different equipment, the noise levels for +5VSB were completely different (very low). Please click here to see the results. The only explanation we have is that our equipment was somehow interfering with the results. This way the comments above about the +5VSB output should not be taken at face value.
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.
Now let’s see if we could pull more than 750 W from this unit.
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, giving us a total of 66 A.
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 and it shut down when we tried to overload it.
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 83%. As you can see Corsair could have labeled this unit as a 900 W power supply, but they didn’t do this because otherwise they wouldn’t be able to get the 80 Plus Gold certification.
32 A (384 W)
32 A (384 W)
15 A (75 W)
15 A (49.5 W)
3 A (15 W)
0.5 A (6 W)
% Max Load
Corsair HX750W power supply specs include:
* Researched at Newegg.com on the day we published this review.
Corsair HX750W is an impressive power supply, being to this date one of the power supplies with the highest efficiency that we’ve tested to date, easily beating all other 750 W power supplies we’ve tested, including those also based on a DC-DC design on the secondary like Antec TruePower New.
Not only the DC-DC design proved to be superior, but Corsair/CWT decided to use only high-end components inside this unit, which features only Japanese capacitors and solid caps on the DC-DC converters in charge of the + 5 V and +3.3 V outputs.
We could also pull up to 910 W at 46° C from this unit, which is really impressive.
The number of cables available is perfect for a 750 W product (12 SATA power connectors, eight peripheral power connectors and four six/eight-pin video card power connectors), allowing you to build a very high-end system with two very high-end video cards (more video cards are supported if you use adapters to convert standard peripheral power plugs into video card power connectors).
The seven-year warranty – losing only to BFG’s lifetime warranty – is also another reason to pick this product over competitors.
Corsair HX750W is a very good choice for users looking for a 750 W power supply with one of the highest efficiencies around.