Cooler Master GX 750 W Power Supply Review

[nextpage title=”Introduction”]

GX is the new mainstream power supply series from Cooler Master that is arriving on the market today, so far featuring 550 W, 650 W and 750 W models. Let’s see if the 750 W model is a good buy.

GX series is manufactured by Seventeam. Keep in mind that Cooler Master uses several different vendors, so not all power supplies from them are manufactured by Seventeam.

Figure 1: Cooler Master GX 750 W power supply.

Figure 2: Cooler Master GX 750 W power supply.

Cooler Master GX 750 W is very short, especially for a 750 W product, being only 5 ½” (140 mm) deep, using a 120 mm fan on its bottom and active PFC circuit, of course.

Being a mainstream product, the reviewed power supply doesn’t have a modular cabling system. All cables have nylon sleevings, that come from inside the power supply housing.

The cables included are:

• Main motherboard cable with a 20/24-pin connector, 19 ¼” (49 cm) long.
• One cable with two ATX12V connectors that together form one EPS12V connector, 24” (61 cm) long.
• Four cables with one six/eight-pin connector for video cards each, 20” (51 cm) long.
• Two cables with four SATA power connectors each, 17 ¾” (45 cm) to the first connector, 4 ¼” (11 cm) between connectors.
• One cable with one SATA power connector, three standard peripheral connectors and one floppy disk drive power connector, 17 ¾” (45 cm) to the first connector, 5 7/8” (15 cm) between connectors.

This configuration is very good for a 750 W product, providing four connectors for video cards, allowing you to connect two video cards that require two power connectors each.

Figure 3: Cables.

All cables use 18 AWG wires, which is the minimum recommended, but the main motherboard cable uses thicker 16 AWG wires for the +3.3 V output (orange wires), which is great.

Now let’s take an in-depth look inside this power supply.

[nextpage title=”A Look Inside The Cooler Master GX 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.

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. 

This power supply is flawless on this stage, with one coil, two Y capacitors and one X capacitor more than the minimum required, plus two X capacitors after the rectifying bridge.

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 Cooler Master GX 750 W.

[nextpage title=”Primary Analysis”]

On this page we will take an in-depth look at the primary stage of Cooler Master GX 750 W. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.

This power supply uses two GBU1006 rectifying bridges connected in parallel in its primary, each one supporting up to 10 A at 100° C. At 115 V this unit would be able to pull up to 2,300 W from the power grid; assuming 80% efficiency, the bridges would allow this unit to deliver up to 1,840 W without burning themselves out. Nice overspecification! Of course, we are only talking about these components, and the real limit will depend on all the other components in this power supply.

On the active PFC circuit two SPW20N60C3 power MOSFET transistors are used, 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 in pulse mode at 25° C. These transistors present a 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.

This power supply uses a capacitor from Su’scon labeled at 85° C to filter the output from the active PFC circuit.

In the switching section, two SPP20N60C3 power MOSFETs are used on the traditional two-transistor forward configuration. The specs fo
r these transistors are published above.

Figure 9: Rectifying bridges, switching transistors and active PFC transistors.

The switching transistors are controlled by the famous PFC/PWM combo controller CM6800.

Figure 10: PFC/PWM combo controller.

Now let’s take a look at the secondary of this power supply.

[nextpage title=”Secondary Analysis”]

This power supply has eight Schottky rectifiers on its secondary.

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%.

The +12 V output is produced by four PFR60L60CT Schottky rectifiers, each one supporting up to 60 A (30 A per internal diode at 120° C, 0.65 V maximum voltage drop), giving us a maximum theoretical current of 171 A or 2,057 W for the +12 V output. That’s what we call overspecification!

The +5 V output is produced by two PFR30V30CT Schottky rectifiers, each one supporting up to 30 A (15 A per internal diode at 120° C, 0.45 V maximum voltage drop), giving us a maximum theoretical current of 43 A or 214 W.

The +3.3 V output is produced by another two PFR30V30CT Schottky rectifiers, giving us a maximum theoretical current of 43 A or 141 W.

All these numbers are theoretical. The real amount of current/power each output can deliver is limited by other components, especially by the coils used on each output.

Figure 11: +3.3 V, +5 V and +12 V rectifiers.

The outputs are monitored by a PS113 integrated circuit, which supports only OVP (over voltage protection) and SCP (Short-Circuit Protection). Any other protection this power supply may have is implemented outside this circuit. In fact this unit has two thermistors. When this configuration is used it usually means the unit has over temperature protection (OTP).

Figure 12: Monitoring circuit.

All capacitors from the secondary are also from Su’scon.

[nextpage title=”Power Distribution”]

In Figure 13, you can see the power supply label containing all the power specs.

Figure 13: Power supply label.

As you can see, according to the label this unit has a single +12 V rail, so there is not much to talk about here.

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 +12VA and +12VB inputs listed below are the two +12 V independent inputs from our load tester. During this test both inputs were connected to the power supply single rail (+12VB input was connected to the power supply EPS12V connector and all other cables were connected to the load tester +12VA input).

Input	Test 1	Test 2	Test 3	Test 4	Test 5
+12VA	5 A (60 W)	11 A (132 W)	16 A (192 W)	22 A (264 W)	27 A (324 W)
+12VB	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 (30 W)	8 A (26.4 W)	10 A (33 W)
+5VSB	1 A (5 W)	1.5 A (7.5 W)	2 A (10 W)	2.5 A (12.5 W)	3 A (15 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	148.8 W	300.0 W	448.9 W	596.7 W	742.8 W
% Max Load	19.8%	40.0%	59.9%	79.6%	99.0%
Room Temp.	45.3° C	45.2° C	46.4° C	46.6° C	48.4° C
PSU Temp.	45.8° C	46.2° C	47.8° C	51.2° C	55.5° C
Voltage Regulation	Pass	Pass	Pass	Pass	Pass
Ripple and Noise	Pass	Pass	Pass	Fail on +3.3 V and +5VSB	Fail on +3.3 V and +5VSB
AC Power	180.6 W	356.6 W	543.3 W	745.0 W	966.0 W
Efficiency	82.4%	84.1%	82.6%	80.1%	76.9%
AC Voltage	115.3 V	113.7 V	111.9 V	109.2 V	107.2 V
Power Factor	0.978	0.988	0.993	0.995	0.997
Final Result	Pass	Pass	Pass	Fail	Fail

 

Cooler Master GX 750 W can really deliver its labeled wattage at high temperatures.

Efficiency is decent for a mainstream product if you pull up to 80% of the unit’s labeled capacity (i.e., up to 600 W), between 80% and 84%. At full load efficiency drops below the 80% mark, at 77%. This unit is 80 Plus certifie
d and as we have been exhaustively explaining in our reviews, Ecos Consulting, the company behind 80 Plus, tests power supplies at 25° C, while we test them between 45° C and 50° C, and efficiency drops with temperature. Therefore our tests are more rigorous (and more realistic) that those conducted in order to get the 80 Plus certification (click here to learn more). By the way, the 80 Plus website doesn’t list this power supply.

Voltage regulation was superb, especially for a mainstream product: all positive voltages were within 3% of their nominal values all the times. This is good because voltages are closer to their official values than allowed by ATX12V specification, which sets a 5% tolerance (10% for -12 V). The only exception was during test five, where +5VSB went a little bit out of this tighter range, but still within the allowed range.

The problem with Cooler Master GX 750 W was noise and ripple. During tests four and five noise level on +3.3 V and +5VSB outputs was higher than the maximum allowed (50 mV): 57.4 mV and 70 mV for +3.3 V during tests four and five, respectively, and 51.6 mV and 58.4 mV for +5VSB during tests four and five, respectively. Below you can see the screenshots from our oscilloscope during test five. The maximum allowed for +12 V is 120 mV. All these numbers are peak-to-peak figures.

Figure 14: +12VA input from load tester during test five at 742.8 W (66.6 mV).

Figure 15: +12VB input from load tester during test five at 742.8 W (72.4 mV).

Figure 16: +5V rail during test five at 742.8 W (37.4 mV).

Figure 17: +3.3 V rail during test five at 742.8 W (70.0 mV).

Let’s see if we could pull even more from Cooler Master GX 750 W.

[nextpage title=”Overload Tests”]

The maximum we could pull from this power supply with it still working is shown below. It is interesting to notice that with the unit hot it would shut down after some seconds, showing the over temperature protection in action, which is great. That is why we collected the data below at a lower temperature. During this test +3.3 V and +5VSB outputs were still with noise above the maximum allowed.

Input	Maximum
+12VA	30 A (360 W)
+12VB	30 A (360 W)
+5V	13 A (65 W)
+3.3 V	13 A (42.9 W)
+5VSB	3 A (15 W)
-12 V	0.5 A (6 W)
Total	836.8 W
% Max Load	111.6%
Room Temp.	35.2° C
PSU Temp.	40.9° C
AC Power	1,115 W
Efficiency	75.0%
AC Voltage	107.0 V
Power Factor	0.998

[nextpage title=”Main Specifications”]

Cooler Master GX 750 W power supply specs include:

• ATX12V 2.31
• Nominal labeled power: 750 W.
• Measured maximum power: 836.8 W at 35.2° C.
• Labeled efficiency: 85% typical (i.e., at 50% load, 375 W)
• Measured efficiency: Between 76.9% and 84.1% at 115 V (nominal, see complete results for actual voltage).
• Active PFC: Yes.
• Modular Cabling System: No.
• Motherboard Power Connectors: One 20/24-pin connector and two ATX12V connectors that together form an EPS12V connector.
• Video Card Power Connectors: Four six/eight-pin connectors on individual cables.
• SATA Power Connectors: Nine in three cables.
• Peripheral Power Connectors: Three in one cable.
• Floppy Disk Drive Power Connectors: One.
• Protections: Over voltage (OVP, not tested), under voltage (UVP, not tested), over power (OPP, not tested), over temperature (OTP, not tested) and short-circuit protection (SCP, tested and working).
• Warranty: Five years
• More Information: https://www.coolermaster.com
• Average prince in the US: USD 120.00

* Researched at Newegg.com on the day we published this review.

[nextpage title=”Conclusions”]

Cooler Master GX 750 W looks like a good option for users looking for a mainstream 750 W power supply, however it has a major flaw that prevents us from recommending it: noise level at +3.3 V and +5VSB outputs were above the maximum allowed when we pulled 600 W and above from this unit. High noise levels overload and can even damage components on your computer.

If you are looking for a mainstream 750 W power supply our recommendation is still Seventeam ST-750P-AF, which is USD 20 cheaper than Cooler Master GX 750 W and provides better performance (namely higher efficiency and lower noise levels). Of course GX 750 W offers as advantages having four power cables for video cards and more SATA power connectors, but it is only safe to run this new Cooler Master unit up to 450 W.