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[nextpage title=”Introduction”]

This 750 W power supply from Coolmax can be found at Newegg.com for only USD 60, a price way below the one from good 750 W units we’ve already reviewed. Let’s see if you can save money buying this unit or if it is better to stick with a product from a different brand.

This power supply is manufactured by a company called LongYi Electronics, which is so small that they don’t even have a website.

Coolmax CUL-750B 750 W power supplyFigure 1: Coolmax CUL-750B 750 W power supply.

Coolmax CUL-750B 750 W power supplyFigure 2: Coolmax CUL-750B 750 W power supply.

Coolmax CUL-750B 750 W is 6 19/64” (160 mm) deep, using a 135 mm fan on its bottom (the product box says this unit uses a 140 mm fan, but this information is wrong). This unit features active PFC.

A small modular cabling system is available, with only three connectors: one red for a video card power cable and two black for peripheral and SATA power cables. Most cables are permanently attached to the power supply, and these cables use a nylon protection that comes from inside the unit. All cables use 18 AWG wires, which is the correct gauge to be used. The cables included are:

  • Main motherboard cable with a 20/24-pin connector, 14 ½” (37 cm) long (permanently attached to the power supply).
  • One cable with two ATX12V connectors that together form one EPS12V connector, 15 3/8” (39 cm) long (permanently attached to the power supply).
  • One six/eight-pin connector for video cards, 16 1/8” (41 cm) long (permanently attached to the power supply).
  • One six-pin connector for video cards, 16 1/8” (41 cm) long (modular cabling system).
  • Two cables with two SATA power connectors each, 19 ¼” (49 cm) to the first connector, 5 7/8” (15 cm) between connectors (one cable permanently attached to the power supply, one cable on the modular cabling system).
  • One cable with two standard peripheral power connectors, 15 ¾” (40 cm) to the first connector, 5 7/8” (15 cm) between connectors (permanently attached to the power supply).
  • One cable with two standard peripheral power connectors and one floppy disk drive power connector, 15 ¾” (40 cm) to the first connector, 5 7/8” (15 cm) between connectors (modular cabling system).

This configuration is simply ridiculous for a 750 W power supply. Most cables are too short, there is only four SATA power connectors and the ATX12V/EPS12V has only two +12 V wires coming from the power supply, instead of four, which would be the correct configuration.

Coolmax CUL-750B 750 W power supplyFigure 3: Cables.

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

[nextpage title=”A Look Inside The Coolmax CUL-750B 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.

Coolmax CUL-750B 750 W power supplyFigure 4: Overall look.

Coolmax CUL-750B 750 W power supplyFigure 5: Overall look.

Coolmax CUL-750B 750 W power supplyFigure 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 unit has all the required components on this stage.

Coolmax CUL-750B 750 W power supplyFigure 7: Transient filtering stage.

In the next page we will have a more detailed discussion about the components used in the Coolmax CUL-750B 750 W.

[nextpage title=”Primary Analysis”]

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

This power supply uses one PBU1005 rectifying bridge installed on a heatsink, which supports up to 10 A at 100° C if a heatsink is used. At 115 V this unit would be able to pull up to 1,150 W from the power grid; assuming 80% efficiency, the bridge would allow this unit to deliver up to 920 W without burning itself out. Of course, we are only talking about these components, and the real limit will depend on all the other components in this power supply.

Coolmax CUL-750B 750 W power supplyFigure 8: Rectifying bridge.

Two SPP20N60C3 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.

Coolmax CUL-750B 750 W power supplyFigure 9: Active PFC transistors and diode.

The electrolytic capacitor used to filter the output from the active PFC circuit is labeled at 105° C, which is great, and manufactured by a company called “CS.”

The reviewed power supply uses another two SPP20N60C3 power MOSFETs on its switching section, installed on the traditional two-transistor forward configuration. The specs for these transistors were already published above.

Coolmax CUL-750B 750 W power supplyFigure 10: Switching transistors. The transistor on the right is from the +5VSB power supply.

The primary is controlled by a CM6805 PFC/PWM combo controller.

Coolmax CUL-750B 750 W power supplyFigure 11: 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 four Schottky rectifiers attached to its secondary heatsink.

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 two S30C60C Schottky rectifiers connected in parallel, each one supporting up to 30 A (15 A per internal diode at 125° C, 0.58 V maximum voltage drop), giving us a maximum theoretical current of 43 A or 514 W for the +12 V output – a limit that is simply too low for a unit labeled at 750 W.  

The +5 V output is produced by one S40D45C Schottky rectifier, which supports up to 40 A (20 A per internal diode at 100° C, 0.55 V maximum voltage drop), giving us a maximum theoretical current of 29 A or 143 W for the +5 V output.

The +3.3 V output is produced by another S40D45C Schottky rectifier, giving us a maximum theoretical current of 29 A or 94 W for the +3.3 V output.

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.

Coolmax CUL-750B 750 W power supplyFigure 12: +12 V, +5 V and +3.3 V rectifiers.

The outputs are monitored by a PS113 integrated circuit, which supports only OVP (over voltage protection) and UVP (under voltage protection) protections.

Coolmax CUL-750B 750 W power supplyFigure 13: Monitoring integrated circuit.

The electrolytic capacitors from the secondary are marked as being from a company called “GL” and a company called “CS.”

[nextpage title=”Power Distribution”]

In Figure 14, you can see the power supply label containing all the power specs.

Coolmax CUL-750B 750 W power supplyFigure 14: Power supply label.

As you can see, according to the label this unit has four +12 V rails, but this information is not correct. Even though internally the wires are divided into four groups, they lack current sensors (“shunts”) and the monitoring circuit does not support over current protection (OCP). Having these components is a requirement for the unit to have more than one +12 V virtual rail, therefore this unit has, in fact, a single +12 V rail. Click here to understand more about this subject.

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.  

In our tests we connect the power supply being tested inside our thermal chamber and leave it there running with a load representing 60% of the power supply capacity until the temperature inside the chamber has increased to around 47° C. The first sample we’ve got from this unit burned after only a few minutes under this condition, not allowing us to even test it. We replaced the defective unit with the store we bought it from and, because what happened with the first sample, we decide to increase load little by little until we could see the maximum amount of power we could extract from the reviewed unit.

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.

Input Test 1 Test 2 Test 3 Test 4 Test 5
+12VA 3 A (36 W) 3.5 A (42 W) 4.5 A (54 W) 5.5 A (66 W) 6.25 A (75 W)
+12VB 2.5 A (30 W) 3.25 A (39 W) 4 A (48 W) 5 A (60 W) 6 A (72 W)
+5V 1 A (5 W) 1 A (5 W) 1.5 A (7.5 A) 1.5 A (7.5 A) 2 A (10 W)
+3.3 V 1 A (5 W) 1 A (5 W) 1.5 A (4.95 W) 1.5 A (4.95 W) 2 A (6.6 W)
+5VSB 1 A (5 W) 1 A (5 W) 1 A (5 W) 1 A (5 W) 1 A (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

87.3 W 102.6 W 128.5 W 152.8 W 178.4 W
% Max Load 11.6% 13.7% 17.1% 20.4% 23.8%
Room Temp. 40.8° C 40.7° C 41.2° C 41.8° C 43.3° C
PSU Temp. 46.5° C 46.3° C 46.5° C 47.0° C 47.9° C
Voltage Regulation Pass Pass Pass Pass Pass
Ripple and Noise Pass Pass Pass Pass Pass
AC Power 107.3 W 124.3 W 153.2 W 181.4 W 211.1 W
Efficiency 81.4% 82.5% 83.9% 84.2% 84.5%
AC Voltage 114.6 V 114.4 V 114.1 V 113.8 V 113.0 V
Power Factor 0.977 0.978 0.982 0.986 0.989
Final Result Pass Pass Pass Pass Pass
Input Test 6 Test 7 Test 8 Test 9 Test 10
+12VA 7.5 A (90 W) 8.25 A (99 W) 9.25 A (111 W) 10 A (120 W) 11 A (132 W)
+12VB 7 A (84 W) 8 A (96 W) 9 A (108 W) 10 A (120 W) 11 A (132 W)
+5V 2 A (10 W) 2.5 A (12.5 W) 2.5 A (12.5 W) 3 A (15 W) 3 A (15 W)
+3.3 V 2 A (6.6 W) 2.5 A (8.25 W) 2.5 A (8.25 W) 3 A (9.9 W) 3 A (9.9 W)
+5VSB 1 A (5 W) 1 A (5 W) 1 A (5 W) 1 A (5 W) 1 A (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 205.9 W 230.5 W 254.8 W 279.9 W 303.8 W
% Max Load 27.5% 30.7% 34.0% 37.3% 40.5%
Room Temp. 44.3° C 45.3° C 46.6° C 48.0° C 43.4° C
PSU Temp. 48.6° C 49.9° C 41.6° C 53.3° C 44.9° C
Voltage Regulation Pass Pass Pass Pass Pass
Ripple and Noise Pass Pass Pass Pass Pass
AC Power 242.4 W 272.7 W 301.3 W 332.3 W 360.8 W
Efficiency 84.9% 84.5% 84.6% 84.2% 84.2%
AC Voltage 112.5 V 113.2 V 112.7 V 111.9 V 108.8 V
Power Factor 0.992 0.994 0.995 0.996 0.996
Final Result Pass Pass Pass Pass Pass
Input Test 11 Test 12 Test 13 Test 14 Test 15
+12VA 12 A (144 W) 13 A (156 W) 14 A (168 W) 15 A (180 W) 16 A (192 W)
+12VB 11.75 A (141 W) 12.75 A (153 W) 13.5 A (162 W) 14.5 A (174 W) 15.5 A (186 W)
+5V 3.5 A (17.5 W) 3.5 A (17.5 W) 4 A (20 W) 4 A (20 W) 4.5 A (22.5 W)
+3.3 V 3.5 A (11.55 W) 3.5 A (11.55 W) 4 A (13.2 W) 4 A (13.2 W) 4.5 A (14.85 W)
+5VSB 1 A (5 W) 1 A (5 W) 1 A (5 W) 1 A (5 W) 1 A (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 328.2 W 351.8 W 377.4 W 400.4 W 428.9 W
% Max Load 43.8% 46.9% 50.3% 53.4% 57.2%
Room Temp. 46.2° C 48.8° C 45.6° C 49.4° C 44.4° C
PSU Temp. 48.2° C 52.0° C 48.2° C 53.2° C 46.4° C
Voltage Regulation Pass Pass Pass Pass Pass
Ripple and Noise Pass Pass Pass Pass Pass
AC Power 392.6 W 422.5 W 454.1 W 485.9 W 521.4 W
Efficiency 83.6% 83.3% 83.1% 82.4% 82.3%
AC Voltage 108.4 V 108.3 V 107.9 V 107.9 V 107.5 V
Power Factor 0.996 0.996 0.996 0.997 0.996
Final Result Pass Pass Pass Pass Pass
Input Test 16 Test 17 Test 18
+12VA 17 A (204 W) 18 A (216 W) 19 A (228 W)
+12VB 16.5 A (198 W) 17.25 A (207 W) 18.5 A (222 W)
+5V 4.5 A (22.5 W) 5 A (25 W) 5 A (25 W)
+3.3 V 4.5 A (14.85 W) 5 A (16.5 W) 5 A (16.5 W)
+5VSB 1 A (5 W) 1 A (5 W) 1 A (5 W)
-12 V 0.5 A (6 W) 0.5 A (6 W) 0.5 A (6 W)
Total 451.6 W 476.8 W Fail
% Max Load 60.2% 63.6% Fail
Room Temp. 48.0° C 49.4° C Fail
PSU Temp. 39.4° C 40.5° C Fail
Voltage Regulation Pass Pass Fail
Ripple and Noise Pass Pass Fail
AC Power 554.0 W 590.0 W Fail
Efficiency 81.5% 80.8% Fail
AC Voltage 106.8 V 106.6 V Fail
Power Factor 0.997 0.996 Fail
Final Result Pass Pass Fail

Coolmax CUL-750B 750 W burned while we tried to pull 500 W from it at high temperatures. It burned exactly on the same point our first sample burned. The components that burned were one of the switching transistors and one of the +12 V rectifiers. Therefore, this unit shouldn’t be labeled as a 750 W unit, but as 475 W instead. Note that this unit has no protections to prevent it from burning. On next page we will show you the video from this power supply burning when delivering 500 W.

Efficiency was always above 80%, peaking 84.9% when we pulled around 200 W from it. Voltages were always within the allowed range and noise and ripple were always low. For instance, during test 18, with the unit delivering 500 W and before it burned noise level on +12VA was at 47.6 mV, on +12VB was at 51.4 mV, on +5 V was at 38.2 mV and on +3.3 V was at 37.4 mV. The maximum allowed is 120 mV on +12 V outputs and 50 mV on +3.3 V and +5 V outputs. All numbers are peak-to-peak figures.

[nextpage title=”The Power Supply Burning”]

Below you can see the power supply burning while we were pulling 500 W from it. After it burned, we tried to turn it back on, at no avail. Opening it we discovered that one of the switching transistors
and one of the +12 V rectifiers were the components that burned.

[nextpage title=”Main Specifications”]

Coolmax CUL-750B 750 W power supply specs include:

  • ATX12V 2.3
  • Nominal labeled power: 750 W
  • Measured maximum power: 476.8 W at 49.4° C.
  • Labeled efficiency: Not informed.
  • Measured efficiency: Between 80.8% and 84.9% at 115 V (nominal, see complete results for actual voltage).
  • Active PFC: Yes.
  • Modular Cabling System: Yes, partial.
  • Motherboard Power Connectors: One 20/24-pin connector and two ATX12V connectors that together form an EPS12V connector (permanently attached to the power supply).
  • Video Card Power Connectors: One cable with a six-pin connector (modular cabling system) and one cable with a six/eight-pin connectors (permanently attached to the power supply).
  • SATA Power Connectors: Two cables with two connectors each (one permanently attached to the power supply, one on the modular cabling system).
  • Peripheral Power Connectors: Two cables with two connectors each (one permanently attached to the power supply, one on the modular cabling system).
  • Floppy Disk Drive Power Connectors: One (on the peripheral cable from the modular cabling system).
  • Protections: over voltage (OVP) and short-circuit (SCP). Under voltage protection (UVP) present but not listed by the manufacturer.
  • Warranty: Three years.
  • Real Manufacturer: LongYi Electronics
  • More Information: https://www.coolmaxusa.com
  • Average price in the US*: USD 60.00.

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

[nextpage title=”Conclusions”]

We can’t understand how a manufacturer label a 450 W power supply as a 750 W and get away with it. We thought we had already got rid of this kind of thing here in the US, but we guess we were wrong.

Even if Coolmax CUL-750B could deliver its promised wattage it would still be a bad product, since its cables are short and it has only four SATA power connectors.

On the other hand, voltage regulation and noise/ripple levels are excellent, and efficiency stayed all the times above 80%. But please keep in mind that two samples from this unit burned when we pulled 500 W from it, so probably this unit would present efficiency below 80% if we were able to pull its labeled wattage.

If you take a closer look at our tests, you will see that this unit achieved its efficiency peak at 200 W. Efficiency is usually plotted as an inverse parabola, reaching its peak somewhere between 40% and 60% of its maximum wattage. Therefore, using the maximum efficiency we’ve seen in our tests, we could easily calculate that the maximum wattage for this power supply would be somewhere between 333 W and 500 W – matching with a fantastic precision what we found during our tests.

By the way: the manufacturer says the fan from this unit is 140 mm, but this is wrong, this unit uses a 135 mm fan.