CWT 750VH 750 W Power Supply Review
By Gabriel Torres on October 23, 2008


Introduction

Hardware Secrets Silver Award

CWT is the manufacturer behind several well known power supply brands, in particular Corsair and Thermaltake. Today we are going to do an in-depth test with their CWT-750VH model, which is a 750 W unit with modular cabling system and a 140 mm fan, to see if it can really deliver its labeled power and if it is identical to any power supply from Corsair or Thermaltake. Check it out.

We have already reviewed Corsair TX750W and disassembled Thermaltake Toughpower 750 W, which are also manufactured by CWT and also labeled at 750 W. So we will be able to see if the reviewed model from CWT is internally identical to Corsair TX750W or to Thermaltake Toughpower 750 W. Externally there are some differences. This model from Corsair does not feature a modular cabling system, feature present on the reviewed model from CWT and on Thermaltake Toughpower 750 W.

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Figure 1: CWT 750VH power supply.

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Figure 2: CWT 750VH power supply.

As you can see in Figure 1, this power supply uses a big 140 mm ball bearing fan on its bottom and a big mesh on the rear side where traditionally we have an 80 mm fan. We like this design as it provides not only a better airflow but the power supply produces less noise, as the fan can rotate at a lower speed in order to produce the same airflow as an 80 mm fan.

This power supply has active PFC, so it can be sold in Europe, and because of that it also features auto voltage selection. CWT says this unit has 80% minimum efficiency. Of course we will measure efficiency during our tests.

The main motherboard cable uses a 20/24-pin connector and comes inside the power supply housing, while all other cables use the available modular cabling system. On some power supplies with modular cabling systems  the EPS12V, ATX12V and video card auxiliary power cables come from inside the power supply housing, not using the modular cabling system, which is not the case with the reviewed unit.

This power supply comes with eight peripheral power cables: one EPS12V/ATX12V cable, two 6-pin auxiliary power cables for video cards, one 6/8-pin auxiliary power cable for video cards, three cables with two SATA power connectors each, one cable with four peripheral power connectors and one floppy disk drive power connector and one cable with three peripheral power connectors and one floppy disk drive power connector.

It is important to note that while this power supply comes with three auxiliary cables for video cards it has only two connectors for this kind of cable on its modular cabling system, so you can only install one or two video cards directly to this power supply, not three as you could assume (of course you can add more using adapters on the standard peripheral power plugs).

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Figure 3: Cables from the modular cabling system.

While the number of connectors provided by this power supply is enough for the average user, we think that users looking for a power supply on the 750 W range certainly have at least two video cards and since high-end video cards require two auxiliary power cables each it would be better if the manufacturer added support for four power cables for video cards instead of only two.

On this power supply all wires are 18 AWG. It would be nice to see thicker 16 AWG wires on a 750 W product.

On the aesthetic side the manufacturer used nylon sleevings on all cables, but the sleeving used on the main motherboard cable does not come from inside the power supply housing.

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

A Look Inside The CWT-750VH

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.

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Figure 4: Overall look.

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Figure 5: Overall look.

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Figure 6: Overall look.

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 stage this power supply is flawless, providing four extra Y capacitors, one extra X capacitor and one extra coil.

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Figure 7: Transient filtering stage (part 1).

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Figure 8: Transient filtering stage (part 2).

In the next page we will have a more detailed discussion about the components used in the CWT-750VH.

Primary Analysis

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

This power supply uses one GBJ1506 rectifying bridge in its primary, which support up to 15 A at 100° C. This component is clearly overspec'ed: at 115 V this unit 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 this component. Of course we are only talking about this component and the real limit will depend on all other components from the power supply.

The active PFC circuit from this power supply uses two power MOSFET transistors (20N60C3 – the same one used by several other power supplies we took a look). Each 20N60C3 can handle up 300 A @ 25° C each in pulse mode (which is the case) or 45 A @ 25° C or 20 A @ 110° C in continuous mode.

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Figure 9: Rectifying bridge and active PFC transistors.

On the switching section other two 20N60C3 power MOSFET transistors in two-transistor forward configuration are used. These are the same transitors used on Corsair TX750W but Thermaltake Toughpower 750 W uses a different set of transistors (IRFP460A), which has a lower current limit (80 A vs. 300 A in pulsating mode, both rated at 25° C). In other words, at least in theory the primary stage from the reviewed power supply can deliver more current (and thus power) to the secondary stage than Toughpower 750 W. The primary section of CWT-750VH is identical to TX750W’s.

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Figure 10: Switching transistors.

The primary is controlled by a CM6800G integrated circuit, which is a PWM/active PFC controller and is physically located on a small printed circuit board attached to the main board.

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Figure 11: Active PFC and PWM controller.

The electrolytic capacitor used on the active PFC circuit is Japanese from Hitachi, which is terrific. It is rated at 85° C (see Figure 11).

Secondary Analysis

This power supply uses four Schottky rectifiers on its secondary.

The +12 V output is produced by two STPS6045CW Schottky rectifiers connected in parallel. Each rectifier supports up to 60 A at 150° C (30 A per internal diode). The maximum theoretical current the +12 V 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 (which in this case is made by two 30 A diodes in parallel). Just as an exercise, we can assume a typical duty cycle of 30%. This would give us a maximum theoretical current of 86 A or 1,029 W for the +12 V output. The maximum current this line can really deliver will depend on other components, in particular the coil used.

The +5 V output is produced by one STPS40L45CW Schottky rectifier, which supports up to 40 A at 130° C (20 A per internal diode). The maximum theoretical current the +5 V 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 (which in this case is made by one 20 A diode). Just as an exercise, we can assume a typical duty cycle of 30%. This would give us a maximum theoretical current of 29 A or 143 W for the +5 V output. The maximum current this line can really deliver will depend on other components, in particular the coil used. This is the same rectifier used by Corsair TX750W and Thermaltake Toughpower 750 W.

The +3.3 V output is produced by another STPS40L45CW Schottky rectifier, which supports up to 40 A at 130° C (20 A per internal diode). The maximum theoretical current the +12 V 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 (which in this case is made one 20 A diode). Just as an exercise, we can assume a typical duty cycle of 30%. This would give us a maximum theoretical current of 29 A or 94 W for the +3.3 V output. The maximum current this line can really deliver will depend on other components, in particular the coil used.

Even though this power supply uses a separated rectifier for the +3.3 V output, it is generated from the same transformer output that feeds the +5 V rail, so the maximum current the +5 V and +3.3 V outputs can deliver together is limited by the transformer.

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

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Figure 13: Thermal sensor and +12 V rectifier.

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 model on the manufacturer’s website.

The thermal sensor is attached to the secondary heatsink and you can see it in Figure 13. This sensor is used to control the fan speed according to the power supply internal temperature. During our tests we could see the power supply fan gradually increasing its speed as the power supply temperature increased.

This power supply uses capacitors from Samxon on the secondary, a company from Hong Kong with manufacturing facilities in China. They are all rated at 105° C, as usual.

Power Distribution

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

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Figure 14: Power supply label.

According to the label, this power supply has four virtual rails. The difference between a single-rail power supply and a multi-rail one is the presence of individual OCP (over current protection) circuits on each +12V rail. On the manufacturer’s website there is no mention to OCP, however in our tests we could see that this protection was really there and active (at a value that we think was too high, however: 30 A; it should be set around 20 A in our opinion).

The rails are distributed like this:

Now let’s see if this power supply can really deliver 750 W of power.

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. Then we tried to pull even more power from this unit and the results for this test are in the next page.

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.

+12V2 is the second +12V input from our load tester and during our tests we connected the power supply EPS12V connector to it, which is connected to the power supply +12V1 and +12V2 rails. The +12V1 input from our load tester was connected to the +12V3 (main motherboard cable), +12V4 (peripheral connectors) and +12V2 (video card auxiliary cable) rails at the same time.

Input

Test 1

Test 2

Test 3

Test 4

Test 5

+12V1

5 A (60 W)

11 A (132 W)

17 A (204 W)

24 A (288 W)

33 A (396 W)

+12V2

5 A (60 W)

10 A (120 W0

15 A (180 W)

20 A (240 W0

22 A (264 W)

+5V

2 A (10 W)

4 A (20 W)

6 A (30 W)

8 A (40 W)

8 A (40 W)

+3.3 V

2 A (6.6 W)

4 A (13.2 W)

6 A (19.8 W)

8 A (26.4 W)

8 A (26.4 W)

+5VSB

1 A (5 W)

1 A (5 W)

1.5 A (7.5 W)

2 A (10 W)

2 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)

0.8 A (9.6 W)

Total

149.5 W

299.4 W

450.8 W

613.8 W

754.2 W

% Max Load

19.9%

39.9%

60.1%

81.8%

100.6%

Room Temp.

47.9° C

47.9° C

48.8° C

48.6° C

50.9° C

Voltage Stability

Pass

Pass

Pass

Pass

Pass

Ripple and Noise

Pass

Pass

Pass

Pass

Pass

AC Power

175.0 W

340.0 W

518.0 W

717.0 W

906.0 W

Efficiency

85.4%

88.1%

87.0%

85.6%

83.2%

Final Result

Pass

Pass

Pass

Pass

Pass

This power supply could not only deliver its labeled power at 50° C, but more than that (see results in the next page).

Efficiency was the highlight from this product. It could deliver at least 83% efficiency when fully loaded, peaking 88% when delivering 40% of its nominal 750 W capacity (300 W).

Ripple and noise where below the maximum allowed, even though models from some other manufacturers can deliver a lower noise level at their +12 V outputs when their units are delivering their full load. Just to remember, the maximum allowed for the +12 V outputs is 120 mV peak-to-peak and the maximum allowed for the +5 V and +3.3 V outputs is 50 mV peak-to-peak. Below you see noise levels for the reviewed power supply delivering 750 W (test number five).

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Figure 15: Noise level at +12V1 input from our load tester with the reviewed unit delivering 750 W (74.2 mV).

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Figure 16: Noise level at +12V2 input from our load tester with the reviewed unit delivering 750 W (83 mV).

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Figure 17: Noise level at +5 V input from our load tester with the reviewed unit delivering 750 W (14.2 mV).

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Figure 18: Noise level at +3.3 V input from our load tester with the reviewed unit delivering 750 W (21.6 mV).

Now let’s see how much power we could pull from this unit keeping it working inside ATX specs.

Overload Tests

Before overloading the power supply we always test to see if the over current protection (OCP) circuit is active and at what level it is configured.

To test this we configured our load tester to pull only 1 A from its +12V1 input and 33 A from its +12V2 input, which this time was connected to the power supply ATX12V connector (we used the ATX12V connector because it was the only cable connected to the power supply +12V1 rail; the EPS12V connector is connected to the +12V1 and +12V2 rails at the same time). Under this scenario (i.e., pulling a lot of current from one of the power supply’s +12V rails) the power supply have to shut down, what happened, so OCP was present and active. We started lowering the current until we could turn on the power supply to determine at what level OCP was configured. This was accomplished at 29 A. We think that OCP was configured at a level that is too high, in our opinion it should be configured at a lower value like 20 A.

Connecting the EPS12V connector back on our load tester and starting from test number five shown in the previous page, we started increasing currents to see the maximum amount of power we could pull from this power supply with it still working inside ATX specs.

The maximum we could pull from this unit is summarized in the table below.

Input

Maximum

+12V1

33 A (396 W)

+12V2

33 A (396 W)

+5V

14 A (70 W)

+3.3 V

14 A (46.2 W)

+5VSB

3 A (15 W)

-12 V

0.8 A (9.6 W)

Total

940 W

% Max Load

125.3%

Room Temp.

45° C

AC Power

1,170 W

Efficiency

80.3%

We were impressed by these results. This power supply could not only deliver 25% more than labeled (940 W) but also could keep efficiency at 80% under this extreme condition.

On the other hand, this power supply doesn’t seem to have over load protection (OPP or OLP), as usually when a power supply has this protection it shuts down if you try to pull more than 20% of its labeled power.

Ripple and noise increased a lot, to 90 mV at +12V1 input from our load tester and to 97 mV at +12V2 input from our load tester. Even though these numbers are high, they are still under the maximum allowed (120 mV).

Short circuit protection (SCP) worked fine for both +5 V and +12 V lines.

Main Specifications

CWT-750VH power supply specs include:

Conclusions

This is a good power supply, using the same design as Corsair TX750W and Thermaltake Toughpower 750 W. It not only delivers 750 W at 50° C but we could pull up to 940 W at 45° C from this unit, which is remarkable. So basically you are buying a 750 W power supply and bringing home a 940 W unit.

 It is an excellent alternative to these two popular products, especially because of its lower price tag. It is internally identical to Corsair TX750W, but coming with a modular cabling system, feature not found on Corsair’s model. Thermaltake Toughpower 750 W also has a modular cabling system, but this power supply from Thermaltake uses transistors with lower current limits on the switching section (all other components are identical), so CWT-PV750 is a better product.

The highlight of this product is its efficiency, always above 80%, even when we overloaded it (a lot, by the way). If you pull around 40% of its nominal power (300 W) you will see an outstanding 88% efficiency. At full load, it presented 83% efficiency, which is a terrific result.

Noise and ripple could be lower, but they were still below the maximum allowed.

This power supply, however, isn’t flawless: there are two drawbacks you should be aware of. First, apparently it doesn’t feature over load protection (OPP/OLP). This feature is desirable but since it can deliver far more than it is labeled, almost nobody will miss this. The second problem is the support for only two video card power cables. This is a major problem, because high-end video cards require two auxiliary power connectors, so you will be able to connect directly to your power supply only one high-end video card or two mid-range models. A power supply on this power range should come with four video card power connectors. Of course you can solve this issue installing adapters on the peripheral power connectors, but this will lower the number of available peripheral power plugs.

If what we said above does not bother you, you can go ahead and buy this product.

Originally at http://www.hardwaresecrets.com/article/CWT-750VH-750-W-Power-Supply-Review/635


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