ASUS Power Supply Units: Three Models Reviewed

We have already tested ASUS power supply units once in our lab back in the far away 2004. Today we are going to check out three new products from this manufacturer ranging from 550 to 750 W.

by Oleg Artamonov
01/15/2009 | 10:45 AM

We tested power supplies from ASUS as long ago as 2004. ASUS has forgotten this market since then or its PSUs just have not come our way. But today we are going to introduce to you three new PSUs from that brand: two mainstream 550W models and a top-end 750W one. The difference between them goes far beyond the numbers on the labels!

Testing Methodology


Click the following link for a description of our testing methodology and equipment and a brief explanation of what the specified and tested parameters of power supplies mean: X-bit Labs Presents: Power Supply Units Testing Methodology In-Depth. If you feel overwhelmed with the numbers and terms this article abounds in, check out an appropriate section of the mentioned article for explanation.

You can check out other PSU models we have tested in our labs in the Cooling/PSU section of our site.

ASUS A-55GA (550W)

The 550W A-55GA power supply is a medium-wattage model in ASUS’s product range.

It comes in a small blue-painted cardboard box. A brief specification is listed on the back of the box: the allowable loads and currents on every of the power rails. Running a little ahead, I want to note that a splittable 20+4-pin mainboard connector is listed among the PSU’s features but my sample had a monolithic 24-pin connector.

The PSU is designed in the standard form-factor with a length of 140 millimeters. There won’t be any installation problems even with very cramped system cases. There are no remarkable features in the appearance of the A-55GA except for the orange On/Off switch. Such a switch can be seen with many PSUs manufactured by AcBel Polytech.

One look inside the PSU is enough to identify it as an AcBel. The A-55GA is based on AcBel’s popular platform that is used in PSUs selling under AcBel’s own brand as well as in PSUs from other brands, e.g. Cooler Master. The platform is not very new but meets the basic requirements of today.

The PSU is based on an ML4800CP chip which is a controller of both active PFC and main regulator. The input circuitry looks odd but does not differ fundamentally from other PSUs. There are just two parallel-connected high-voltage capacitors instead of one, and an E-core choke (it looks like the PSU’s power transformer) of active PFC instead of a toroidal choke. The output circuitry follows the classic design with joint voltage regulation.

Capacitors from LTEC Ltd. are installed at the output.

I can find no fault with the assembly quality of this PSU. AcBel has always been good in this respect.

The A-55GA is cooled with an MGT12012HB-025 fan from Protechnic Electric and equipped with the following cables and connectors:

The selection of connectors is good except that the SATA cable is only one. In many system cases it is going to be impossible to connect both a hard drive and a DVD drive without adapters. The abundance of Molex connectors is going to be uncalled for, however. I guess the manufacturer should have replaced one of the PATA power cables with a SATA one.

The PSU has a high load capacity of the +5V and +3.3V rails but the +12V power rail, the most demanded one in a modern computer, has a maximum current of 30A only. It means that the maximum effective output power of this PSU, i.e. the power it can yield considering the typical distribution of load among the different power rails, is about 400 watts.

The PSU worked without problems at a load of 515W in my tests.

When working with an APC SmartUPS SC 620, the PSU was rather unstable: the UPS switched to the batteries normally at a load of 350W but then produced a gurgling sound, reported overload and shut down. It was only at a load of 300W and lower that the pair could work normally on the batteries. In this case, when the switching to the batteries is performed well but the operation is unstable, you can solve the problem by using a more expensive UPS with a sinusoidal output voltage.

The output voltage ripple is within the allowable limits, but there are also high occasional spikes in the oscillogram.

The output voltages are not very stable. The voltages on the +12V and +5V rails sag under load. And if the sagging +5V voltage is not a problem for a modern PC, the reduction of the +12V voltage by over 3% below the nominal value may make some graphics cards unstable. Thus, the maximum reasonable load on this PSU’s +12V rail is 250-300 watts. This is more than enough for a configuration with a fast CPU and one mainstream graphics card, though.

The PSU is about 80% efficient. It even hits 82% at one point of the graph but then its efficiency droops again as the load grows up. That’s an acceptable result, but you can see PSUs with an efficiency of 85-87% today.

The PSU is very quiet under low loads but the fan starts to accelerate linearly at loads above 150W, reaching 1700rpm at 300W. As a result, the A-55GA is just average in terms of noisiness.

Summing it up, the A-55GA is a typical midrange power supply suitable for configurations with one mainstream graphics card. The effective output power of this PSU is about 400W or even less considering the instability of the output voltages. It is not quiet and there is nothing exceptional in its other characteristics. And it has got a lot of cheaper competitors in its product class.

ASUS P-55GA (550W)

Differing from the previous model with only one letter in its name, the P-55GA is actually quite a different power supply.

The box is the same size but more colorful and attractive than the previous model’s, but how many customers choose their power supply by its box, anyway? PSUs selling apart of a system case are usually bought by experienced users who repair or upgrade their PCs with their own hands. For such users it is the parameters, not the appearance, that are the most important.

The PSU is as large as the previous model (140 millimeters in length) but the case has changed dramatically, starting from the color of the On/Off switch and ending with the design of the cover. When you are taking this model apart, you must move the cover backward rather than lift it up.

The internal design has little to do with the A-55GA, too. Although the overall functionality and characteristics of the two models coincide, the P-55GA uses a different platform from a different manufacturer. The latter can be easily identified by the UL certificate number on the PSU label. It is Delta Electronics. The PSU uses a common design with an active PFC device at the input and joint voltage regulation at the output. The assembly quality is high.

CapXon capacitors are installed in the PSU’s low-voltage section.

The PSU is cooled by a 120x120x25mm fan (Yate Loon D12SH-12) with a rated speed of 2200rpm.

It is equipped with the following cables and connectors:

There are obvious improvements over the A-55AG: two SATA cables and universal 6+2-pin connectors for graphics cards. A modern configuration can be connected to this PSU without any adapters.

The load capacity of this PSU doesn’t differ much from that of the A-55GA. The allowable load on the +12V rail is the same 360W, so the total effective output power of this PSU, i.e. the power it can yield considering the typical distribution of load among the different power rails, is about 400 watts.

The PSU worked without problems at loads up to 527W in my tests.

Together with an APC SmartUPS SC 620 the PSU could work at loads up to 410W when powered by the mains but was not stable when powered by the batteries (the UPS reported overload and shut down after 10-15 seconds of battery operation even at a load of 200W).

The output voltage ripple is within the permissible limits at full load.

The cross-load diagram is not a very pretty sight. The +12V voltage goes through the entire range of allowable values from 11.4V to 12.6V. Thus, this PSU should not be loaded by more than 250-300W. Anyway, even with this limitation the P-55GA can power up a gaming system with a mainstream graphics card whose power draw is not higher than 100W. The PSU will have a sufficient reserve of output power then.

The PSU is just as efficient as the previous model, reaching 83% at one point of the graph. But its average efficiency is about 80%. That’s a normal result as today’s PSUs go.

The fan is rotating at a constant speed of about 1150rpm at loads up to 300W. Its noise is within comfortable limits then. The A-55GA and P-55GA are similar in terms of noisiness. The former is quiet under low loads while the latter sounds more agreeable at medium and high loads.

Summing up, I can tell you that the P-55GA differs from the A-55GA with the cables: its selection of cables and connectors suits a modern PC better. The rest of the differences are insignificant. Both PSUs offer an effective output power of 350-400W. Both are average in terms of noisiness and have low stability of the output voltages. The P-55GA was only much inferior to the A-55GA in the UPS test.

ASUS “Vento” U-75HA (750W)

The 750-watt U-75HA will complete this review.

Its box is larger and heavier than the boxes of the previous models. Two model names are indicated on it in large print: U-75HA and U-65HA. You have to look at the table on the side panel to learn what exactly model is hidden within.

The PSU itself is larger, too. Its length is 180 millimeters as opposed to the previous models’ 140. Well, I don’t think anyone would try to install a 750W power supply into a microATX system case, so the large size shouldn’t be a problem.

There is no free space inside. The PCB occupies the entire case. The power supply is equipped with active PFC and, unlike the previous models, features dedicated voltage regulation. You can see this by the three (instead of two) large toroidal chokes in the low-voltage section (the top right of the photograph).

I have no complaints about the assembly quality of this model, either. Everything is neat, the cables are tied up with straps, and large elements are secured with drops of glue.

Widely reputed KZH series capacitors from United Chemi-Con are installed at the PSU’s output. Expensive PSUs make use of the KZE series as a rule, but the KZH series differs with even lower equivalent resistance, which means better filtering of high-voltage pulsation.

One component draws your attention with a loud click whenever you turn the PSU on or off. It is an electromagnetic relay capable of switching up to 250VAC at a current up to 16A.

The relay helps reduce the power consumed by the PSU in idle mode. Although the main regulator is shut down then, the active PFC device of a computer PSU goes on working uselessly, consuming some power. But PFC is not necessary at all for idle mode because the +5V standby source is powered directly by the mains.

To avoid this loss, the U-75HA has a relay in its high-voltage section right before PFC. This relay can disable the PFC device when necessary.

The PSU is cooled by a 135x135x25mm fan (Adda ADN512UB-A91). The impeller looks interesting: nine wide blades with small gaps in between.

The PSU is equipped with the following cables and connectors:

There is nothing to find fault with: the PSU is meant for two graphics cards (which corresponds to its output power) and offers a sufficient number of SATA connectors on two cables. Every cable is sleeved.

The PSU can yield a total of almost 650W across its four “virtual” +12V lines, which is close to its total output power. This is good because the effective output power of the above-discussed A-55GA and P-55GA is far lower than the specified one due to the low load capacity of the +12V rail.

The actual maker of this PSU can be identified by the UL certificate number (in the bottom left of the label). It is Delta Electronics. In fact, Delta offers the GPS-750AB-A model which is identical to the U-75HA in specs and connectors.

The PSU worked continuously under a load of 725W in my tests. It was also stable with my APC SmartUPS SC 620 at loads up to 365W and 335W when powered by the mains and the batteries, respectively.

The output voltage ripple is close to the allowable maximum on the +3.3V rail but low on the other two rails.

The +12V voltage is very stable, only deflecting by 1% in a very small part of the diagram. The +5V is somewhat worse, keeping within a 3% deflection. The +3.3V voltage sagged below the permissible limit but only under extremely high loads. This is a visible difference from the previous two models!

The PSU reaches 80% efficiency at 100W (13% from the maximum) and then goes as high as 87%. The power factor is very close to 1.

The fan speed is no higher than 900rpm at loads below 500W, making the PSU quiet, almost silent. Then the speed grows up. The rumble of the impeller and the hiss of the airflow are audible at a load of 600W (fan speed of 1200rpm). The PSU is quiet overall.

Thus, the ASUS Vento U-75HA boasts good characteristics including high output power (I mean the effective, not just specified, output power), good stability of the output voltages, high efficiency, and quiet operation. This PSU can be recommended for top-end gaming stations as it can easily handle a couple of graphics cards (or one dual-GPU card like Radeon HD 4870 X2).


I have in fact tested two classes of power supplies in this review: the relatively inexpensive A-55GA and P-55GA, and the high-wattage U-75HA.

The first two PSUs are similar to each other and are meant for mainstream PC configurations. They don’t look like a good buy to me because their parameters are not exactly good and their specified output power is overstated (the real effective output power of these PSUs is about 400W). They also seem to be somewhat more expensive then their market alternatives.

As opposed to them, the Vento U-75HA is free from serious drawbacks. Its effective output power corresponds to the specification, and it is stable and quiet. Hopefully, its pricing will make ASUS competitive to other brands on the PSU market.