ASUS U-75HA 750 W Power Supply Review
By Gabriel Torres on October 30, 2009
ASUS is the number on motherboard manufacturer in the world and they’ve been expanding to other business for several years, recently reaching the power supply market. Though ASUS power supplies are not sold in the US, this didn’t prevent us from getting our hands on their 750 W product, which is sold throughout the world. Like the 500 W (P-50GA) and 650 W (U-65GA), models that we’ve already reviewed, this unit is manufactured by Delta Electronics. Does this power supply carry ASUS high-quality standards? Let’s see.
ASUS U-75HA is longer than the 650 W version, being 7 1/8” (180 mm) deep, having a 140 mm fan on its bottom (the 650 W comes with a 120 mm model, explaining why the 750 W model is externally bigger), active PFC and no modular cabling system.
All cables have a nylon protection, but sleevings don’t come from inside the power supply, as you can see in Figure 2. The cables included on U-75HA are:
The main difference between the 750 W and the 650 W models here is the presence of one additional cable with two six/eight-pin connectors for video cards on the 750 W version.
The cables have 19 ¾” (50 cm) between the power supply housing and the first connector on the cable and 5 ¾” (14.5 cm) between connectors, on cables with more than one connector.
All wires are 18 AWG. Interesting enough the 650 W model uses thicker 16 AWG wires on the +3.3 V outputs from the main motherboard cable, feature not duplicated on the 750 W model.
The number of connectors available is enough for you to build a mainstream PC or even a high-end system with two high-end video cards, but we’d prefer to see each video card power connector installed on an individual cable instead of sharing the same cable.
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.
Here we could see that the 650 W and 750 W models are not 100% identical. Although they share some similarities – which is normal to happen when you get two power supplies from the same manufacturer – we could spot enough differences to tell that the 750 W model is not the 650 W model with upgraded components.
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.
The metal component that looks like a regular power socket in Figure 7 is in fact a complete filtering circuit, with all required components listed above inside it, but the MOV.
ASUS U-75HA also has three ferrite coils, two X capacitors and four Y capacitors more than the minimum required, but it doesn’t come with a MOV (component in charge of removing spikes coming from the power grid), which is a sin. We expected it to have one, since it carries the name ASUS.
In the next page we will have a more detailed discussion about the components used in the ASUS U-75HA 750 W.
On this page we will take an in-depth look at the primary stage of ASUS U-75HA 750 W. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.
This power supply uses two D15XB60 rectifying bridges connected in parallel. Each bridge can deliver up to 15 A at 100° C if a heatsink is used (which is the case) or up to 3.2 A at 25° C is a heatsink is not used. So in theory you would be able to pull up to 3,450 W from the power grid; assuming 80% efficiency, the bridges would allow this unit to deliver up to 2,760 W without burning themselves out. Talk about 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.
Three 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.
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. ASUS U-75HA 750 W uses two 330 µF x 450 V capacitors in parallel; this is equivalent of one 660 µF x 450 V capacitor.
These electrolytic capacitors are from CapXon and rated at 85° C.
In the switching section, another two SPW20N60C3 power MOSFET transistors are used.
Instead of using one PFC/PWM combo chip, this power supply uses separated controllers. For controlling the active PFC circuit one ICE1PCS02 PFC controller is used, while for controlling the switching transistors one UC3845B PWM controller is used.
The primary from U-75HA is a little bit different from the primary from U-65GA. The 750 W version uses two 15 A bridges while the 650 W model uses a single 25 A component. The active PFC and switching transistors are identical, but the 750 W model has three transistors on the active PFC circuit and not only two like the 650 W model.Now let’s take a look at the secondary of this power supply.
ASUS U-75HA 750 W uses eight Schottky rectifiers on the secondary, making it to be completely different from the 650 W model from the same manufacturer (which has four rectifiers on the 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 three S60SC6M Schottky rectifiers (60 A, 30 A per internal diode at 118° C, 0.67 V voltage drop), one in charge of the positive part of the rectification and two in charge of the “freewheeling” part of the rectification (i.e., to discharge the coil). For our theoretical exercise we have to consider the path with the lower current limit, which is the direct rectification one, giving us a maximum theoretical current of 86 A or 1,029 W for the +12 V output.
The +5 V output is produced by two STPS30L45CW Schottky rectifiers connected in parallel (30 A, 15 A per internal diode at 135° C, maximum voltage drop of 0.50 V). This gives us a maximum theoretical current of 43 A or 214 W for the +5 V output.
The +3.3 V output is produced by another two STPS30L45CW Schottky rectifiers in parallel, giving us a maximum theoretical current of 43 A or 141 W for the +3.3 V output.
The eighth rectifier, an STPS2045CT (20 A, 10 A per internal diode at 155° C, maximum voltage drop of 0.57 V), is used for the standby (+5VSB) power supply.
Instead of using a monitoring integrated circuit this power supply implements a discrete solution using LM339 voltage comparators, so we couldn’t check what protections this power supply really has. The small daughterboard located on the secondary is in charge of providing the protections, controlling the fan, generating the power good signal and turning the power supply on and off.
In Figure 14, you can see the power supply label containing all the power specs.
This power supply has four rails, distributed like this:
This distribution is perfect, as it separates the CPU (ATX12V/EPS12V), the video card and all the rest on different 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 tests the +12V1 input was connected to the power supply +12V1 and +12V3 rails while the +12V2 input was connected to the power supply +12V2 rail.
5 A (60 W)
11 A (132 W)
16 A (192 W)
22 A (264 W)
31 A (372 W)
5 A (60 W)
10 A (120 W)
16 A (192 W)
21 A (252 W)
23 A (276 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
ASUS U-75HA can really deliver its labeled power at 50° C, which is great. If you pull between 40% and 60% from this power supply labeled power (between 300 W and 450 W) you will see very good efficiency between 83% and 84%. At light load (20% load, i.e., 150 W) efficiency was of 82.8%, not bad at all. At 80% load (600 W) efficiency dropped to 81.6%, still above the 80% mark. And at full load (750 W) efficiency dropped a little bit below 80%, at 79.2%. These are definitely far better results than the ones achieved by the 500 W and by the 650 W power supplies from ASUS.
This unit is 80 Plus-certified, but you need to keep in mind that this organization tests power supplies at 23° C (a temperature that is too low in our opinion), while we tested this power supply at 50° C. The higher the temperature, the lower efficiency is.
Voltage regulation was one of the highlights from ASUS U-75HA. During tests one through four all voltages stayed within 3% from their nominal values, i.e., closer to their nominal values than required, as the ATX specification allows voltages to be up to 5% from their nominal values. This includes -12 V, an output that usually doesn’t like to stay so close to its nominal value. During test five the +3.3 V output went outside this tighter range, but was still inside the maximum allowed (5%).
Noise and ripple levels stayed low all the times. Below you can see the results for test number five, when the power supply was delivering 748.9 W. The maximum allowed is up to 120 mV for +12 V outputs and up to 50 mV for +5 V and +3.3 V outputs. All numbers are peak-to-peak.
When we tried to pull more than 23 A from the +12V2 rail from this power supply the over current protection (OCP) entered in action shutting down the power supply. Since our load tester has only two inputs (and each one limited to 33 A), we were limited by our instrument, since we could “only” pull up to 33 A from the +12V1 input (which was connected to the +12V1 and +12V3 rails) and 23 A from the +12V2 input (more than that OCP would kick in, as explained). Thus it is possible that ASUS U-75HA can deliver even more than we were able to pull.
The main goal of our overload test is to see if the power supply burns or explodes and if its protections are active. Thus ASUS U-75HA 750 W passed this test.
33 A (396 W)
23 A (276 W)
28 A (140 W)
28 A (92.4 W)
3 A (15 W)
0.5 A (6 W)
% Max Load
ASUS U-75HA power supply specs include:
ASUS U-75HA uses a different project from ASUS U-65GA, achieving a better performance. From the three power supplies offered by ASUS, this one is the best.
For what its intended target audience – mainstream users – this unit provides a satisfactory performance, with efficiency between 83% and 84% if you pull between 150 W and 450 W from it, dropping to 82% at 600 W and 79.2% at 750 W. Voltage regulation was one of the highlights from ASUS U-75HA, with all voltages within 3% from their nominal values if you pull up to 600 W from it, i.e., closer to their nominal values than required, as the ATX specification allows voltages to be up to 5% from their nominal values. This includes -12 V, an output that usually doesn’t like to stay so close to its nominal value. During our 750 W test the +3.3 V output (and only this one) went outside this tighter range, but was still inside the maximum allowed (5%). Noise and ripple were very low at all timess. And plus it has its protections working just fine.
Of course coming from a brand like ASUS, we’d expected a product with a better performance – and probably that is why ASUS doesn’t offer this product in the United States – but they are clearly targeting more mainstream users on other markets.
There are many 750 W power supplies on the market better than this one, but if you are on budget it may be an option.