by Oleg Artamonov
08/30/2005 | 02:07 PM
Power supplies are usually tested in one of two ways: the PSU is either installed into a real computer or put on a special testbed. In the first case the tester has almost no control over the load on the PSU, so the results are usually not very informative and don’t allow making a general statement about the overall quality of a given power supply.
In the second case the tester fully controls the load and can try the PSU in any allowable mode of operation, but many readers think that the latter method produces purely academic results which allow comparing different PSUs between each other, yet do not make it clear how the PSU is going to behave in a real computer.
For this review I decided to combine both methods. That is, I will check the PSU on our testbed and then will connect it to a real computer, measure the power consumption and put appropriate marks on the diagrams I’ll have drawn based on the testbed data.
This way we’ll have a detailed view of the power supply’s characteristics and, on the other hand, we will see how these characteristics meet the demands of modern computer systems.
I want to note that I measure the consumption of each component independently, for different power rails of the PSU, since this is the only way of getting data appropriate for comparison with the numbers obtained on the testbed. If you want to know the total consumption of the whole system case, you should count in the PSU’s efficiency factor (typically about 70-80%) besides the numbers shown in the review.
I assembled two computers for my tests. They differ in their processors and, accordingly, in their mainboards and memory. These are typical configurations of modern top-end gaming systems, i.e. a single top-end graphics card, one gigabyte of memory, and a well-overclocked processor (the last thing may be not very typical, I should confess). The systems lack a DVD-RW drive, but this device doesn’t have a big share in the total power consumption of a computer much (moreover, it is usually used when the CPU and the graphics card are idle; I mean few people play Doom 3 burning DVDs in the background).
So, the Athlon 64 platform consisted of:
The Pentium 4 computer was assembled out of:
To measure the consumption, I cut four standard 75-millivolt/20-amperes shunt resistors (that is, the voltage drop of a 20 amperes current is 75 millivolts on this shunt) into the power supply’s cables (in +5, +3.3, and in both +12V rails; I will denote the mainboard and hard disk’s +12V line as 12V1 and the processor’s as 12V2). The shunts don’t affect the work of the system as the voltage drop on them is much lower than the allowable fluctuations of the PSU’s voltages. In other words, the computer just doesn’t notice them. I measured voltage on the shunts with a Uni-Trend UT70D multimeter at an 80-millivolt margin. The multimeter’s accuracy is ±(0.05% + 1 position); the declared accuracy of the shunts is 0.5%, but this is quite acceptable for our purpose.
The main danger for the tester is that the multimeter is intended for measuring direct current, while the current consumed by a computer includes periodic fluctuations as well as separate impulses at various frequencies besides the constant constituent. For example, the graphics card changes its power consumption a little depending on the frame rate and movements of the disk drive’s heads generate a group of pulses at frequencies up to several megahertz, and so on. This is why precise measurements of the consumed current can only be done with an oscilloscope with a passband of 10MHz (I don’t invent this number, but take it from the manufacturers’ recommendations). But we are not interested in measuring the consumption with an up to a fraction of a watt precision. This precision makes no sense for us as it doesn’t give us new and useful information, so it is quite possible to use a multimeter instead of an oscilloscope, but with the following reservation: the computer must be tested in a sustained state, i.e. in periods of uniform CPU and graphics card loads, with idle heads of the hard disk drives and so on.
That’s why I won’t test transient processes like when the computer is powered up or the hard drive is spinning up. Each test was running continuously, and in the case of game tests I performed the measurements on a second launch of the game since at the first launch the game usually loads up from the hard drive parts of the game level as it advanced through it.
I installed Windows XP on both computers and this OS served as the first test – power consumption in the idle mode. Then I used two CPU-loading tests (S&M and BurnK7) and two tests that loaded both the CPU and the graphics card (Doom 3 and 3DMark03).
The table below shows you consumption currents on different rails in all the mentioned tests. Just to remind you: the +12V1 line corresponds to the mainboard, graphics card and hard disk drive, and the +12V2 line to the processor.
It is already clear that the computer mostly loads the +12V rail. The currents on the +5V and +3.3V rails are too low in comparison with the load capacity of these lines (they are 25 and 20 amperes, respectively, even for a weak 250W ATX12V 1.2 power supply). As you may remember, it was high loads on the +3.3V rail that made the developers of the ATX standard introduce this power source into the power supply. Earlier, in the AT standard, this voltage had been generated right on the mainboard out of +5V. The load capacity of the +3.3V rail was growing steadily for the first four years of the existence of the ATX standard, reaching a maximum of 28 amperes (for a 300W unit), and special attention was given to the stability of this voltage (the standard described compensation of a voltage drop under loads only for this power rail).
But today it seems like we can soon abandon the +3.3V line altogether. The Athlon 64 mainboard still consumes a little power from this line, but the consumption of the LGA775 mainboard is negligible, being less than 3 watts.
It’s similar with the +5V rail: there was a time when it used to bear the main load and top-end power supplies allowed a current of up to 40-50 amperes on it, but now the load on the +5V line is less than 5 amperes. Well, this passing away of the low-voltage power lines is quite logical: consumed power being the same, consumption current is lower if voltage is higher. If modern processors and graphics cards were powered via the +5V or, worse yet, +3.3V rail, the number of wires in the power connectors would have to be doubled.
For better readability, I drew diagrams of the consumed power (not of the currents, as in the table above). The diagrams are drawn in the same scale, so you can compare the two computers.
Besides the obvious and repeatedly voiced statement that the Athlon 64 generates less heat than the Pentium 4, the diagrams also make the difference between the consumption on the +12V rail and on the +3.3 with +5V rails stand out. If you summed up 12V1 and 12V2 in one column (these rails are actually a single source inside the power supply), the columns of the low-voltage rails would be just lost in comparison.
These tests done, we can get closer to FSP’s power supply.
FSP’s BlueStorm series includes units for retail with wattage ranging from 350 to 460W. The junior models are identical to the well-known THN series that is distributed among OEMs. The senior model, AX500-A, with a wattage of 460W doesn’t have a direct analog among OEM products.
The power supply is shipped in a deep-blue cardboard box.
The same deep-blue or even violet color scheme is applied to the unit at large as well as to its elements: the Power On/Off button is highlighted with blue at work, the fan is also blue (but of a lighter hue than the case), and even the cables are put into plaited pipes of a blue color. The case has a matte coating.
The internals of this PSU have the typical look of FSP Group’s products. We’ve got the same PCB as with other THN series units and the same components, too. So, even though the AX500-A doesn’t have “THN” in its model name, we can regard it as belonging to this series all the same.
The PSU is equipped with a passive PFC device (you can see this coil in the upper left corner of the snapshot). The high-voltage rectifier’s capacitors have a 1000µF rating. The line filter is complete. The ribs of the heatsinks aren’t tall, but their foundation is thick enough, about 5 millimeters.
The unit offers the following cables:
A 12cm Protechnic Electric MGA12012HS-025 fan is installed in this PSU. The max rotational speed of this fan is 2500rpm at 38.3dB noise. My measurements proved that the fan reaches its max speed (and noise) when there is the highest load on the power supply:
On one hand, the fan speed is adjusted very effectively. It is steadily increased depending on the load (you may remember that some older models from FSP used to change the speed of their fan in a sudden jump from min to max at a load of about 150-250 watts). On the other hand, this power supply is not quiet. The fan is rather fast, and its air stream is audible even at 1500rpm.
The PSU’s efficiency is good, although not the best possible (about 75-80%). The power factor is quite typical for a product with passive PFC, reaching 0.83 at the maximum. To tell you the truth, it doesn’t differ much from the power factor of PSUs without any kind of correction.
As I wrote above, the AX500-A is a 460W unit, despite the number in its name. The allowable load power on the +5V and +3.3V rails is up to 150W (well, my measurements above have already shown that modern computers would be satisfied even with half of that power); the maximum allowable currents on +5V, +3.3V and +12V rails are 28, 30 and 15+16 amperes, respectively (like in other ATX12V 2.0 units, the +12V rail is split in two outputs here).
The cross-load characteristics of the PSU are good, but like other units without dedicated voltage regulation it cannot boast an ideal stability of the output voltages. The +12V voltage fluctuates the most, from 11.4 to 12.6V.
On the other hand, if we take a look at the four dots on the diagram that correspond to the min and max consumption of the above-described Athlon 64 and Pentium 4 systems (the measurements I performed at the beginning of the article were done for these four dots), you can see that the output voltages of the PSU are the same for each dot – even the relatively unstable +12V voltage changes by only about 1% from one dot to another.
This PSU is obviously not just sufficient, but even excessive for such powerful configurations, even though its wattage is considered rather ordinary today now that some manufacturers have offered 600W and higher products. Even if you add a second graphics card into the system it will only increase the power consumption on the +12V rail by 75 watts at most, and the output voltages will still be within acceptable ranges.
By the way, the diagram suggests that the problem with powering modern top-end computers is not about consumed currents, but rather about low quality of many power supplies. Even if we took an old ATX12V 1.2 unit with a maximum allowable current of 15 amperes, the Athlon 64 system would be satisfied (the Pentium 4 is more voracious). Thus, the problem is not about the formal wattage of the unit but about its ability to meet its own specification and to distribute those watts along the output power rails. For example, if we take a 300W unit of the ATX12V 2.0 standard with an allowable load on the +12V rail of 22 amperes or 264 watts, it will be enough even for the Pentium 4, on the condition that the power supply can really yield those 264 watts.
Talking about the BlueStorm AX500-A again, this PSU maintains stability of the output voltages well enough for a PSU of its class. It does deliver the specified power and can be used in advanced configurations that include two graphics cards and a top-end processor. Its output power is even excessive for a system with a single graphics card and one CPU.
The output voltage ripple at the frequency of the PWM regulator is very low when the PSU is under full load: about 15 millivolts on the +5V rail and about 40 millivolts on the +12V rail. This is three times below the maximum allowable value. Alas, the high-frequency ripple was accompanied with low-frequency one (at the double frequency of the power grid or 100Hz in our case) at loads above 300 watts.
If we add this ripple, the pulsation amplitude becomes almost 45 millivolts on the +5V rail and 60 millivolts on the +12V rail. These values are still within the acceptable range, but close to the limit on the +5V rail.
The tested BlueStorm AX500-A power supply is in fact a continuation of the successful THN series with the maximum output wattage increased to 460W. The PSU is neatly manufactured and has good characteristics; it is powerful even for very serious systems with two graphics cards and a top-end processor.
On the other hand, the AX500-A is not an ideal product since it is rather noisy at work. You won’t hear the air stream from its fan only in relatively low-power systems, but why would you want a powerful PSU for a weak configuration?
Thus, the AX500-A, like the rest of PSUs from FSP Group for that matter, is what I would call a typical “workhorse”. It is a reliable and inexpensive power supply for users whose priorities are stable operation and good quality.
The results of my measuring the power consumption of the test computers show that the +5V and +3.3V voltages have in fact become auxiliary in modern computer systems. The load on these rails is negligible in comparison with the load on the +12V rail. It means that power supplies of the ATX12V 1.2 and 1.3 standards, designed for high loads on the low-voltage rails, are becoming obsolete. Their capacity is ineffectively distributed along the output rails, so you’ll need a 400-500W power supply of the older standard where a 300W ATX12V 2.0 unit would be quite enough. Unfortunately, some unscrupulous manufacturers are already “redesigning” their older power supplies by editing their labels. They put a 24-pin connector and declare two +12V rails with, say, 10 and 15amp currents, but also mention that a combined load on both +12V rails cannot exceed 216W or 18 amperes, while a true power supply of the ATX12V 2.0 standard must be able to output the full power on both +12V rails for indefinitely long. And this full power in this theoretical example would be (10+15)*12=300 watts. The difference is really big, as you can see.
So, a power supply that fully complies with the ATX12V 2.0 standard – like the reviewed AX500-A – must not only have a 24-pin power connector and two +12V rails, but also be able to yield much more power on these two rails than an older-standard unit. Keep this fact in mind when you go shopping for your new PSU!