Test Setup

All measurements were performed using two Chroma 6314A mainframes equipped with the following electronic loads: six 63123A [350 W each], one 63102A [100 W x2], and one 63101A [200 W]. The aforementioned equipment is able to deliver 2500 W of load, and all loads are controlled by a custom-made software. We also used a Picoscope 3424 oscilloscope, a Picotech TC-08 thermocouple data logger, a Fluke 175 multimeter, and a Yokogawa WT210 power meter. We also included a wooden box, which, along with some heating elements, was used as a hot box. Finally, we had at our disposal four more oscilloscopes (Rigol 1052E and VS5042, Stingray DS1M12, a second Picoscope 3424, and a CEM DT-8852 sound level meter). In this article, you will find more details about our equipment and the review methodology we follow. Finally, we conduct all of our tests at 40-45°C ambient in order to simulate with higher accuracy the environment seen inside a typical system, with 40-45°C being derived from a standard ambient assumption of 23°C and 17-22°C being added for the typical temperature rise within a system.

Primary Rails Voltage Regulation

The following charts show the voltage values of the main rails, recorded over a range from 60W to the maximum specified load, and the deviation (in percent) for the same load range.

5VSB Regulation

The following chart shows how the 5VSB rail deals with the load we throw at it.

Hold-up Time

The hold-up time is a very important characteristic of a PSU and represents the amount of time, usually measured in milliseconds, that a PSU can maintain output regulations as defined by the ATX spec without input power. In other words, it is the amount of time that the system can continue to run without shutting down or rebooting during a power interruption. The ATX spec sets the minimum hold-up time to 16 ms at maximum continuous output load. In the following screenshot, the blue line is the mains signal and the yellow line is the "Power Good" signal. The latter is de-asserted to a low state when any of the +12V, 5V, or 3.3V output voltages fall below the undervoltage threshold, or after the mains power has been removed for a sufficiently long time to guarantee that the PSU cannot operate anymore.



The PSU failed this test. It didn't achieve a hold up time of 16 ms or higher. Unfortunately, this is the case for most PSUs we have tested to date, so it is not a big surprise to us.

Inrush Current

Inrush current or switch-on surge refers to the maximum, instantaneous input-current drawn by an electrical device when first turned on. Because of the charging current of the APFC capacitor(s), PSUs produce large inrush-current right as they are turned on. Large inrush current can cause the tripping of circuit breakers and fuses and may also damage switches, relays and bridge rectifiers; as a result, the lower the inrush current of a PSU right as they are turned on, the better.



The small APFC cap, although it didn't enable the PSU to pass the hold-up time test successfully, contributed significantly to the low inrush current the unit registered.

Voltage Regulation and Efficiency Measurements

The first set of tests revealed the stability of the voltage rails and the efficiency of the PowerX-550. The applied load was equal to (approximately) 20%, 40%, 50%, 60%, 80%, 100%, and 110% of the maximum load that the PSU can handle. In addition, we conducted two more tests. In the first test, we stressed the two minor rails (5V and 3.3V) with a high load while the load at +12V was only 2 A, and, in the second test, we dialed the maximum load that the +12V rail could handle while the load on the minor rails was minimal.


The PSU easily delivered its full power at over 45°C, proving that it can deal efficiently with heat. Also, its fan profile isn't aggressive, and we had to stress the PSU really hard to make the fan operate at full speed, nor did the high operating temperatures prevent the rails from registering very good voltage regulation, which is impressive since the unit belongs to the mainstream category. The 5V rail surprised us by managing to surpass the corresponding performance of many high-end PSUs that cost a lot more. To be frank, we didn't expect to find a rail with a deviation below 1% in a budget-oriented PSU.

The PowerX-550's efficiency doesn't impress, since it registered the normal efficiency readings we would expected from a typical Bronze unit with 230VAC input. Its sweet spot is around 40-50% of its maximum-rated-capacity load, where it registers close to 86% efficiency. Users with a 115VAC power grid should also expect 1-1.5% lower efficiency throughout most load ranges.