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 an Instek GPM-8212 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. 40 - 45°C are derived from a standard ambient assumption of 23°C and 17 - 22°C are added for the typical temperature rise within a system.

Primary Rails Voltage Regulation

The following charts show the voltage values of the main rails, which were recorded over a range of 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 without input power as defined by the ATX spec. It is, in other words, 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 ST45SF-G registered a hold-up time a little lower than the 16 ms threshold that the ATX spec sets; however, none of the PSUs we tested so far managed to surpass the above threshold.

Inrush Current

Inrush current or switch-on surge refers to the maximum, instantaneous input-current drawn by an electrical device when it is 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 or relays; as a result, the lower the inrush current of a PSU right as they are turned on, the better.

Voltage Regulation and Efficiency Measurements

The first set of tests revealed the stability of the voltage rails and the efficiency of the ST45SF-G. 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.


As you can see from the corresponding column of the above table, the fan starts with a rather high RPM and makes its presence felt, but it wasn't annoyingly loud until it surpassed 3000 RPM; that is, to our ears which aren't that sensitive. Efficiency at 20% load is on the low side for a Gold PSU, which may make someone wonder how this unit earned the 80 Plus Gold certification. Well, for starters, 80 Plus tests at a much lower ambient of only 23°C and efficiency is, at such low loads, actually higher with 115VAC, but 230VAC input takes the lead as the load increases. Efficiency never surpassed the 90% mark, but the 40%-60% load range was really close to it.

Regarding voltage regulation, the small PSU managed to keep it tight on all rails even when we dialed a load equal to 110% of its max-rated capacity.