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, and 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 with the 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 hold-up time is ridiculously low and currently the lowest we have ever measured. The APFC capacitor is apparently too small for the task. Unfortunately, it is a common practice for most manufacturers to use a small APFC cap in order to save on cost since an increase in capacity and quality of these particular caps (the hold-up ones) comes with an increase in cost. Manufacturers should, nevertheless, also respect the ATX spec requirements by providing a hold-up time that is at least close to 16 ms, not way under 10 ms.

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 only advantage of a small APFC cap, besides its obviously low cost, is the low inrush current that it creates during the PSU's startup phase, but inrush current can also be suppressed effectively with high capacity hold-up caps if a proper design is utilized.

Voltage Regulation and Efficiency Measurements

The first set of tests revealed the stability of the voltage rails and the efficiency of the ST50F-ES. The applied load was equal to (approximately) 20%, 40%, 50%, 60%, 80%, 100%, and 110% of the maximum load that the PSU can handle. We conducted two additional 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. 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 small PSU managed to deliver its full power (and more) at operating temperatures over even 45°C. Here we should note that the low efficiency rating of the PSU is a cause for high energy dissipation, so we had to shut the heating elements of our hot box down at high loads to keep the temperature close to those normal to our testing environment.

Voltage regulation on all rails is surely not the best we have ever seen, but you can't call it bad either since all outputs, except for 5VSB, were within 3%. We can't expect such a budget unit to generate a voltage regulation below 1%, so we are satisfied with its overall performance in this area. Unfortunately, the fan profile is aggressive at high ambient temperatures, which we can't fault because the PSU generates a lot of heat through its internals. The fan had to move almost 110 W of heat out of the PSU's enclosure during our full load test, for example, which is, without a doubt, a daunting task. The fan essentially had to work over hours at high RPM to keep the unit safe.