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. 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 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. 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 hold-up time meets ATX spec requirements, which means that the proper capacity hold-up cap was used.

Inrush Current

Inrush current or switch-on surge refers to the maximum, instantaneous input-current drawn by an electrical device as 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.



The SS-520FL's inrush current is the lowest we have ever measured with our new equipment. This is a clear indication of a well-designed platform that effectively suppresses high inrush currents during the start-up phase.

Voltage Regulation and Efficiency Measurements

The first set of tests revealed the stability of the voltage rails and the efficiency of the SS-520FL. 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 also 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 unit managed to deliver its full power (and even more) at 43°C ambient. We tried to pull its full power at even higher temperatures, but anything above 44-45°C triggered OTP. Apparently, Seasonic preferred to stay on the safe side by setting the OTP trigger-point to a temperature much lower than 50°C. The good thing here is that the OTP works well by safely shutting the unit down before it overheats significantly, which will garner its components some more longevity. It took a while to raise the ambient temperature of our hot box because the SS-520FL operates at incredibly low temperatures due to its extra-high efficiency. The PSU's efficiency really is pretty amazing and exceeded 93% in five out of the nine tests. This is stellar even for a Platinum PSU. Seasonic is probably paving the ground for a new Titanium efficiency level!

Voltage regulation is also nothing less than great since all three major rails registered very low deviations. SS-520FL's 5V and 3.3V efficiency also managed to take the lead by beating out Corsair's high-end models that use digital control on those rails. As you can see, a top-notch design utilizing conventional components/methods can provide equally great or better performance than a DSP controlled design. The only downside of the SS-520FL's performance is its low PF at 20% load, which should exceed 0.9. However, a high PF is not that important to residential customers since they only pay for Real Power, not Reactive Power. Yet a high PF is still somewhat desirable because it would put less stress on the mains power grid.