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. You will find more details about our equipment and the review methodology we follow in this article,. Finally, we conduct all of our tests at 40°C-45°C ambient to simulate the environment seen inside a typical system as accurately as possible, with 40°C-45°C being derived from a standard ambient assumption of 23°C and 17°C-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 60 W 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

Hold-up time is a very important PSU characteristic. It represents the amount of time a PSU can maintain output regulations as defined by the ATX spec without input power and is usually measured in milliseconds. In other words, it is the amount of time 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 measured hold-up time is close to the minimum allowed time the ATX spec specifies, so we won't whine a lot about its failure to pass this test. However, we will still deduct some performance points for missing the 16 ms threshold by 1 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 it is turned on, the better.



The registered inrush current was higher than it would normally be for a medium-capacity PSU, which is pretty strange because the NTC thermistor is quite large. But 40 A doesn't pose a threat to properly working circuit breakers and fuses, so you have nothing to worry about.

Voltage Regulation and Efficiency Measurements

The first set of tests revealed the stability of the voltage rails and the efficiency of the V550S. 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 0.10 A. This test reveals whether the PSU is Haswell ready or not. In the second test, we dialed the maximum load that the +12V rail could handle while the load on the minor rails was minimal.

Efficiency was crazily high for a Gold unit, especially up to our 60% load test, but it started to decrease at higher loads, which goes to show that this unit's efficiency is tuned around lower loads. Also, despite the 40°C limit on papers, the PSU had no problem whatsoever delivering its full power and even more at 45°C-46°C. This PSU runs a fairly relaxed fan profile, and we had to push the unit really hard for the control circuit to nearly maximize RPM, where the noise output was significant. The fan this small unit is equipped with is powerful, too strong for such a highly efficient PSU, which is the reason for its unusually low RPM under normal conditions. Finally, voltage regulation on all rails was very good—the +12V rail registered a reading within 1%.