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 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 very good and easily exceeds the minimum allowed by the ATX spec. Many much more expensive PSUs fail this test, but the VP550F didn't have any problems at all.

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 inrush current of the VP550F is higher than that of other units with a similar capacity, but its values are still at normal levels.

Voltage Regulation and Efficiency Measurements

The first set of tests revealed the stability of the voltage rails and the efficiency of the VP550F. The applied load was equal to (approximately) 20%, 40%, 50%, 60%, 80% and 100% 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.


For starters, the PSU didn't allow us to draw more than its rated capacity. 10% more than its full power had voltages on all rails, especially at +12V, drop dead low and ripple go sky-high. This is a clear indication that the unit's capacity of 550 W is on the verge of this platform's capabilities. Take a look at the +12V voltage regulation at full power and during our CL2 tests and you will most likely figure out that a maximum capacity of 450 W would net this unit much better overall performance. Also, the horrible results in the CL1 test, where we deliberately applied the minimum load to the +12V rail and the maximum load to the minor ones, shows that this unit isn't compatible with the new C6 and C7 sleep states of the Haswell processors; it failed Intel's testing procedure.

Voltage regulation on this rail managed to stay within 3% regardless of the significant drop at +12V during the 80% to 100% load test. Voltage regulation was very good on all the other rails and can easily compete with much more expensive units. The PSU also had no problems delivering its full power at even above 45°C ambient, and the registered efficiency was high enough for a unit that is only certified for the basic 80 Plus level.