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. The AC source is a Chroma 6530, and it can deliver up to 3 kW of power. We also used a Keysight DSOX3024A oscilloscope, a Rigol DS2072A oscilloscope kindly sponsored by Batronix, a Picoscope 3424 oscilloscope, a Picotech TC-08 thermocouple data logger, two Fluke multimeters (models 289 and 175), a Keithley 2015 THD 6.5 digit bench DMM, and a lab-grade N4L PPA1530 3-phase power analyzer along with 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 three more oscilloscopes (Rigol VS5042, Stingray DS1M12, and a second Picoscope 3424) and a Class 1 Bruel & Kjaer 2250-L G4 Sound Analyzer we equipped with a type 4189 microphone that features a 16.6-140 dBA-weighted dynamic range. You will find more details about our equipment and the review methodology we follow in this article. We also conduct all of our tests at 40 °C - 45 °C ambient to simulate the environment seen inside a typical system more accurately, 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.

To control the Chroma 6530 source, we use a GPIB-USB controller, which avoids its very picky Serial port. This controller was kindly provided by Prologix.



We use an OLS3000E online UPS with a capacity of 3000VA/2700W to protect our incredibly expensive Chroma AC source.

Primary Rails Load 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 and represents the amount of time, usually measured in milliseconds, a PSU can maintain output regulations as defined by the ATX specification without input power. 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 specification sets the minimum hold-up time to 17 ms with the maximum continuous output load.

According to the ATX specification, the PWR_OK is a "power good" signal. This signal should be asserted as high, at 5V, by the power supply to indicate that the +12V, 5V, and 3.3V outputs are within the regulation thresholds and that sufficient mains energy is stored by the APFC converter to guarantee a continuous power operation within specifications for at least 17 ms. Conversely, PWR_OK should be de-asserted to a low state, 0V, when any of the +12V, 5V, or 3.3V output voltages fall below their under voltage threshold or when mains power has been removed for a sufficiently long time such that the PSU's operation cannot be guaranteed. The AC loss to PWR_OK minimum hold-up time is set at 16 ms, a lower period than the hold-up time described in the paragraph above, but the ATX specification also sets a PWR_OK inactive to DC loss delay which should be more than 1 ms. This means that the AC loss to PWR_OK hold-up time should always be lower than the PSU's overall hold-up time, which ensures that the power supply will never continue to send a power good signal while any of the +12V, 5V and 3.3V rails are out of spec.

In the following screenshots, the blue line is the mains signal, the green line is the "Power Good" signal, and the yellow line represents the +12V rail.







The PSU's hold-up time is long enough even though its bulk cap has a relatively low capacity, which has it meet the ATX spec's hold-up time requirements.

Inrush Current

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



Measured inrush current was quite high even though this unit has an NTC thermistor with a bypass relay.

Load Regulation and Efficiency Measurements

The first set of tests revealed the stability of the voltage rails and the SF-750F14EG's efficiency. The applied load was equal to (approximately) 10%-110% of the maximum load the PSU can handle, in 10% steps.

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 in the maximum load the +12V rail can handle. The load on the minor rails was minimal.

Load regulation is great on all rails but 5VSB, where it doesn't really matter as long as it is within the ATX spec's range. The PSU also easily delivered its full power for quite a while at a very high ambient temperature of almost 46 °C. The efficiency levels are satisfactory, but the fan's profile at light loads could afford to be more relaxed. We enabled the semi-passive option, but the fan was running at even 10% of the PSU's maximum-rated-capacity load because ambient temperatures were at 38 °C.