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 capable of delivering 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.

We use a GPIB-USB controller to control the Chroma 6530 source, 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 loads 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 a 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, PWR_OK is a "power good" signal. This signal should be asserted as high on the 5V rail 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 system's continuous operation for at least 17ms. 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 the under-voltage threshold or when mains power has been removed for long enough to guarantee that a power supply isn't operating anymore. The AC loss to PWR_OK minimum hold-up time is set to 16 ms, which is less than the hold-up time described above, but the ATX specification also sets a PWR_OK inactive-to-DC loss delay that should be higher than 1 ms. This means that the AC loss to PWR_OK hold-up time should be lower than the PSU's overall hold-up time to ensure that the power supply doesn't send a power good signal once any of the +12V, 5V and 3.3V rails are out of spec.

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







This PSU's hold-up time is incredibly long. Seasonic wasn't messing around when it stated that it exceeds 30 ms.

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.



Inrush current is quite high because of the bulk caps' increased capacity. Seasonic should use a larger thermistor to keep it below 50 A.

Load Regulation and Efficiency Measurements

The first set of tests revealed the stability of the voltage rails and the SSR-750TD'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 the maximum load the +12V rail could handle while the load on the minor rails was minimal.

Load regulation is incredibly tight on all rails, and efficiency is out of this world. Usually, only either load regulation or efficiency is great, but Seasonic's engineers somehow managed both. This platform blows any other Titanium platform out of the water, including those of Super Flower's top-notch Leadex units. The Prime 750 also has no problem at all in delivering astonishing performance under very high ambient temperatures. All parts are way over spec, which has them easily handle such tough conditions.

Take a closer look at the table above and you will notice that the cooling fan actually slows down at 70% and 80%, the unit's max-rated capacity, which also quietens down the fan. This happens because the fan doesn't increase its speed linearly as it should when its input voltage increases from 2.4 V to a little over 3 V; it drops in speed instead. We don't know the reason behind this strange design choice. There's probably a trick to allow for the fan's low start-up voltage since 2.4 V is quite low for a 135mm fan, and it somehow leads to the fan's strange speed curve with low voltage input.