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°C-45°C ambient in order to simulate with higher accuracy the environment seen inside a typical system, 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 of 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, representing the amount of time, usually measured in milliseconds, a PSU can maintain output regulations as defined by the ATX spec 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 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 was very low—lower than 10 ms. The bulk caps are apparently not up to the task.

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.



Though hold-up time was low, a clear indication of lower-than-needed bulk cap capacity, the registered inrush current was close to 45 A. This PSU most likely needs a thermistor with higher resistance to further restrict inrush current.

Voltage Regulation and Efficiency Measurements

The first set of tests revealed the stability of the voltage rails and the efficiency of the CMX850. The applied load was equal to (approximately) 20%, 40%, 50%, 60%, 80%, 100%, and 110% of the maximum load 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 the +12V rail could handle while the load on the minor rails was minimal.

Voltage regulation is nothing to write home about, and the 3.3V rail also cracked the 4% threshold, which is not good at all. The fan was definitely working extra hard during these tests to keep the unit cool, which produced a bunch of noise sure to annoy most of you. But we didn't have any problems during the full-load and overload tests, irregardless of the ambient inside our hotbox reaching more than 45°C. Efficiency registered by the CMX850 was also at the levels Cougar promised, which is pretty satisfactory for a modern Bronze unit. The PSU hit 89% efficiency with typical loads and 230 VAC, so it really will save the average user enough energy to make investing in a much more expensive Gold or Platinum unit a waste of money.