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Enermax Digifanless 550W Power Supply Review

Transient Response Tests

In these tests, we monitor the response of the PSU in two different scenarios. First, a transient load (10A at +12V, 5A at 5V, 5A at 3.3V and 0.5A at 5VSB) is applied to the PSU for 200ms while the PSU is working at 20-percent load. In the second scenario, the PSU is hit by the same transient load while operating at 50-percent load. In both tests, we use our oscilloscope to measure the voltage drops caused by the transient load. The voltages should remain within the ATX specification's regulation limits.

These tests are crucial since they simulate the transient loads a PSU is likely to handle (such as booting a RAID array or an instant 100-percent load of CPU/GPUs). We call these tests "Advanced Transient Response Tests," and they are designed to be tough to master, especially for a PSU with a capacity below 500W.   

For details on our transient response testing, please click here.

Advanced Transient Response at 20 Percent


Advanced Transient Response at 50 Percent


The Enermax unit responded well to our Advanced Transient Response tests, registering low deviations on all rails. The only one that didn't fare well against the competition was the 5VSB rail. However, with a two-percent deviation, this is still a decent result.

Below are the oscilloscope screenshots we took during Advanced Transient Response testing.

Transient Response At 20-Percent Load

Transient Response At 50-Percent Load

Turn-On Transient Tests

We measure the response of the PSU in simpler scenarios of transient load—during the PSU's power-on phase—in the next set of tests. For the first measurement, we turn the PSU off, dial the maximum current the 5VSB can output and then switch on the PSU. In the second test, we dial the maximum load +12V can handle and start the PSU while it's in standby mode. In the last test, while the PSU is switched off (we cut off power or switch the PSU off by flipping the on/off switch), we dial the maximum load the +12V rail can handle before switching the PSU on from the loader and restoring power. The ATX specification states that recorded spikes on all rails should not exceed 10 percent of their nominal values (+10 percent for 12V is 13.2V and 5.5V for 5V).    

We noticed a small voltage overshoot on the 5VSB rail, but it was much lower (5.2 V) than the ATX specification's upper limit. On the +12V rail we measured small spikes, which are nothing to worry about since they are barely noticeable.

Aris Mpitziopoulos
Aris Mpitziopoulos

Aris Mpitziopoulos is a Contributing Editor at Tom's Hardware US, covering PSUs.