Advanced Transient Response Tests

These tests monitor the response of the PSU in two different scenarios. First, a transient load (10 A at +12V, 5 A at 5V, 5 A at 3.3V, and 0.5 A at 5VSB) is applied to the PSU for 200 ms while the latter is working at 20% load. In the second scenario, the PSU, while working at 50% load, is hit by the same transient load. In both tests, we measure the voltage drops the transient load causes using our oscilloscope. The voltages should remain within the regulation limits defined by the ATX specification. We must stress here that the above tests are crucial because they simulate transient loads a PSU is very likely to handle (e.g., booting a RAID array, an instant 100% load of CPU/VGAs, etc.). We call these tests "Advanced Transient Response Tests", and they are designed to be very tough to master, especially for PSUs with capacities below 500 W.





The V550S performed well, though its relatively small capacity is a significant handicap in these tests. It only registered small deviations on all rails, with +12V, the most important rail of all, being close to 1% on both tests. We were left pretty satisfied by its Advanced Transient Response performance.


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

Transient Response at 20% Load

Transient Response at 50% Load

Turn-On Transient Tests

We measure the response of the PSU in simpler scenarios of transient loads—during the power-on phase of the PSU—in the next set of tests. In the first test, 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 the PSU is in standby mode. In the last test, while the PSU is completely switched off (we cut off power or switch off the PSU's 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% of their nominal values (e.g., +10% for 12V is 13.2V and 5.5V for 5V).


The first two turn-on transient tests went smoothly, but we noticed an unexpected drop on all rails except for 5VSB during the third test, which is a clear indication of a protection kicking in and instantly removing power, but everything did go back to normal afterward. The inrush current control circuit of the PFC controller most likely interfered, and once we dialed a lower load and ran the third turn-on test again, the slope was smooth, so this particular problem only occurs at full load, where inrush current is high. Real-life scenarios are actually unlikely to produce such results, so you have nothing to worry about. However, we will deduct performance points for its failure to comply with our third turn-on transient test, even though this issue is not due to a protection feature kicking in.