Advanced Transient Response Tests

These tests monitor the PSU's response 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. We measure the voltage drops the transient load causes with our oscilloscope in both of these tests. The voltages should remain within the regulation limits the ATX specification defines. We must stress here that these tests are crucial since 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 a capacity below 500 W.





The +12V rail deviated by more than it should for a high-end 650 W PSU, while the 3.3V rail failed to keep its voltage above 3.2 V in both tests. We would like a PSU that belongs to such a highly competitive category to perform better in these tests.

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 PSU's response in simpler scenarios of transient load—during the power-on phase of the PSU—in these tests. In the first test, we turn the PSU off, dial the maximum current the 5VSB can produce, and 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 the PSU off by flipping its on/off switch), we dial the maximum load the +12V rail can handle before switching the PSU on through 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 5VSB slope is smooth, and there is only a small bump in the second test, and while there are larger bumps in the last test, they are nothing to worry about. All in all, good-enough performance in these tests.