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. 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 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 a PSU with a capacity below 500 W.





Most likely because of PWM mode, which the main switches operate in with light loads, the +12V rail deviated significantly once during the first test. As voltage drops dropped significantly afterward, things reverted back to normal for such a high capacity unit in our second test. Voltage drops on all other rails were pretty good, although the 3.3V rail's voltage dropped below 3.2 V for a split second only.


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 PSU's power-on phase—in the next set of tests. In the first test, we turn the PSU off, dial the maximum current 5VSB can output, and switch the PSU on. 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).



We noticed a very small spike during the first test, a not perfectly smooth slope in the second, and a nasty spike in the third test. The third of the above tests is thankfully based on an unrealistic scenario, but we still expected this platform to perform better.