Advanced Transient Response Tests

In these tests, we 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 PSUs with a capacity below 500 W.





Deviations at +12V were normal for the PSU's small capacity, and the 5V and 5VSB rails performed well, but we measured readings below 3.2 V in both tests on the 3.3V rail, which is troubling.


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 load—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 the PSU off by flipping its 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).



Everything was fine in the first two tests, which changed after the rails settled down in the last test as there was a very large voltage drop, as though one of the PSU's protections had just kicked in. The VSM550 based on exactly the same platform exhibited similar behavior, making this a problem particular to this design, and Cooler Master should let Enhance know. Such a huge voltage drop could be a threat to a system's stability. Thankfully, this is a very unlikely scenario in real life, though it should still be looked into nonetheless.