Advanced Transient Response Tests

In these tests, we 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 in a 20% load state. 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 that 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 since they simulate transient loads that 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 lower than 500 W.





Deviations are, except on the 3.3V rail, low on all rails, but a loose voltage regulation drops the voltage on all rails during the application of the transient load at low loads; that is, except for the 5V rail. Also, the extra loose voltage regulation on the 3.3V rail, along with the high deviation that it registers with the application of the transient load, led to a big fail on the second test with the voltage dropping below 3.1 V. Something else we noticed off the scope shots was the significant time the 12V and 5V rails need to stabilize their voltages as compared to other units we have tested in the past. The ACRF topology is most likely responsible for such a long stabilization time. It primarily affects the +12V rail, which in turn causes such a problem on the 5V rail that is generated through a DC-DC converter by the +12V rail. Strangely enough, the 3.3V rail isn't affected.


Below, you will find the oscilloscope screenshots that 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 that the 5VSB can output, and then switch on the PSU. In the second test, we dial the maximum load that +12V can handle and we start the PSU, all 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 that 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 for 5V is 5.5V).


The 5VSB rail registered a small spike which is, however, below the nominal voltage of this rail. At +12V, we measured two spikes with the second one (standby to full test) being the higher by reaching almost 12.5 V, a fairly high reading that is still well below the 13.2 V limit.