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 at 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.





The +12V rail registered very low voltage drops, something that is normal since this rail is extra strong. The 5VSB and 5V rails follow closely with small deviations in performance. In last place is the 3.3V rail which registered a significant deviation that exceeded 4% in both cases. We would like to see a smaller deviation on this rail, but its voltage is, because of its high nominal voltage, kept at good levels.


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 teset. 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).


We measured a small spike that barely exceeded the nominal voltage (5.014V) at 5VSB. On the +12V rail the registered overshoots were larger on the +12V rail and reached, in the worst case, 12.34V, a voltage level that is still far from the upper limit. Also, the slope during the "off to full power 12V" test took some time to settle down, which is contrary to the other +12V test where it settled down almost instantly.