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.





On the +12V rail, the deviation is not high, but the voltage fell to almost 11.8 V during the second test. On the minor rails, we measured larger deviations, with voltage on the 3.3V rail falling to almost 3.2 V during the second test. Thankfully, we didn't measure readings below 3.2 V on the 3.3V rail. Finally, 5VSB registered the smallest deviations of all rails; its voltage was very close to the nominal value in both cases.

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 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 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 5.5V is 5V).


At 5VSB, we measured a very small spike, which is nothing to worry about. We noticed no voltage overshoots on the +12V rail—on both tests. The slope was smooth. However, there is a small spike after the voltage settles down on the "PSU OFF TO FULL 12V" test.