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 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 the above 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 capacities below 500 W.





Voltages on all rails but 3.3V never dropped too low because of initially high readings and relatively small deviations. However, we would like to see slightly lower deviations on at least the +12V rail, especially during the first test. 3.3V, the usual suspect, had its voltage drop very close to the lower limit the ATX spec sets during the second test (3.14 V).


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 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 the 5VSB can output, and 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's on/off switch off), 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).


These tests caused a small voltage overshoot on 5VSB and a slightly bumpy slope on +12V during the last test. None of the above should keep you awake at night, so let's move on to our ripple measurements.