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. In the second scenario, the PSU, while working at 50% load, is hit by the same transient load. We measure the voltage drops the transient load causes in both tests 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 a capacity below 500 W.





The +12 V rail registered high deviations during the first test because it was operating in PWM mode, but deviations on the same rail dropped down to nearly 1% in the second test. The 3.3V rail didn't perform well here since it dropped its voltage very close to the lower limit during the second test, while the other two rails (5V and 5VSB) performed decently.


You will find the oscilloscope screenshots we took during Advanced Transient Response Testing below.

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 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 off the PSU's on/off switch), we dial the maximum load +12V 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).



The 5VSB rail registered a noticeable voltage overshoot which, however, was below the 5.5 V upper limit, though it exceeded the 20 ms rise time limit by a few ms in the other two tests.