Advanced Transient Response Tests

These tests monitor the PSU's response 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 these 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 Advanced Transient Response tests, and they are designed to be very tough to master, especially for a PSU with a capacity below 500 W.





A better result at +12V wasn't possible in the first test because the main switchers were PWM operated; however, the same rail only registered deviations within 1% in the second test. Results were pretty good on the other rails, and to our surprise, voltage drops on the 3.3V rail were well controlled in both cases.

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 load—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 off by flipping its on/off switch), we dial the maximum load the +12V rail can handle before switching the PSU on through 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 slope was very smooth, though the rail took some time before reaching its nominal voltage in the second test, and there were a few small steps between 3-6 V in the third test; however, those are nothing to worry about. Rise times were also within the ATX specification's range of 0.2-20 ms.