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 the ATX specification defines. 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 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 primary switchers most likely operate in PWM mode during the first test. Deviations on the +12V rail were, as such, higher than expected for a 1 kW PSU in the first test. In the second test, the same rail achieved a much better result since its deviation dropped to 0.57%. Voltage drops on the 5V and 5VSB rails were well-controlled, which wasn't the case with the 3.3V rail since it dipped below 3.2V in both tests.

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 these tests. In the first test, we turn the PSU off, dial the maximum current the 5VSB can produce, 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 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 slope is very smooth in the first test, and we only noticed a small bump before the rails stabilized in the second test. The waveform isn't that smooth in the third test, and there is also a tiny spike, but it is nothing to worry about. All in all, satisfactory performance in these tests.