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.





The response of the PSU to transient loads wasn't very good, although voltage drops on all rails never exceed the respective limits. However, check the scope screenshots below and you will notice that the slopes were far from smooth, which shows that the rails were greatly affected by and slow to respond to the transient loads. The 3.3V rail also deviated by more than 5% in both tests, and we don't want to see deviations close to or over 4% on this rail.


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 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 rail registered a smooth voltage overshoot that was even smoother in the second test - to the point of being very difficult to discern. However, the bump was clear but still well below the maximum allowed voltage value in the last test. All in all, decent performance here, although it would have been better had the voltage overshoot in the last test only been lower or smoother.