Advanced Transient Response Tests

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





Deviations of the +12V rail were higher in the first test because the main switchers operate in PWM mode. Performance on this rail improved notably in the second test. The 5V and 5VSB rails managed to control their voltage drops, while the 3.3V rail didn't perform as well since its voltage dropped below 3.2 V 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 PSU's response in simpler scenarios of transient load—during the power-on phase of the PSU—in these 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 outcome here is a tiny spike at 5VSB that is nothing to worry about. Performance during the second test was spotless, but all hell broke loose in the last test as ripple increased and notable voltage deviations took place before the rail settled down.