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 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.





Due to the ACRF topology, we measured increased deviations in excess of 1% on the +12V rail. We do expect PSUs of similar capacity to deviate by no more than 1%, with close to 0.5% being excellent. The 3.3V rail also performed poorly in both tests since its voltage instantly dropped bellow 3.2 V once we applied the transient load.


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 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 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 PSU registered a very small voltage overshoot on its 5VSB rail, and while its slope was smooth in the second test, things didn't fare as well in the third test as ripple was out of control for a short period of time. The ACRF topology obviously couldn't handle the sudden full load during the unit's turn-on phase. Such is thankfully an unlikely real-life scenario since no PSU will be asked to deliver its full power right after it has been turned on; however, since the ATX specification includes the test, we will deduct some performance points for the result.