Advanced Transient Response Tests

In these tests, we monitor the response of the PSU in two different scenarios. First, a transient load (11 A at +12 V, 5 A at 5 V, 6 A at 3.3 V, and 0.5 A at 5VSB) is applied to the PSU for 50 ms, while the latter is working at a 20% load state. 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 that the transient load causes, using our oscilloscope. In any case, 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 that 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 lower than 500 W.





In the first test, the deviation at +12 V was high, most likely because the PSU was working in PWM mode due to the low power level it outputs. In the second test, the deviation on the same rail was much lower and under 1%.
During these tests, the weak chain proved to be the 3.3 V rail which, in the second test, dropped to 3.15 V, a low reading especially for a PSU of this category.


Below you will find the oscilloscope screenshots that we took during Advanced Transient Response Testing.

Transient Response at 20% Load

Transient Response at 50% Load

Turn-On Transient Tests

In the next set of tests, we measured the response of the PSU in simpler scenarios of transient loads, during the power-on phase of the PSU. In the first test, we turned off the PSU, dialed 2 A of load at 5VSB and then switched on the PSU. In the second test, while the PSU was in standby mode, we dialed the maximum load that +12 V can handle and we started the PSU. In the last test, while the PSU was completely switched off (we cut off power or switched off the PSU's on/off switch), we dialed the maximum load that the +12 V rail can handle and then we switched on the PSU from the loader and we restored power. The ATX specification states that recorded spikes on all rails should not exceed 10% of their nominal values (e.g., +10% for 12 V is 13.2 V and for 5 V is 5.5 V).


On the 5VSB rail, the waveform ramped up smoothly and there were no voltage overshoots. During the other two tests, although the waveform ramped up smoothly again, it needed quite some time to settle down at the nominal voltage while forming noticeable ripples.