Advanced Transient Response Tests

In these tests we monitor the response of the PSU in two different scenarios. First a transient load (11A at +12V, 5A at 5V, 6A at 3.3V and 0.5A at 5VSB) is applied for 50 ms to the PSU, while the latter is working at a 20% load state. In the second scenario the PSU, while working with 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 voltages should remain within the regulation limits specified 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. starting of 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 500W.





During the first test the +12V rail registered a significant drop, because the main switchers are working in PWM mode at such low loads and the response to transient loads is kind of slow. The situation improved in the second test, where the main switchers are operating in FM mode and the voltage drop was normal this time for a PSU of this capacity. In general the unit performed decently here, considering its low capacity which poses a major handicap in these tests.

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 measure the response of the PSU in simpler scenarios of transient loads, during the turn on phase of the PSU. In the first test we turn off the PSU, dial 2A load at 5VSB and then switch on the PSU. In the second test, while the PSU is in standby, we dial the maximum load that +12V can handle and we start the PSU. In the last test, while the PSU is completely switched off (we cut off power or switch off the PSU's On/Off switch), we dial the maximum load that +12V can handle and then we switch on the PSU from the loader and we restore 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 for 5V is 5.5V).


On the 5VSB rail we didn't record any voltage spikes and the slope is smooth. On the contrary, at +12V, on both tests, the waveform doesn't ramp up smoothly but makes a big step on the middle. Also in the second test a noticeable spike exists and removes some performance points.