Advanced Transient Response Tests

In these tests we monitor the response of the PSU in two different scenarios. First a transient load (10A at +12V, 5A at 5V and 6A at 3.3V) 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 (with the exception now that load at 3.3V is increased by 4A). In both tests, we measure the voltage drops that the transient load causes, using a Labjack that is attached to our loader and the Stingray 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.





Voltage drops on the +12V and 5V rails are low. Especially the +12V rail performed really well. For a PSU that can handle over 100A at +12V this comes sort of natural. On the contrary 3.3V failed to pass the really tough second part of the tests, but this happened mainly because of the already low voltage of the specific rail and not due to a high voltage drop.

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).



At 5VSB we did not notice any voltage overshoots but the rise time* is a little above the 20ms limit that the ATX specification sets, however this is not something to worry about. At the other two tests we also didn't encounter any voltage overshoots, but the slope of +12V in both cases is not smooth and at 1.7-1.9V it not only stalls but for a few ms it turns to negative. ATX specification states that "the smooth turn-on requires that, during the 10% to 90% portion of the rise time, the slope of the turn-on waveform must be positive" so this is a problem, however not dead serious since we shouldn't forget that in both tests we draw almost 100A from the +12V line, a very high load.

*Rise time is defined as the time it takes any output voltage to rise from 10% to 95% of its nominal voltage.