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.





All tests completed successfully, however the 3.3V rail showed a weak point since in both cases the deviation exceeded 5%. Nevertheless due to the high initial voltage value at this rail the drop was kept above the lower voltage regulation limit, so we consider both tests as successfully passed.

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).


As you can see from the above scope shots we didn't measure any voltage overshoot at all three tests. There is only a small step in the +12V waveform in both tests but this is nothing to worry about. Also the rise time close, but below, 20 ms; the uppervlimit that ATX spec sets.