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.





As you can see from the tables above, the deviations are low, especially if we take into account the small capacity of this unit. What troubled us during these tests was that the unit kept shutting down once we applied the transient load. Apparently a protection kicked in once we engaged the transient load generator and we had to tune the scope accordingly in order to "catch" the transient waveforms, before the protections engaged. In order to see if there is a problem with dynamic loads we programmed our electronic loads in a variable loading pattern and the unit worked fine there without shutting down, so most likely our transient load generator has some compatibility issues with this PSU. However since this is the first unit that doesn't get along with our transient generator, we will mark it down as a problem in our conclusion.

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, the 5VSB rail did not register any voltage overshoots and the same applies to the +12V rail, too. However the latter rail, on both tests, registered a small step in the waveform and during the standby-to-full-load test it needs over 20ms to stabilize its voltage level. Nevertheless all above are nothing to worry about and the most significant here is that none of the above rails registered any spikes.