Advanced Transient Response Tests

In these tests, we monitor the response of the PSU in two different scenarios. First, a transient load (10 A at +12V, 5 A at 5V, 5 A at 3.3V, and 0.5 A at 5VSB) is applied to the PSU for 200 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. 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.





The response of the +12V rail to transient loads is very good. Its deviation stayed under 1% on both tests. Voltage drops at 5V and 5VSB were low as well, but the deviations of the 3.3V rail lead to readings below 3.2V, especially on the second test. We don't like seeing such low readings at 3.3V, even if they are instantaneous on 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

We measure the response of the PSU in simpler scenarios of transient loads - during the power-on phase of the PSU - in the next set of tests. In the first test, we turn the PSU off, dial the maximum current that the 5VSB can output, and switch the PSU on. In the second test, we dial the maximum load that +12V can handle and start the PSU while the PSU is in standby mode. 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 the +12V rail can handle before switching the PSU on from the loader and restoring 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 5.5V for 5V).


We measured a small voltage overshoot at 5VSB, while the slope was almost perfect on the second test. The slope made a large step at around 6.5 V on the last test and produced a nasty sawtooth wave when it reached the nominal voltage. Definitely not the ideal waveform according to the ATX spec, but all the registered spikes were still well below the limit.