Advanced Transient Response Tests

In these tests, we monitor the response of the PSU in two different scenarios. First, a transient load (11 A at +12V, 5 A at 5V, 6 A at 3.3V, and 0.5 A at 5VSB) is applied to the PSU for 50 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. In any case, 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 deviations on these tests are not so large. However, the low initial voltage of the 3.3V rail leads to a big fail on the second test, with its voltage dropping far below the 3.14V limit. Thankfully, the +12V rail registered controlled voltage drops on both tests.


Below, you will see the oscilloscope screenshots that we took during the 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 power-on phase of the PSU. In the first test, we turn off the PSU, dial 2 A of load at 5VSB and then switch on the PSU. In the second test, while the PSU is in standby mode, 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 the +12V rail can handle, 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).


Only at 5VSB we measured a voltage overshoot, which however doesn't exceed the nominal voltage of this rail. On the +12V rail both slopes ramp up smoothly, with the rise time being within range (0.2-20ms) on both cases.