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 +12 V, 5 A at 5 V, 6 A at 3.3 V, and 0.5 A at 5 VSB) 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.





All voltage drops are well controlled, and the deviations are small even on the 3.3 V rail, which is usually the weak chain in these tests. Apparently, CWT did a good job in this platform and the response of all rails at dynamic loads is very good.

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 5 VSB and then switch on the PSU. In the second test, while the PSU is in standby mode, we dial the maximum load that +12 V 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 +12 V rail 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 12 V is 13.2 V and for 5 V is 5.5 V).


On the 5VSB rail, there is a minor spike, while on the "standby to full 12V" test the waveform ramps up smoothly. The worst result was obtained in the final test where the waveform made a noticeable wave, which, however, did not exceed the nominal voltage at its peak. Thankfully, the rise time on all three tests we conducted was constantly within the range specified by the ATX specifications (0.2-20 ms).