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 20% load. 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 the transient load causes using our oscilloscope. The voltages should remain within the regulation limits defined by the ATX specification.

In the real world, a PSU is always working with loads that change, depending on whether the CPU or graphics cards are busy. So it is of immense importance, for the PSU, to be able to keep its rails within the predefined, by the ATX spec, ranges. The smaller the rail deviations the more steady the system will be; hence less stress will be applied to its components.

We should note that the ATX spec requires for capacitive loading during the transient rests, but in our methodology we chose to apply the worst case scenario with no extra capacitance on the rails. Although the ATX spec asks for this capacitance, this doesn't mean that your system (meaning the mainboard and the rest of the parts) will provide it, so we have to keep this scenario in mind as well.






Transient response at +12V is quite good, and the same goes for 5V and 5VSB. 3.3V results were unsatisfactory, especially in the second test where the voltage dropped below 3.2 V.

Below are the oscilloscope screenshots 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 more straightforward scenarios of transient load 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 the 5VSB can output, and then switch on the PSU. In the second test, we dial the maximum load +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 the PSU off by flipping its on/off switch), we dial the maximum load 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 13.2 V and 5.5 V for 5V).

Ripple Measurements

Ripple represents the AC fluctuations (periodic) and noise (random) found in the DC rails of a PSU. Ripple significantly decreases the life span of capacitors since it increases their temperature; a 10 °C increase can cut into a capacitor's life span by 50 percent. Ripple also plays an important role in overall system stability, especially when it is overclocked. The ripple limits, according to the ATX specification, are 120 mV (+12V) and 50 mV (5V, 3.3V, and 5VSB).


Ripple suppression could be better at 3.3V.

Ripple at Full Load

Ripple at 110% Load

Ripple at Crossload 1

Ripple at Crossload 2