Advanced Transient Response Tests

In these tests, we monitor the response of the PSU in two different scenarios. First, a transient load (15 A at +12V, 6 A at +5V, 6 A at +3.3V, and 0.5 A at 5VSB) is applied to the PSU for 20 ms while it 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, our oscilloscope measures the voltage drops caused by the transient load. All voltages should remain within the regulation limits defined by the ATX specification.

During real-world usage, a PSU always operates under changing loads, depending on whether the CPU or graphics card is busy. It is of immense importance that the PSU is able to keep its rails within the limits defined by the ATX specification. Smaller deviations reduce stress being applied to system components.

We should note that the ATX specification requires for capacitive loading during the transient tests, but in our methodology, we chose to apply the worst-case scenario with no extra capacitance on the rails. Although the ATX specification asks for this capacitance, your system—the mainboard and its other parts—may not provide it, which we have to keep in mind as well.





With 50% load as a starting point at which the primary switching FETs operate in FM mode, the transient response at +12 V is below 1%, which is pretty good. The 5 V and 5VSB rails also perform well, but the 3.3 V rail drops its voltage below 3.2 V in both tests.

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 in the next set of tests—during the power-on phase of the PSU. 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).

Inrush Current

Inrush current, or switch-on surge, refers to the maximum, instantaneous input current drawn by an electrical device when it is first turned on. Large enough inrush current can cause the tripping of circuit breakers and fuses and may also damage switches, relays, and bridge rectifiers. As a result, the lower the inrush current of a PSU right as it is turned on, the better.


Inrush current with 230 V is at pretty high levels.