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. We measure the voltage drops the transient load causes with our oscilloscope in both tests. Voltages should remain within the regulation limits defined by the ATX specification.

Real-world usage always has a PSU work with loads that change depending on whether the CPU or graphics cards are busy, which makes whether the PSU can keep its rails within the ranges defined by the ATX specification important. The smaller the deviations, the steadier the system will be, which results in less stress being applied to its 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 specifications 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 20% load, the transient response at +12 V is slightly over 1%, but the voltage is still much higher than 12 V. With 50% load, the deviation is below 1%, which is as expected for a modern high-capacity platform. My only concern here is the deviation above 4% in both 3.3 V tests, but the voltage is still above 3.2 V in both cases.

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).

Power Supply Timing Tests

The table above lists all required power supply timing values.




Both the Power-on (T1) and Power_OK delay (T3) periods are quite long. I would like to see values below 100 ms for T1 and within 100–150 ms for T3.

Ripple Measurements

Ripple represents the AC fluctuations (periodic) and noise (random) found in the DC rails of PSUs. Ripple significantly decreases the life span of capacitors because it increases their temperature; a 10 °C increase can cut into a capacitor's life span by up to 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 is great on all rails even though there are no in-cable caps.

Ripple at Full Load

Ripple at 110% Load

Ripple at Crossload 1

Ripple at Crossload 2