In the real world, a PSU is always working with loads that change, depending on whether the CPU or graphics cards are busy. So, a decent PSU review should contain some tests with dynamic or transient loads. We conduct a variety of transient tests:
- While the PSU is working at a 20 percent load state, a transient load is applied to the PSU for 200ms (15A at +12V, 6A at 5V, 6A at 3.3V, and 0.5 A at 5VSB).
- While working at 50 percent load, the PSU is hit by the same transient load.
- In the next tests we use again the same starting points, 20 and 50 percent load states, but, we increase the load-changing repetition rate from 5 Hz (200ms) to 50 Hz (20ms) and 1 kHz (1ms). This way we push even harder the PSU.
In ATX v3.0 compatible PSUs, the transient response tests are even more demanding, as shown in the table below.
We measure the voltage drops that the sudden load change causes in all tests. The voltages should remain within the ATX specification's regulation limits. Finally, we should also note that the latest ATX spec requires a load-changing repetition state of 50 Hz to 10 kHz for transient response testing. The only reason that we decided to keep the "slow" 5 Hz tests is to retain compatibility with our database.
We also conduct three transient tests where we measure the response of the PSU in its simpler turn-on phase. We turn off the PSU in the first test, dial a full load at 5VSB, and then switch on the PSU. In the second test, while the PSU is on standby, we dial the maximum load that +12V can handle and 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 +12V can handle, then switch on the PSU and restore power.
The ATX specification states that recorded spikes on all rails should not exceed 10 percent of their nominal values (+10 percent for 12V is 13.2V and 5.5V for 5V ).
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