Transient Response Tests
Advanced Transient Response Tests
For details on our transient response testing, please click here.
Ιn these tests, we monitor the ST85F-PT's response 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 for 200 ms while the PSU works at 20 percent load. In the second scenario, the PSU is hit by the same transient load while operating at 50 percent load. In both tests, we use our oscilloscope to measure the voltage drops caused by the transient load. The voltages should remain within the ATX specification's regulation limits.
These tests are crucial because they simulate the transient loads a PSU is likely to handle (such as booting a RAID array or an instant 100 percent load of CPU/GPUs). We call these tests "Advanced Transient Response Tests," and they are designed to be very tough to master, especially for a PSU with a capacity of less than 500 W.
Advanced Transient Response at 20 Percent
| Voltage | Before | After | Change | Pass/Fail |
|---|---|---|---|---|
| 12V | 12.236V | 12.115V | 0.99% | Pass |
| 5V | 5.082V | 4.989V | 1.83% | Pass |
| 3.3V | 3.351V | 3.210V | 4.21% | Pass |
| 5VSB | 5.057V | 5.004V | 1.05% | Pass |
Advanced Transient Response at 50 Percent
| Voltage | Before | After | Change | Pass/Fail |
|---|---|---|---|---|
| 12V | 12.183V | 12.062V | 0.99% | Pass |
| 5V | 5.053V | 4.952V | 2.00% | Pass |
| 3.3V | 3.323V | 3.190V | 4.00% | Pass |
| 5VSB | 5.018V | 4.974V | 0.88% | Pass |
The +12V rail stays within 1 percent in both tests. This is a decent result, though we'd be more satisfied by deviations closer to 0.6 percent.
The 5V and the 5VSB rails demonstrate low voltage drops, while the 3.3V rail doesn't do as well (it's in the 4 percent region). This specific rail usually registers the worst performance in our tests, mostly because of its low initial voltage. Even a moderate drop imposes significant deviations. Just to illustrate, a 0.1 V voltage drop translates to 0.83 percent at +12V, 2 percent at 5V, and 3 percent at 3.3V.
Here are the oscilloscope screenshots we took during Advanced Transient Response Testing:
Transient Response At 20 Percent Load
Transient Response At 50 Percent Load
Turn-On Transient Tests
In the next set of tests, we measure the response of the PSU in simpler transient load scenarios—during the PSU's power-on phase.
For the first measurement, we turn off the ST85F-PT, dial in the maximum current the 5VSB can output, and switch the PSU on. In the second test, we dial the +12V rail's maximum load and start the PSU while it's in standby mode. In the last test, while the PSU is completely switched off (we cut off the power or switch the PSU off by flipping its switch), we dial the maximum load the +12V rail can handle before switching the ST85F-PT on from the loader and restoring power. The ATX specification states that recorded spikes on all rails should not exceed 10 percent of their nominal values (+10 percent for 12 V is 13.2 V, and 5.5 V for 5 V).
The 5VSB slope is perfect. The +12V slope isn't as pretty, but it doesn't exhibit any spikes or voltage overshoots either.