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Transient Response Tests
Advanced Transient Response Tests
For details on our transient response testing, please click here.
In these tests, we monitor the response of the PSU in two different scenarios. First, a transient load (10A at +12V, 5A at 5V, 5A at 3.3V and 0.5A at 5VSB) is applied for 200ms 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 them "Advanced Transient Response Tests," and they are designed to be very tough to master, especially for PSUs with less than 500W capacity.
Advanced Transient Response at 20 Percent
| Voltage | Before | After | Change | Pass/Fail |
|---|---|---|---|---|
| 12V | 12.148V | 12.001V | 1.21% | Pass |
| 5V | 5.051V | 4.952V | 1.96% | Pass |
| 3.3V | 3.347V | 3.196V | 4.51% | Pass |
| 5VSB | 5.026V | 4.959V | 1.33% | Pass |
Advanced Transient Response at 50 Percent
| Voltage | Before | After | Change | Pass/Fail |
|---|---|---|---|---|
| 12V | 12.079V | 11.934V | 1.20% | Pass |
| 5V | 5.001V | 4.909V | 1.84% | Pass |
| 3.3V | 3.308V | 3.145V | 4.93% | Pass |
| 5VSB | 4.969V | 4.913V | 1.13% | Pass |
The deviations on the +12V rail are higher than we would like to see; the same applies to the 3.3V rail, which dropped below 3.2V in both tests. Only the 5V and 5VSB rails managed to register good enough performance.
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 PSU's response in simpler transient load scenarios—during its power-on phase.
For the first measurement, we turn off the PSU, dial in the maximum current the 5VSB can output and switch on the PSU. In the second test, we dial the maximum load the +12V can handle 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 off the PSU by flipping its on/off switch), we dial the maximum load the +12V rail can handle before switching on the PSU 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 12V is 13.2V, and 5.5V for 5V).
The 5VSB slope isn't exactly smooth; however, there are no spikes or voltage overshoots. Things look better in the second test, although the rail needs some time until it reaches the nominal voltage. Finally, in the third test, we notice two small periods with excess ripple. Most likely the primary switching FETs have a hard time catching up with the sudden transition to full load operation from power-off mode.
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