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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 monitored the PSU's response in two different scenarios. First, a transient load (10A at +12V, 5A at 5V, 5A at 3.3V and 0.5A at 5VSB) was applied for 200ms while the supply was working at 20 percent load. In the second scenario, we hit it with the same transient load while operating at 50 percent capacity. In both tests, we used our oscilloscope to measure the voltage drops caused by the transient load. The voltages should have remained 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 tough to master, especially for a PSU with a capacity of less than 500W.
Advanced Transient Response at 20 Percent
| Voltage | Before | After | Change | Pass/Fail |
|---|---|---|---|---|
| 12V | 12.275V | 12.188V | 0.71% | Pass |
| 5V | 5.155V | 5.075V | 1.55% | Pass |
| 3.3V | 3.360V | 3.241V | 3.54% | Pass |
| 5VSB | 5.120V | 5.063V | 1.11% | Pass |
Advanced Transient Response at 50 Percent
| Voltage | Before | After | Change | Pass/Fail |
|---|---|---|---|---|
| 12V | 12.210V | 12.118V | 0.75% | Pass |
| 5V | 5.132V | 5.061V | 1.38% | Pass |
| 3.3V | 3.340V | 3.221V | 3.56% | Pass |
| 5VSB | 5.083V | 5.045V | 0.75% | Pass |
Performance in these tests was pretty good; the +12V rail stayed well below one percent in both cases, while deviations on the other rails were normal for this PSU's capacity. Ideally, we would like to see a deviation within three percent at 3.3V; however, this rail is lightly used by modern systems, so we are satisfied by the 3.56 percent result.
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 measured the response of the PSU in simpler transient-load scenarios—during its power-on phase.
For the first measurement, we turned the supply off, dialed in the maximum current the 5VSB could output and switched on the PSU. In the second test, we dialed the maximum load the +12V could handle and started the PSU while it was in standby mode. In the last test, while the PSU was completely switched off (we cut off the power or switched off the PSU by flipping its on/off switch), we dialed the maximum load the +12V rail could 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).
At 5VSB, the spike was quite high at 5.46V, reaching close to the ATX spec's upper limit. Sirfa should look into this. On the "standby to full +12V" load test, performance was good, while in the last test we noticed a pretty low voltage overshoot and a small dive at the start of the waveform.
Overall, we'll call this performance mediocre, with the most significant problem being the 5VSB spike.
