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Transient Response Tests
Advanced Transient Response Tests
For details on our transient response testing, please click here.
Ιn these tests, we monitor the ST45SF's response in several scenarios. First, a transient load (10A at +12V, 5A at 5V, 5A at 3.3V, and 0.5A at 5VSB) is applied for 200ms as the PSU works at 20 percent load. In the second scenario, it's hit by the same transient load while operating at 50 percent load.
In the next sets of tests, we increase the transient load on the major rails with a new configuration: 15A at +12V, 6A at 5V, 6A at 3.3V, and 0.5A at 5VSB. We also increase the load-changing repetition rate from 5 Hz (200ms) to 50 Hz (20ms). Again, this runs with the PSU operating at 20 and 50 percent load.
The last tests are even tougher. Although we keep the same loads, the load-changing repetition rate rises to 1 KHz (1ms).
In all of the tests, we use an 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 "Advanced Transient Response Tests," and they are designed to be very tough to master, especially for a PSU with a capacity of less than 500W.
Advanced Transient Response at 20 Percent – 200ms
| Voltage | Before | After | Change | Pass/Fail |
|---|---|---|---|---|
| 12V | 12.134V | 11.876V | 2.13% | Pass |
| 5V | 5.065V | 4.977V | 1.74% | Pass |
| 3.3V | 3.358V | 3.220V | 4.11% | Pass |
| 5VSB | 5.130V | 5.035V | 1.85% | Pass |
Advanced Transient Response at 20 Percent – 20ms
| Voltage | Before | After | Change | Pass/Fail |
|---|---|---|---|---|
| 12V | 12.134V | 11.739V | 3.26% | Pass |
| 5V | 5.065V | 4.944V | 2.39% | Pass |
| 3.3V | 3.357V | 3.213V | 4.29% | Pass |
| 5VSB | 5.130V | 5.037V | 1.81% | Pass |
Advanced Transient Response at 20 Percent – 1ms
| Voltage | Before | After | Change | Pass/Fail |
|---|---|---|---|---|
| 12V | 12.135V | 11.847V | 2.37% | Pass |
| 5V | 5.065V | 4.903V | 3.20% | Pass |
| 3.3V | 3.357V | 3.207V | 4.47% | Pass |
| 5VSB | 5.130V | 5.033V | 1.89% | Pass |
Advanced Transient Response at 50 Percent – 200ms
| Voltage | Before | After | Change | Pass/Fail |
|---|---|---|---|---|
| 12V | 12.091V | 11.811V | 2.32% | Pass |
| 5V | 5.037V | 4.944V | 1.85% | Pass |
| 3.3V | 3.333V | 3.200V | 3.99% | Pass |
| 5VSB | 5.093V | 5.012V | 1.59% | Pass |
Advanced Transient Response at 50 Percent – 20ms
| Voltage | Before | After | Change | Pass/Fail |
|---|---|---|---|---|
| 12V | 12.092V | 11.673V | 3.47% | Pass |
| 5V | 5.037V | 4.935V | 2.03% | Pass |
| 3.3V | 3.333V | 3.171V | 4.86% | Pass |
| 5VSB | 5.093V | 5.037V | 1.10% | Pass |
Advanced Transient Response at 50 Percent – 1ms
| Voltage | Before | After | Change | Pass/Fail |
|---|---|---|---|---|
| 12V | 12.092V | 11.793V | 2.47% | Pass |
| 5V | 5.037V | 4.881V | 3.10% | Pass |
| 3.3V | 3.333V | 3.169V | 4.92% | Pass |
| 5VSB | 5.093V | 5.007V | 1.69% | Pass |
The transient response of the +12V rail isn't good at all, although we don't see any voltages below 11.75V. The deviations at 5V are notable as well, while the 5VSB rail performs much better. Finally, the worst rail of all is the 3.3V one, where we measure deviations of up to 5%. At least the ST45SF keeps its rails within the ATX spec. This is quite a feat.
Here are the oscilloscope screenshots we took during Advanced Transient Response Testing:
Transient Response At 20 Percent Load – 200ms
Transient Response At 20 Percent Load – 20ms
Transient Response At 20 Percent Load – 1ms
Transient Response At 50 Percent Load – 200ms
Transient Response At 50 Percent Load – 20ms
Transient Response At 50 Percent Load – 1ms
Turn-On Transient Tests
In the next set of tests, we measured the ST45SF's response in simpler transient load scenarios—during its power-on phase.
For the first measurement, we turned the ST45SF off, dialed in the maximum current the 5VSB could output, and switched the PSU back on. In the second test, we dialed the maximum load the +12V rail could handle and started the 450W supply 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 it back on from the loader and restoring the 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 5.5 V for 5V).
The 5VSB rail's response is almost perfect, while the +12V rail doesn't respond so well in the last test. Its voltage overshoot is much lower than the limit, though, so there is nothing to worry about.
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