Transient Response
Transient response in power supplies refers to how quickly and effectively the PSU stabilizes its output voltage during sudden changes in load demand, such as when a CPU or GPU increases its power draw. It’s measured by the time and voltage deviation during these shifts. A faster, smaller transient response ensures stable power, preventing instability and component damage.
20% Load – 20ms
| Voltage | Before | After | Change | Pass/Fail |
| 12V | 12.126V | 11.931V | 1.61% | Pass |
| 5V | 5.031V | 4.934V | 1.93% | Pass |
| 3.3V | 3.325V | 3.192V | 3.99% | Pass |
| 5VSB | 5.101V | 5.060V | 0.80% | Pass |
50% Load -20ms
| Voltage | Before | After | Change | Pass/Fail |
| 12V | 12.099V | 12.005V | 0.77% | Pass |
| 5V | 5.025V | 4.923V | 2.03% | Pass |
| 3.3V | 3.318V | 3.178V | 4.22% | Pass |
| 5VSB | 5.067V | 5.012V | 1.09% | Pass |
The transient response is average at 12V, primarily because of the high deviation at a 20% starting load. This is because the LLC resonant converter operates at PWM loads and at light loads to offer high efficiency; in contrast, the standard frequency modulation (FM) mode is inefficient under these conditions due to high switching frequencies and associated losses. So it is common to see PSU designs that switch from FM to PWM at light loads to control the output voltage by varying the duty cycle rather than the switching frequency. This affects the transient response, though, when the PSU operates at a light load before the transient load is applied.
Transient Response ATX v3.1 Tests
[Note] For PSUs without a 12+4 pin connector, the maximum applied load for the transient response tests is 150%, rather than 200%.
The PSU passes all ATX v3.1 transient response tests, but the 12V and 3.3V rails drop low at higher loads.
The 12V rail’s performance is mediocre in these tests.
