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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 RM750x'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 | 12.171V | 12.045V | 1.04% | Pass |
| 5V | 5.026V | 4.992V | 0.68% | Pass |
| 3.3V | 3.300V | 3.234V | 2.00% | Pass |
| 5VSB | 5.004V | 4.956V | 0.96% | Pass |
Advanced Transient Response at 20 Percent – 20ms
| Voltage | Before | After | Change | Pass/Fail |
|---|---|---|---|---|
| 12V | 12.171V | 11.969V | 1.66% | Pass |
| 5V | 5.026V | 4.981V | 0.90% | Pass |
| 3.3V | 3.300V | 3.220V | 2.42% | Pass |
| 5VSB | 5.003V | 4.975V | 0.56% | Pass |
Advanced Transient Response at 20 Percent – 1ms
| Voltage | Before | After | Change | Pass/Fail |
|---|---|---|---|---|
| 12V | 12.173V | 12.017V | 1.28% | Pass |
| 5V | 5.027V | 4.983V | 0.88% | Pass |
| 3.3V | 3.300V | 3.222V | 2.36% | Pass |
| 5VSB | 5.004V | 4.962V | 0.84% | Pass |
Advanced Transient Response at 50 Percent – 200ms
| Voltage | Before | After | Change | Pass/Fail |
|---|---|---|---|---|
| 12V | 12.155V | 12.096V | 0.49% | Pass |
| 5V | 5.019V | 4.983V | 0.72% | Pass |
| 3.3V | 3.293V | 3.225V | 2.06% | Pass |
| 5VSB | 4.986V | 4.953V | 0.66% | Pass |
Advanced Transient Response at 50 Percent – 20ms
| Voltage | Before | After | Change | Pass/Fail |
|---|---|---|---|---|
| 12V | 12.155V | 12.057V | 0.81% | Pass |
| 5V | 5.019V | 4.975V | 0.88% | Pass |
| 3.3V | 3.293V | 3.214V | 2.40% | Pass |
| 5VSB | 4.986V | 4.956V | 0.60% | Pass |
Advanced Transient Response at 50 Percent – 1ms
| Voltage | Before | After | Change | Pass/Fail |
|---|---|---|---|---|
| 12V | 12.158V | 12.088V | 0.58% | Pass |
| 5V | 5.019V | 4.979V | 0.80% | Pass |
| 3.3V | 3.293V | 3.214V | 2.40% | Pass |
| 5VSB | 4.987V | 4.984V | 0.84% | Pass |
The RM750x performed well, though the older RM750x's +12V rail was even better. Perhaps its larger dimensions allowed for more capacitance on the secondary side, which could have affected the PSU's transient response.
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 RM750x’s response in simpler transient load scenarios—during its power-on phase.
For our first measurement, we turned the RM750x off, dialed in the maximum current the 5VSB rail 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 750W 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 the PSU off), we dialed the maximum load the +12V rail could handle before switching it back 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 12V is 13.2V, and 5.5 V for 5V).
There was a minor voltage overshoot on the 5VSB rail, while the waveform was smooth during the second test. The result wasn't so good during our last test, though; the +12V rail needed more than 20ms to reach its nominal voltage.
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