XFX XTR Series 850W PSU Review

Transient Response Tests

Advanced Transient Response Tests

For details on our transient response testing, please click here.

Ιn 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 metrics 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

Advanced Transient Response at 50 Percent

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The +12V rail stays within 0.7% in both tests, so it performs pretty well. The 5V and 5VSB rails also respond well, while the 3.3V rail exceeds 3%, registering the highest deviations. The good thing is that the 3.3V rail's voltage levels stay well above 3.2V.

Here are the oscilloscope screenshots we took during Advanced Transient Response Testing:

Transient Response At 20 Percent Load

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Transient Response At 50 Percent Load

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Turn-On Transient Tests

In the next set of tests, we measured the response of the PSU in simpler transient load scenarios—during the PSU's power-on phase.

For the first measurement, we turn off the PSU, dial in the maximum current the 5VSB can output and switch the PSU on. In the second test, we dial the maximum load the +12V rail 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 the power or switch the PSU off by flipping its on/off switch), we dial the maximum load the +12V rail can handle before switching the PSU 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 5.5V for 5V).    

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The 5VSB slope is almost perfect, without any notable spikes or voltage overshoots. In the second test, the +12V rail takes some time until it settles down. The same goes for the last test, with the addition of a very small spike. In general, the performance of the XTR 850W is quite good in these tests.