Seasonic Snow Silent 750W Power Supply Review

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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 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) was applied to the PSU for 200 milliseconds while the PSU was working at 20 percent load. In the second scenario, the PSU was hit by the same transient load while operating at 50 percent load. 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 that a PSU is likely to handle (like booting a RAID array or an instant 100 percent CPU/GPU load). We call these tests "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

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VoltageBeforeAfterChangePass/Fail
12 V12.133V12.050V0.68%Pass
5 V5.031V4.966V1.29%Pass
3.3 V3.348V3.258V2.69%Pass
5VSB5.095V5.060V0.69%Pass

Advanced Transient Response at 50 Percent

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VoltageBeforeAfterChangePass/Fail
12 V12.121V12.044V0.64%Pass
5 V5.027V4.963V1.27%Pass
3.3 V3.345V3.254V2.72%Pass
5VSB5.060V5.016V0.87%Pass

The unit easily handled all of the hurdles we threw at it, showing that, besides excellent load regulation, it also has a great response in transient loads that come close to real-life conditions. A PSU will need to handle transient loads for most of its lifetime, so good performance in these tests is significant.

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 the PSU's power-on phase.

For the first measurement, we turned off the PSU, dialed in the maximum current that the 5VSB could output and switched on the PSU. In the second test, we dialed the maximum load that 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 that the +12V rail could handle before switching on the PSU 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 5.5V for 5V).    

The 5VSB rail took a little longer than usual to reach the nominal voltage, but it still did so in less than 20ms as the ATX spec dictates. In the other two tests, the slopes didn't ramp up smoothly. We noticed some spikes, but they weren't high enough to make us worry. These results were not top-notch, but we can't call them bad or even mediocre.

Contributing Editor

Aris Mpitziopoulos is a Contributing Editor at Tom's Hardware US, covering PSUs.