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Seasonic Prime 850 W Titanium PSU Review

Seasonic made an impressive entry in the 80 PLUS Titanium category with its Prime series. This line's current flagship, offering 850W capacity, is being reviewed today. Besides high efficiency, it sports quiet operation and top performance.

Transient Response Tests

Advanced Transient Response Tests

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

Ιn these tests, we monitor the SSR-850TD's response 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, Seasonic's 850W Prime 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 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 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

VoltageBeforeAfterChangePass/Fail
12V12.130V12.039V0.75%Pass
5V5.040V4.947V1.85%Pass
3.3V3.341V3.189V4.55%Pass
5VSB4.999V4.957V0.84%Pass

Advanced Transient Response at 50 Percent

VoltageBeforeAfterChangePass/Fail
12V12.118V12.039V0.65%Pass
5V5.033V4.940V1.85%Pass
3.3V3.334V3.180V4.62%Pass
5VSB4.973V4.934V0.78%Pass
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The transient response of the +12V rail is good enough, though we'd like to see closer to 0.5% in both tests. The 5V and 5VSB rails do pretty well. However, the 3.3V rail needs more work in order to keep its deviations below 3%.

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 its power-on phase.

For the first measurement, we turned off the SSR-850TD, dialed in the maximum current its 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 850W Prime while it was in standby mode. In the last test, while the PSU was completely switched 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 5.5V for 5V).    

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We observe a small voltage overshoot at 5VSB, along with some waves and not-so-rough spikes in the next two tests. Overall, Seasonic's performance is good, though not perfect.

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