After the SFX-L form factor SGX units, Seasonic updated its small form factor offerings with the new SPX and SGX lines following the SFX form factor. The first includes two models with 750 W and 650 W maximum output, while the latter has three members with capacities ranging from 550 W to 750 W maximum output. In today's review, I evaluated the flagship Seasonic SFX offering. I compared it with the best SFX units available today, including the Corsair SF750 Platinum, which holds the performance crown in this category. Despite Seasonic's experience in designing PSUs, the SF750 Platinum created by Great Wall in cooperation with Corsair is the far better overall performer. That said, the SPX-750 has better transient response at +12 V, which is of immense importance, and a longer warranty. It also registers much higher efficiency with 2% load since the SF750 is based on an older design, so the SF750 isn't compatible with the newer ATX requirements.

That having been said, the SPX-750 has a messy fan-speed profile, which results in a noisy operation at even medium loads. Moreover, PF readings with 230 V input are low, EMI emissions should be lower, and the 5VSB rail is inefficient. I won't say a lot about the hold-up time below 17 ms since the SF750 does not have a long hold-up time, either. There is no space to install larger bulk caps in these high-capacity SFX units, so achieving a higher hold-up time is very tough to impossible.

The primary issue with the SPX-750 is that both my samples died during the over-temperature evaluation. The way I test OTP is straightforward: I remove the top cover and heat up the secondary side while the PSU is idle using a heat gun. At the same time, I monitor temperatures on the secondary side with a temperature logger. Most PSUs shut down at a specific temperature, and once they cool down, they start again without issue, which was not the case with the SPX-750. Both samples shut down after a while, but neither would start, so the increased temperatures on the secondary side led to a failure. The OTP should be adjusted to ensure this does not happen. It is weird to have a powerful SFX PSU with high power density fail the OTP evaluation since increased internal temperatures aren't unexpected. I am sure Seasonic will take care of this, now that I made them aware of it.

It took me a while to post this review because I was waiting on new samples from Seasonic, and in the interim, I came across several reviews, but none mentioned the OTP issue I had found, which is odd. On the other hand, most PSU reviews don't evaluate the protection features thoroughly because it requires good knowledge of what you are doing and can be dangerous. I have seen PSUs burning or exploding during this procedure, and on some days, I do break thousands of dollars worth of PSUs. I am always sad when hardware fails while in my hands, but this is my job, and having it happen in my hands is definitely preferable to having it happen in yours.

I am looking forward to more Seasonic reviews since anyone can benefit from a second opinion when it comes to improving one's product, including top-rated, successful manufacturers. Unfortunately, I cannot recommend this product because of the OTP problems I came across.