I've been eagerly anticipating the HDPLEX 400w for quite awhile. It's the first DC-ATX style PSU (like the PICO PSU) that can deliver enough power to adequately feed a high end system. Please watch the video above for my in-depth review.
The HDPLEX is called (by its creators) a DC to ATX PSU. Traditional PSUs like ATX/SFX take alternating current (AC) from the wall, convert it to direct current (DC). Then they break up the power into 3.3v, 5v, and 12v sources (over simplified) and supply that power to your PC. What makes the HDPLEX different is that the conversion from AC to DC power is done somewhere else. The HDPLEX takes in DC power and then turns it into "ATX," hence the classification of DC to ATX PSU.
So where does the AC to DC conversion happen? In an external AC to DC adapter. The one I used in this review is a 330W Dell that was used to power some alienware desktops and laptops.
The short answer is that there is no widely available or relatively affordable 400w + AC/DC adapter.
Determining how much power the HDPLEX can deliver is a little confusing. We have two power ratings, one for the PSU (HDPLEX) and another for the AC/DC adapter. A regular ATX PSU has a single wattage and efficiency rating. For example, if a 1000w PSU has a 90% efficiency, it would (in theory) need to pull 1,111w from the wall to deliver 1000w to the PC. (in reality the efficiency rating is not determined at 100% load, but this is just an example)
With the HDPLEX 400w, the power first comes from the AC adapter. The Dell 330w adapter is rated at 87% efficient, so (again in theory) to deliver a full 330w, it would need to pull 380w from the wall. The HDPLEX 400w is 95% efficient, so of the 330w provided by the Dell, 313.5w are available to the PC. This is the number you want to keep your system TDP under.
To test the HDPLEX, I tried to assemble the highest performing system that I could build and remain within the power limits of the AC/DC adapter and PSU.
I estimated the total system TDP to be around 290w at its absolute peak. This would result in the AC/DC adapter pulling around 350w from the wall. In practice, I was only pulling on average 300w from the wall. I overclocked the CPU and GPU and this number grew to about 320w. The video above goes into these results in detail.
The HDPLEX is a great product and I would recommend it to anyone looking for a small form factor PSU. The ability to power top tier components in such a small package is quite frankly revolutionary. GPU's are more powerful and using less energy than ever before. I hope that HDPLEX 400W and/or products like it will become more standard in small form factor cases. It's a fairly niche product right now because most PC cases only support ATX and SFX style PSUs. I expect this to change in the future.