Introduction

We would like to thank Seasonic for supplying the review sample.


Seasonic is among the few OEMs offering fanless power supplies. Building a PSU without any active cooling is challenging, especially if high-wattage, since thermal loads have to be as low as possible to ensure it operates properly under all operating conditions. Such a PSU usually uses parts that far exceed requirements to ensure increased temperatures don't cause failures if these parts are stressed. It goes without saying that all protection features should operate flawlessly, including over temperature protection.

The Prime Fanless 700 W is the strongest passive power supply that money can get you today. It uses the Prime Titanium platform with a vast wattage overhead. In essence, the platform can deliver way more than 700 W, but is set to 700 W because there is no cooling fan to remove the heat, and the higher the power output, the higher the thermal load. The best way to build a passive PSU is to use a highly efficient, strong platform and limiting power output to where the parts won't have a problem handling the heat for prolonged periods of time even without any cooling. You also have to pay extra attention to the chassis and its airflow. Hot air goes up, so you either use a beefy heatsink on top or make the top cover look like Swiss cheese. Finally, keep in mind that you should not install passive PSUs on their side or with the top exhaust grille facing down because it will trap the hot air internally, which puts a lot of stress on parts and shuts the PSU down by triggering OTP. As such, not all cases are suitable for passive PSUs, including those with a PSU shroud without any ventilation holes.

Specifications

Photos

There is only a small photo of the product on the back of the box. You will only find the model description, efficiency badge, and warranty period at the face of the package, the last of which is incredibly long at twelve years.


Protection inside the box is good, and the bundle is rich. Besides the usual stuff, we have Velcro straps, zip ties, a connector to jump-start the PSU, and the case badge. Seasonic also gives you the opportunity to win a $50 Steam e-gift by performing a few tasks.



The chassis is full of perforations for hot air to easily leave the PSU's internals. Since there is no cooling fan, this is the easiest way to keep temperatures under control.


Since there is no space on the sides for labels, there is one at the base of the PSU, leaving room for some more exhaust grilles.


The modular panel has 13 sockets; two for the 24-pin ATX connector, five for the peripheral cables, and six for the EPS and PCIe cables.


The silver Prime badge stands out on the top grille. Chassis dimensions are on the larger side of average to host the large PCB.

Cables and Connectors

The are lots of cables and connectors, with the peripheral cables different lengths to choose the best ones for your system, unless you have tons of peripheral devices. All cables are long, and only the ATX one has inline caps for better ripple suppression.


On two of the three SATA cables, the distance between connectors is long, and the same goes for the Molex to SATA 3.3 adapter. It would be nice to see the 4-pin connectors spaced apart 150 mm, too; however, I won't complain (a lot) since 125 mm is decently close to that figure.

Component Analysis

Before reading this page, we strongly suggest looking at this article, which will help you better understand the insides of a PSU.



The PCB is large to increase the space between parts for better airflow. Moreover, there are heatsinks everywhere since there is no active cooling. Even the main transformer is fitted with a heatsink, which is good since it can get quite hot. To offer better load regulation on the minor rails and increase efficiency through lower energy losses on power transfers, the DC-DC converters handling these rails are on the modular PCB. A thick cable is used to feed the VRMs of the minor rails 12 V—a bus bar would have been better.


The transient/EMI filter stages include five Y and three X caps. Two CM chokes and a Champion CM02X discharge IC are also used.


A Metal Oxide Varistor (MOV) handles voltage surges, while an NTC thermistor and relay combination suppresses large inrush currents.


The bridge rectifiers are powerful, able to handle up to 50 A of current, which is way more than what this power supply will require.


The APFC converter uses two Infineon FETs and a CREE boost diode, which I expected to be stronger since all parts for this PSU are over spec. The bulk caps are by Chemi-Con, and their combined total capacity is 940 uF, so the hold-up time will easily exceed 17 ms.


The APFC controller is an On Semiconductor NCP1654. It is on a vertical daughter-board, which looks strange since it is quite small, so it would easily fit on the main PCB. Seasonic probably did this for ease of swappability.


The four main FETs are by Infineon and installed in a full-bridge configuration.


The unit's main transformer has a large heatsink on top of it to help it keep its temperatures low.


This photos shows the unit's resonant controller and resonant tank, parts of the LLC resonant converter.


The FETs regulating the +12 V rail are installed on the solder side of the main PCB. Heatsinks on the other side of the PCB handle their thermal load.


Two DC-DC converters generate the minor rails. They are installed on the modular PCB to be closer to the peripheral and main ATX sockets for lower voltage drops.


Seasonic used topnotch caps since there is no active cooling and their operating temperatures will be higher than typical. Many polymer caps are used, and these are significantly more tolerant to high operating temperatures than electrolytic ones.


The 5VSB circuit uses three MCC ORing diodes to combine current from the 5 V circuit and deliver it to the 5VSB rail.


The standby PWM controller is a Leadtrend LD7750R.


The supervisor IC is a Weltrend WT7527V, which provides all essential protection features but OTP. The latter is implemented through another circuit.


Several polymer caps are installed on the face of the modular PCB. The supervisor IC and TPS54231 buck-converter IC are also installed here.


Soldering quality is good.