The cables are rather short and there is no 6-pin power connector for the graphics card, but otherwise, I have no complaints about the cables. The 24-pin connector with a detachable 4-pin part allows using this power supply with new as well as older mainboards (that have a 20-pin power connector on board).

This model complies with the ATX12V 2.0 standard – the low load capacity of the +5V and +3V rails is well compensated with the 22amp +12V rail (which is split in two with over-current limiters).

The most interesting test for a fanless power supply is checking it at a sustained full load, i.e. at 280 watts in this case (I loaded only the +5V, +3.3V and +12V lines). The temperature of the PSU heatsinks was steadily growing up for several hours and then stopped at 90°C for the hottest of them (the heatsink with the diode packs). The heatsinks of the high-voltage section were less hot, but the heatsink of the active PFC device may reach the same temperature at an 110V input voltage (thanks to the same active PFC the reviewed power supply supports input voltages from 90 to 264V without any switches).

This temperature may seem high, especially since the power supply was not installed in a system case but was lying on my desk (the temperature would be 10-15°C higher if the PSU were installed in a computer). On the other hand, even if the temperature of the cases of the diode packs becomes 125°C, they will still be able to work with an up to 15amp current (the maximum allowable current diminishes as the temperature of the case of a diode pack grows up; 150°C is the maximum temperature of the die of the pack, not of its case, and the temperature difference between the die and the case depends on the current. In other words, the temperature of the die will be 150°C at 125°C case temperature and 15amp current). The packs are connected in parallel, so we can assume the current for such a “pack made of packs” to be 22.5 amperes (it is not correct to double the currents at parallel connection. To ensure stable operation we must assume that each additional parallel-connected element increases the load capacity by only 50-70%). But any of the output currents of this PSU is below 20 amperes, so there will be no problems here.

So, the power supply can yield its maximum output power without problems. Let’s now check the stability of the output voltages depending on how the total load is distributed among them.

The diagram above shows you the loads at which all the main output voltages of the PSU are within the acceptable ranges, i.e. within ±5% of the nominal value. The X axis shows the load on the +12V rail, and the Y axis shows the combined load on the +5V and +3.3V rails (in each point the load current on the +3.3V rail is 50% of the load current on the +5V rail; thus, the load power of the +3.3V rail is about 25% of the total load power, like in a typical today’s computer). The deflection of the voltages from the nominal in percents is marked in different colors as explained in the legend in the top right corner of the diagram.

The FSP Zen behaves quite predictably for a PSU with dedicated voltage regulation: the +5V voltage deflects from the nominal by less than 1% and the +3.3V and +12V voltages by less than 2% at any allowable load. As you can see, the power supply maintains the required levels of the output voltages at any allowable load.

The only negligible drawback is that the PSU is unstable at highly misbalanced loads. For example, its protection circuitry wakes up when there’s a 250W load on the +12V rail and less than 7 watts on the +5V rail. That’s why the diagram is shifted a little away from the axes – I usually start with a load of 5W, but here the initial load is bigger. Anyway, so greatly misbalanced loads just can’t possibly occur in a real computer system.