The next parameter we must check out is high-frequency voltage ripple on the PSU output. It turned out that the result depended greatly on the load. For example, there was a strong ripple at high loads on the +5V rail. The oscillogram below was shot at a load of 280W on the whole power supply of which 100W fall on the +5V and 20W more on the +3.3V power rail:

The voltage ripple on the +5V power rail reached 75 millivolts – 50% above the maximum acceptable value. But if the load on the +5V is reduced to 70W, the PSU “calms down” immediately:

The ripple doesn’t exceed the acceptable limit of 50 millivolts. And if the whole load is transferred to the +12V rail, the ripple almost vanishes – this PSU is obviously designed with such working situations in mind, quite expectedly for an ATX12V 2.0 model.

I didn’t perform my fan speed test due to obvious reasons, but I did measure the efficiency and power factors. FSP promises an efficiency of no less than 89% for this model and it is really so:

The efficiency is 89.3% at the maximum and this is an excellent performance. But I want to remind you once again that I perform my tests at 200V input voltage and if you connect the PSU to an 110V power grid, its efficiency is going to drop due to loss in the active PFC circuitry. It means FSP Group doesn’t tell you the whole truth – the efficiency of this PSU is really up to 89%, but only in 220V power grids.

The power factor is rather low (for a PSU with an active PFC device). It is a little above 0.95, while in theory active power factor correction allows for a power factor of 0.99. Yet, 0.95 is still good if compared with PSUs without PFC (0.65-0.7) and with PSUs with passive PFC (0.7-0.75).

I would like to tell you about one mistake users and testers often do as they put together the efficiency factor and the power factor of a power supply. These two parameters do not relate to each other in any way. I put them in one diagram for the sake of convenience only (as they can be shown well on a same-scale diagram and are also measured with the same tools). You can’t calculate the power factor basing on the efficiency whatever formulas you use. Moreover, the efficiency value is not used at all to calculate the power factor.

So, the FSP Zen is quite an appealing product. Despite the lack of any fans, it quite successfully works under the full load (but of course, the top of the system case right above the PSU will hardly be cool to the touch). This model can work at input voltages from 90V to 264V (you are going to appreciate this if the voltage of the power grid in your area lacks stability). Using dedicated regulation of the output voltages, the power supply keeps them stable at any allowable load, while the load capacity of its +12V rail should be sufficient for all midrange computer systems and even for some top-end configurations. Of course, it wouldn’t cope with a system with two GeForce 7800 graphics cards, but people who own such SLI configurations have other things to worry about than a noisy power supply…

As for drawbacks, the cables of this PSU are rather short, and there’s a big voltage ripple at high +5V loads. The latter thing doesn’t matter much for modern computers, however, as they mostly load the +12V power rail.