The voltage ripple under the maximum load was rather strong, but not above the normal range: 45 millivolts on both +5V and +12V rails, the acceptable maximums being 50 and 120 millivolts, respectively.

As I said above, the fan of our sample of the PSU began to rotate after the unit had worked for 15 minutes under a 150W load. The speed of the fan was steadily increasing from the initial 1100rpm as the load was growing up:

Topower claims the noise from the fan doesn’t exceed 22dB and is not heard against the rest of the computer’s noises. Alas, it is not quite true: at the maximum speed of 2560rpm the stream of air is not loud, but quite audible. A Yate Loon D80SH-12 fan on sleeve bearings is employed here.

The efficiency of this PSU reached 82% at the maximum which is worse than that of the FSP Zen. The power factor is about 0.65-0.68, like in other PSUs without power factor correction circuitry.

So, it would be an overstatement to regard the TOP-420NF as a fanless power supply. It is an ordinary PSU with the fan speed control set up in such a way that the fan starts to work only at a certain temperature of the heatsinks (about 70°C). The external heatsink helps somewhat, but I think the efficiency of passive cooling would be better if more massive heatsinks, specially designed for fanless operation, were employed. On the other hand, if there were fewer obstacles on the path of the air stream (the outer panel of the PSU has very small vent openings because of the external fan), active cooling could be made more efficient, too, and a slower fan might be used then.

On the other hand, if you view the TOP-420NF as an ordinary, but low-noise power supply, much fewer complaints remain. The PSU is really silent under small loads. At higher loads the noise from its fan isn’t too loud and many users won’t notice it at all. The electrical parameters of this model are average. From this point of view, however, the price of the PSU (above $100) may look somewhat too high.