In my tests the fan turned on not at a 250W load, but after twenty minutes of working at 150W load when the temperature of the heatsink with the diode packs reached about 70°C. The fan is going to turn on even earlier in a real computer where it will be additionally heated up with warm air from the processor and the graphics card.

The external heatsink is covered with a protecting grid which is not very necessary in fact. The heatsink is not hotter than 60°C even at the maximum load, so you won’t have a chance to scorch your fingers.

Otherwise, the power supply presents nothing particularly interesting. It is a typical design on a TL494 PWM controller (located on a separate card), without any power factor correction or additional regulation of the output voltages.

The PSU offers you the following cables:

• A 45cm cable with a 20+4-pin mainboard’s connector (the 4-pin part is detachable and the resulting 20-pin connector can be used with older mainboards)
• A 47cm ATX12V cable with a 4-pin connector
• A 46cm cable with a 6-pin graphics card power connector and an additional LC-filter (two 10µF capacitors, two 0.1µF capacitors, and a ferrite ring on the cable)
• Two cables with three Molex connectors and one floppy mini-plug (49cm from the PSU to the first connector and 15cm more to each next connector)
• A cable with four SATA power connectors (47cm to the first connector and 15cm more to each next connector)

The mainboard’s power cables are sheathed into plaited pipes, the graphics card cable – into a flexible plastic tube (this cable has additional screening, which is not however connected to ground). The wires in the other cables are twisted like in a twisted pair. So, the cables of the TOP-420NF are overall better than with the above-described FSP Zen – they are longer and have more connectors. Of course, you can use adapters and splitters, but it’s handier to do without them.

This model formally belongs to the ATX12V 1.3 standard, despite the 24-pin mainboard’s connector, but this version of the standard does not describe power supplies with wattage higher than 300W, so I can only say that the TOP-420NF surpasses the requirements of the standard in every parameter. On the other hand, the PSU is obviously intended for high loads on the +5V rail which is not as important for modern computer configurations as the +12V rail, and the +12V rail of the TOP-420NF has the same acceptable load as the one of the considerably less powerful FSP Zen.

The cross-load characteristics of this PSU don’t look that beautiful. First, the +5V voltage is too high. It will be at about 5.2-5.3V in modern computers where the load on this power rail seldom exceeds 30-40W. Second, the +12V and +3.3V voltages deviate rather far from the norm, too. If you compare the TOP-420NF with other, similarly designed PSUs, it will appear an ordinary, average product, of course, but it doesn’t look appealing at all against the ideal diagrams from the FSP Zen power supply with its dedicated voltage regulation.