The cross-load diagram looks exactly how it must for a PSU with independent voltage regulation. The diagram’s border is determined by the maximum allowable load on the PSU rather than by some voltage going out of the allowable limits. On the other hand, the stability of the +3.3V might be better. As a side remark, PSUs with independent voltage regulation have no advantage over PSUs with group regulation as concerns the +3.3V output. In both types of PSUs there is a separate magnetic-amplifier regulator on that rail.

It’s not so well with the FA-550A. The +12V and +5V voltages remained stable as the load was being increased, but the +3.3V voltage sunk so heavily that the top and right borders of the graph go along the line where this voltage was below the lowest acceptable value. The graph doesn’t look very pretty as a result.

It doesn’t mean that the FA-380A is better than the FA-550A, though. These units have the same circuit design, but the FA-380A had a higher +3.3V voltage initially, i.e. under the minimal load, and reached the maximum output power before that voltage violated the recommended limits. Things may be different with two other samples of these PSUs – some other FA-380A may have a lower initial +3.3V voltage and its cross-load diagram will look exactly as the FA-550A’s. I hope that it was our sample of the FA-550A that was a deviation from the rule rather than otherwise, especially since the mentioned PSU from Antec, designed in generally the same way, had boasted good voltages stability.

The senior model didn’t quite pass the output voltage ripple test, either. At 360W load the voltage ripple was 38 and 60 millivolts on the FA-380A’s +5V and +12V rails (both within the required limits), but the ripple on the FA-550A’s +5V and +12V rails, at 530W load, was 65 and 80 millivolts, respectively, the allowable maximums being 50 and 120 millivolts. The pulsations are half low-frequency (at the double AC frequency or 100Hz in our case) and half high-frequency ones (from the power supply’s inverter).

The fan speed is quite efficiently controlled in these PSUs. It linearly depends on the load and the max speed is almost two times the minimal one. The PSU is not quiet under high loads – the 12cm fan becomes audible at 1500rpm and higher. Moreover, as I said above, the fan’s blades emitted a quiet buzzing sound on the senior PSU model.

The efficiency of these PSUs is average by today’s standards. It is a little over 80%, but only at high loads. The power factor is 98% and more at 150W and higher loads thanks to active PFC.

So the Foxconn WinFast FA-380A and FA-550A power supplies are not definitely good or bad. On one hand, they are neatly assembled, have all the necessary connectors and long enough cables, and are potentially very stable due to the independent voltage regulation. Unfortunately, the potential is not fully revealed in practice. The +3.3V regulator was not very stable in the junior 380W model, but without any consequences. In the 550W unit, however, it was the reason for a very poor cross-load characteristic. The output power of the FA-550A is in fact limited to 350-400W because if the load on the PSU is higher, the +3.3V voltage goes down below acceptable level. The senior model’s output voltage ripple isn’t very good, too. It is higher than normal on the +5V rail and the strong 100Hz ripple betrays poor design or assembly of the high-voltage section.