Output Voltage Stability
The APS-550S delivers stable voltages, especially considering that it lacks dedicated voltage regulation. In fact, the voltages only go out of the prescribed limits when the total load is highly misbalanced towards the +5V and +3.3V rails, which just cannot occur in a real computer. Our reference configurations (marked with the crosses in the diagram) are within the green zone, meaning that the output voltages deflect no more than 3% from the required levels.
Output Voltage Ripple
The output voltage ripple at full load is slightly above the allowable limit on the +5V rail, but you should not worry about those individual short spikes shooting above the 50-millivolt line. They are not going to affect your computer’s stability. The other two rails are all right in this test, too.
This PSU is cooled by a Globe Fan RL4Z S1352512H which is partially covered with a piece of film in order to drive the air into the back of the PSU. Thus, the entire volume of the PSU case is going to be ventilated well.
The fan is rotating at less than 700 RPM under low loads and is virtually silent then. At loads above 200 watts, the fan’s speed grows up linearly, making it audible at loads of 300-350 watts. The maximum speed of the fan is 1430 RPM but the noise remains acceptable even then.
Together with my UPS (the APC SmartUPS SC 620 model) this power supply worked at loads up to 362 watts when the two were powered from the mains, but when they switched to the UPS’s batteries, the UPS would shut down in a couple of seconds at a load of 350 watts and in 5 to 7 seconds at a load of 300 watts. Thus, this PSU has poor compatibility with uninterruptible power supplies. I recommend you to use an UPS with a high reserve of wattage or a sinusoidal output voltage with this PSU.
Efficiency and Power Factor
The PSU has a good but not record-breaking efficiency of 85-86%. It is good that the efficiency does not depend much on the load, falling below 83% at loads of 60 watts only.
The standby source is rated for a current of 2.5 amperes and copes with its job just fine. Its voltage is just a little lower than the required 5 volts at full load.
The only thing I can really find fault with in the Chieftec APS-550S is that its cables are rather short. There is no more than 42 centimeters from the PSU case to the first connector, which may be a problem for large system cases. However, if you’ve got a small system case, the APS-550S will be a good choice as it is a neat, stable, quiet and rather inexpensive power supply.