Efficiency, Differential Temperature and Noise
Efficiency
Efficiency is defined by the power output divided by the power input and is usually expressed as a percentage. If a PSU were a 100% efficient (which none are) 500 watts of AC power going in would result in 500 watts of DC power coming out (with no waste heat to dissipate). In the real world there are always inefficiencies and power is lost in the form of heat during the conversion process. Newer revisions to the ATX12V Power Supply Design Guide V 2.2 have continued to increase the efficiency recommendations for PC switching mode power supplies and now lists both required and recommended minimum efficiencies.
We measured the AC power input to the SX500-LG PSU with an Extech power analyzer while the total DC load was found by adding all the individual +3.3V, +5V, +12V, -12V and +5VSB loads together.
The overall efficiency of the SX500-LG SFX power supply is very good and meets the criteria for 80 Plus Gold certification. The PSU struggled a little while operating on 115 VAC and at elevated temperatures to hit 87% efficiency at 100% load, but we will call this a pass as the 80 Plus Organization tests are conducted at room temperature.
80 Plus Program
Note: Tests conducted at room temperature (25°C)
Differential Temperature and Noise Levels
To simulate a demanding environment, some of the warm exhaust air from the PSU under test is recirculated back to the intake through a passive air duct, which allows the PSU air inlet temperature to increase with load, just like it would in a real PC.
The differential temperature across the power supply was calculated by subtracting the internal case air temperature (T in) from the temperature of the warm exhaust air flowing out the back of the power supply (T out).
Thermocouples were placed at the air inlet and exhaust outlet. The ambient room air temperature was 23ºC (74ºF) +/- 0.5ºC during testing.
T out = temperature of air exhausting from power supply
T in = temperature of air entering power supply
Delta T = T out – T in
Sound pressure level readings were taken 3’ away from the rear of the case in an otherwise quiet room. The ambient noise level was ~28 dBA.
*Fan not spinning
The SilverStone SX500-LG SFX-L power supply features an “intelligent semi-fanless” fan speed control circuit, which is supposed to turn the fan off at low power for silent operation. During testing, the fan stayed motionless through tests one and two and only turned on when we hit test three at 50% load. The fan was very quiet (virtually silent at low to mid power) and did not become noticeable until Test #5 at 500W. As noted previously, the 120mm Globe Fan cooling fan uses sleeve bearings, which can produce very quiet operation but potentially sacrifices the longer-life offered by a high-quality ball bearing fan..
(Courtesy of SilverStone)
An important thing to note
An important thing to note about the ‘fanless’ mode: The entirety of the fan control system is composed of a thermistor attached to the internal heatsink that controls fan voltage, and Silverstone using a fan which happened to have a startup voltage that worked well with the voltage produced by the thermistor and desired startup temperature. The problem being that it is an entirely dumb control system, with only the thermal load of the heatsink (or rather, the thermistor strapped to the heatsing with a short length of heat-shrink tubing) providing any hysterisis.
The upshot being that under idle/low-load conditions, the fan will tend to hover just at the startup voltage. Unfortunately, the fan will also make a lot of noise in this regime, with clicks coming from startup/shutdown and buzzing just below startup as energy is dissipated into the rotor sufficiently to produce audible vibration but insufficient to overcome static friction and start the rotor moving.
Sadly, this makes the PSU in practice a lot noisier than it should be. Infuriatingly, without the ‘fanless mode’ the PSU would be really quite silent. Silverstone (via Tony Ou on the HardOCP forums) even tried their own slim 120mm fan, [url=http://hardforum.com/showpost.php?p=1041479391&postcount=596]and rejected it doe to not spinning down[/url], meaning the PSU is semi-fanless at the expense of actual low-noise operation!
There are quite a few other PSU sellers using the same Sirfa platform that Silverstone are using here, and with different fans, so if they avoid the ‘semi-fanless’ mode those should in practice be quieter. Or the SX500-LG could be modified with Silverstone’s own fan if you’re willing to void the warranty (the fan is soldered to the PSU PCB). Option 3 would be to strap a thick 120mm fan to the outside to keep the PSU cool enough that the thermistor never gets hot enough for the fan to approach the startup voltage in the first place.
I’ve replaced the stock fan
I’ve replaced the stock fan in my SX500LG with the Silverstone FW121 that Tony Ou used. It’s not perfect as the fan still has a quiet buzzing/chatter while spinning, but it’s a good improvement over stock.
Also the stock Globefan was not soldered down. It used a 2 pin fan plug+socket, with some sort of glue on top (not engulfing) to secure it.
My next plan is to swap the FW121 for a Scythe Slip Stream SY1225SL12M as it has a very low starting voltage, although this is a full thickness fan that will need to be mounted to the outside.
I recently purchased this
I recently purchased this power supply and it’s installed in my Xigmatek Nebula ITX case with Z97 mobo, 4690k, GTX970 ITX and even while gaming (Shadow of Mordor – high/ultra settings for example) the fan on the psu still doesn’t spin up. I figured out that because my case has negative air pressure, air is being drawn in from the rear of the power supply keeping it cool enough so it never gets to the target temperature for the fan to kick in – making it passively cooled. Fantastic power supply – I’d just rather it didn’t use semi fanless mode as the Nebula case is designed so that the psu aides the removal of warm air. My case, GPS and cpu temps are all fine, I just think they’d be a little lower with the extra exhaust provided by a spinning psu fan.