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) 800 watts of AC power going in would result in 800 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 SilverStone SX800-LTI 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.
80 Plus Titanium is the highest efficiency certification available to date and requires a power supply to deliver up to 94% efficiency at 50% load. The SX800-LTI power supply is certified to be 80 Plus Titanium compliant but during our testing it struggled to meet the demanding criteria while operating on 120 VAC mains and at elevated temperatures. The SX800-LTI efficiency starts out strong, easily meeting the 80 Plus Titanium specs at the 10% and 20% load marks. However, at the higher loads and increased operating temperatures, efficiency falls off and doesn’t quite hit the mark. Part of the reason is that we test power supplies at elevated temperatures, which decreases the efficiency, whereas the 80 Plus organization conducts their tests at room temperature (25°C).
Looking at the 80 Plus Organization Verification and Testing Report on the Plug Load Solutions website for the SST-SX800-LTI shows the PSU was compliant. But as you can see in the table above the numbers are pretty close to the target values leaving little room for instrument/testing error or elevated operating temperatures.
80 Plus Program
Note: Tests conducted at room temperature (25°C)
Differential Temperature and Noise Levels
And speaking of operating temperatures, we were surprised to find the SX800-LTI ran relatively hot for a very high efficiency power supply. As the efficiency goes up the waste heat generated goes down, which typically results in a power supply running cooler.
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, up to 40°C.
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 ~27 dBA.
*Note: Fan not spinning in fanless Mode
The SX800-LTI power supply starts out in fanless mode and is dead silent. The fan started spinning once we hit the 50% load test and stayed very quiet through mid-power range. At full load with an elevated ambient temperature, the cooling fan did speed up and the noise became noticeable but never became really loud. So overall, the SX800-LTI is pretty quiet. The down side is the warm exhaust air temperatures really warmed up at the higher loads indicating the internal components were also running relatively warm as well. The higher differential temperatures are more than double what we saw last year while testing the SX700-LPT.
Even though the SX800-LTI PSU uses a 120mm fan it is only 15mm deep, which means it does not move as much air as an equivalent 120mm x 25mm fan. Combine this with an apparent fan profile that favors quiet operation and you end up with relatively warm exhaust temperatures, even though the unit is 80 Plus Titanium certified. We would rather see a higher quality fan along with a more aggressive fan speed profile to keep the internal components cooler (especially since 85°C rated electrolytic capacitors are being used) even if it means a little more noise at high loads.
(courtesy of SilverStone)
I do not quite get it. Who
I do not quite get it. Who needs 800W in SFF PC? 800W is required for top/overclocked SLI or Crossfire configuration only.
Perhaps if Corsair comes out
Perhaps if Corsair comes out with something like the TWO…..which I would do an SLI config on to play on the name or something….
Works well with an ATX
Works well with an ATX adapter and would be perfect in an mATX case (which supports SLI) meaning you have more breathing room. Also, more options is never a bad thing.
So, overclocking your CPU and GPU can tax a PSU, and therefore that extra headroom is good to have.
Finally, it’s Titanium rated. Perhaps they didn’t want to put their best components in a 400W PSU, so they are doing more of a “one size fits many” configuration here.
You just don’t see a lot of good quality PSUs in the lower end of the scale, and that’s in part due to a consumer education issue, but also, it costs more for each SKU you pump out. In other words, it’s an efficiency thing on the manufacturing side as well.
I will buy it, not because I
I will buy it, not because I need that extra wattage, but I need to push the silentness to the limit. With only use 200watt on my ITX, means with 800watt psu, It will be in 40% of its fan envelope, which sometime even the fan still not kicked on. Problem with psu without fan, is the hizzing sound
Quite obvious, the sweet spot
Quite obvious, the sweet spot is 50% usage on a PSU. When you get closer to 100% load, there is a chance that capacitors will pop. Also caps lose capacitance over time. I suppose it won’t matter if you upgrade every year though.
I always enjoy your power
I always enjoy your power supply reviews. Very insightful and thoughtful. It almost seems like Silverstone was trying to cut corners a bit on cost here. Using sleeve bearing fan, and cheap Rubycon capacitors is very unfortunate. I agree they should have used a much higher quality fan, and a much better Japanese capacitor. And that warranty is not good either. Very surprised Silverstone did this. They have always had such a good reputation for quality. An otherwise nice SFX power supply though.
This PSU proves there’s no
This PSU proves there’s no need for ATX PSUs anymore, which saves wasted space. You can run two 1080Ti cards and a high-end CPU from this PSU, which already is quite over the top. With more sensible hardware there’s even a lot of room for overclocking.
Some brands have already started releasing mATX and ATX cases that support SFX PSUs only, like Silverstone, Lian-Li and Thermaltake, with more to come.