Efficiency, Differential Temperature and Noise
The overall efficiency of a power supply is very important. The less waste heat generated the better! 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) 750 watts of AC power going in would result in 750 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 Seasonic Gold Series X-750 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 X-750 Gold power supply is excellent and easily meets the criteria for 80Plus Gold certification, even while operating on 115 VAC and at elevated temperatures.
80 Plus Program
Note 1: Power Factor ≥0.90 (50% to 100% Load)
Note 2: Tests conducted at room temperature (25°C)
Differential Temperature and Noise Levels
To simulate real world operation, 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. I was not able to take SPL readings at the higher loads due to the background noise generated by all the DC Load cooling fans cycling on and off.
Note: Testing was conducted in the "Normal" S2FC Silent–Cooling Mode (fan running at all times).
The X-750 Gold uses Seasonic's excellent thermal speed control circuit that when set to the S2FC mode slowly increases the fan speed as the load increases and internal component temperatures heat up. The X-750 is virtually silent at low to mid power even with the fan spinning. Alternately, the fan control can be set to S3FC (fan-less, silent, cooling mode), which prevents the fan from turning on until ~20% load for absolute silence at low power. Personally I like to keep a little air moving at all times but the X-750 gives the end user options to suit their needs and provides one of the best fan speed control circuits on the market today.