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) 1,000 watts of AC power going in would result in 1,000 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.

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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.

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I measured the AC power input to the Antec HCP-1200 PSU with the Extech power analyzer while the total DC load was found by adding all the individual +3.3V, +5V, +12V, -12V and +5VSB loads together. 

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The overall efficiency of the Antec HCP-1200 power supply is very good.  Note that an 80 Plus Gold certified PSU should obtain 90% efficiency while operating at a 50% load.  While the HCP-1200 easily met this requirement when operating on 240 VAC line power, it wasn’t able to reach 90% efficiency when operating on 115 VAC line power.  This actually shouldn’t be a concern because our tests are conducted under simulated real world operating conditions, which include operating temperatures well above the 25°C that the 80Plus Organization uses for their testing.  Higher operating temperatures result in lower efficiency so these results are normal. 

80 Plus Program

There is a growing awareness among users, PC manufacturers and electric utilities regarding the money and natural resources that could be saved by adopting higher efficiency PC power supplies.  One group that is spearheading this movement is the 80 Plus Program, which is supported by the electric utilities industry.  You can learn more about their efforts to promote power supplies with better than 80% efficiency by visiting the 80 Plus Program website.

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    Note 1: Power Factor ≥0.90 (50% to 100% Load)
    Note 2: Tests conducted at room temperature (25°C)

Spending a little more money up front to purchase a high efficiency power supply may very well pay for itself over the lifetime of the PC, especially when you are using this much power… 🙂

Differential Temperature and Noise Levels

To simulate real world operation the HCP-1200 power supply was mounted in a modified mid tower case (Lian Li PC60) during testing.  Some of the warm exhaust air from the PSU under test is recirculated back into the case, which allows the internal case air 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 ~27 dBA. 

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Below 600W output and 30°C inlet air temperature, the HCP-1200 PSU is very quiet; especially for a 1,200W PSU using an 80mm fan!  Temperatures continue to build as the load increases and the cooling fan gradually speeds up to where it becomes noticeable.  Note: I am not able to take SPL readings at the higher loads due to all the programmable DC load cooling fans running.  I got lucky and was able to catch a quick SPL measurement during a brief period when all the load fans had cycled off during the 900W load test.

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