Testing (Cont’d)

Power Factor (PF)


Power factor (PF) is one of those mysterious properties of AC that even most electrical engineers have a hard time explaining.  A thorough technical discussion goes beyond the scope of this review (not to mention this author’s understanding).  For a more detailed discussion about PF, please look here.


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AC Volts x AC Amps = VA (Volt Amp)


Purely Resistive AC Load: VA = Watts (same as DC circuits)

Inductive/Reactive AC Load: VA x PF = Watts


AC Volts x AC Amps x PF = Watts


Power factor is defined as the ratio of true power (measured in watts) to apparent power (measured in Volt Amps).  It measures how effectively AC power is being used by a device.  The difference between true power and apparent power is expressed as the power factor and results from the way true power and apparent power are measured.  Ideally we would like to have true power and apparent power equal to one another, which would result in a PF of 1.00 or 100% effective power utilization. 


I measured the AC Power Factor with an Extech power analyzer.  The BFG Tech 1kW power supply uses Active PFC circuits so as expected, all readings were close to 1.0.


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Note: A power supply with active PFC is more environmentally friendly (doesn’t pollute the AC transmission grid) and will draw less current, but it will not save you money on your monthly electric bill unless you are a commercial user whose bill is based on PF and usage.  




The overall efficiency of a power supply is very important, especially when operating at higher power levels.  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) 600 watts of AC power going in would result in 600 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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The latest 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 the latest revision (Ver 2.2) now lists both required and recommended minimum efficiencies.


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I measured the AC power input to the BFG 1kW PSU with the Extech power analyzer and calculated the combined DC power output by summing the products of all the DC outputs (volts x amps) for six different DC loads.


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The overall efficiency of the BFG Tech 1kW power supply proved to be rather disappointing.  It’s fairly common for most high-end power supplies to be at least 80% efficient these days but the BFG 1kW didn’t meet the recommended minimums.  Keep in mind all that wasted power is converted into heat… and in the next section we’ll see how this power supply deals with that.


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The 80 Plus Computer Power Supply 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 power supplies.  One group that is spearheading this new movement is Ecos Consulting.  You can learn more about their efforts to promote power supplies with better than 80% efficiency by visiting the 80 Plus Program website.


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


The differential temperature across the BFG 1kW power supply was calculated by subtracting the ambient room air temperature (T in) from the temperature of the warm exhaust air flowing out of the power supply (T out). 


Thermocouples were placed at the air inlet and exhaust outlet. The ambient room air temperature was 24ºC (75ºF) +/- 1ºC during testing.


T in = temperature of air entering power supply

T out = temperature of air exhausting from power supply

ΔT = T out – T in


Sound pressure level readings were taken 3′ in front of the PSU in an otherwise quiet room.  The power supply was placed on a foam rubber mouse pad during testing.  The ambient noise level was ~30 dBA. 


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Below 300W output, the BFG 1kW PSU was relatively quiet.  Above 400W, the two cooling fans ramped up noticeably and above 600 watts the power supply was just plain noisy.  Note: I was not able to take accurate SPL readings for the last two loads because all the little cooling fans in the electronic DC loads had kicked in.  Even so, the power supply fans had become what I consider loud (for an office environment) and continued to increase in pitch and noise level as the load increased.


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