AC Ripple, Power Factor and Efficiency

AC Ripple and Noise on the DC Outputs


The amount of AC ripple and noise present on the DC outputs was checked using an oscilloscope.  This AC component may be present in the KHz range where most switching power supplies operate or it may be more prevalent at the 60 Hz line frequency.  I adjusted the O-scope time base to look for AC ripple at both low and high frequencies. 


The ATX12V V2.2 specification for DC output noise/ripple is defined in the ATX12V Power Supply Design Guide.


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Ideally we would like to see no AC ripple (repetitive) or noise (random) on the DC outputs – the cleaner the better!  But in reality there will always be some present.  I measured the amplitude of the AC signal (in millivolts, peak-to-peak) to see how well the power supply complied with the ATX standard.  The following table lists the ripple/noise results during all of the load tests for the four main output voltages of interest.


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The SilverStone OP750 power supply exhibited excellent AC ripple suppression across the board.  This is an area where some high output power supplies have trouble but not the Olympia 750W.  All values stayed well below the recommended limits, even at the maximum rated output – very good!


Power Factor (PF)


Power factor (PF) is one of those mysterious properties of AC that even most electrical engineers have a hard time explaining.  I’m only presenting a brief overview of the subject here – for a more detailed discussion about PF; please see my expanded comments in this review.


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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 at both 115 VAC and 240 VAC input voltages.  The Olympia 750W power supply uses Active PFC circuits so as expected; the majority of 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 with harmonics) 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 now lists both required and recommended minimum efficiencies.


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I measured the AC power input to the SilverStone Olympia 750W 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 Olympia 750W power supply is very good at moderate loads but drops off at higher loads.  The maximum efficiency appears to peak out around 400W, which is where many users are likely to be operating.  We didn’t quite make it up to the advertised efficiency of >80% across the board, but we came very close when operating on 240 VAC.  Note that efficiency will almost always be higher at the 240 VAC line voltage versus 115 VAC (as the voltage goes up the current goes down, and since line/component loses are proportional to current, less current means lower loses).


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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… 🙂


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