AC Ripple and Power Factor
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 main output voltages of interest.

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The SilverStone Element ST70EF 700W power supply exhibited fair AC ripple suppression on all of the primary outputs.   With the +12V outputs fully loaded to 56A, the ST70EF was 86% of the max recommended value.  These results are a little disappointing as up until now the ST70EF has been delivering stellar performance.  Unfortunately this is an area where quite a few PSUs typically fall down.  However, to keep this in perspective, you would never operate this PSU with a 56A +12V load.  At a more realistic load (525W combined) the +12V outputs exhibited less than 60% of the max recommended value while the +3.3V and +5V outputs were under 50%.

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+12V Rail (56A load)

Power Factor (PF)

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.

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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

I measured the AC Power Factor with an Extech power analyzer at both 115 VAC and 240 VAC input voltages.  The SilverStone Element ST70EF 700W PSU uses Active PFC circuits so as expected; the majority of readings were close to 1.00 at the higher loads. 

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Note: PF and efficiency is not the same thing.  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. 


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