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 new 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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Up until know, most of the test results have been very good.  Unfortunately the Quattro 1,000W power supply exhibits a mixed bag when it comes to AC ripple suppression.  The +5V and four +12V rails are OK, starting out low and staying well below the maximum allowable specs even at the full 1kW output.  However, the +3.3V and +5VSB outputs are another story.  The +3.3V rail started out fairly quiet but quickly became active as the load increased.  At 999.9W DC load, the +3.3V rail had reached the maximum allowable limit of 50mV p-p.

Even more disappointing was the AC ripple present on the +5VSB output.  It started out active and quickly went out of spec, increasing to 100mV p-p under the full 1kW load.  The +5VSB section remains powered up and active even when the PC and the rest of the PSU is turned off.  The motherboard uses +5VSB power to turn on the PC when the power-on button is pressed but +5VSB may also be used to power USB headers.  Having a very noisy +5VSB output is not what I want powering the memory chips in a USB flash drive.

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 – 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 Quattro 1,000W 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. 


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