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, all of the test results have been very good.  Unfortunately the ABS Tagan ITZ1300 power supply falls down hard when it comes to AC ripple suppression.  The +3.3V and +5V rails both start off very good but keep going up as the load increases until they are both out of spec by 1,000W.  The +5VSB rail started out quite active and also went out of spec.  Only the +12V rail(s) stayed within the ATX specification but at full load they too were very close to the limit.

This continues to be a problem for many power supply manufacturers, especially at higher output levels.  Having poor quality, electrically noisy DC outputs is undesirable even when they stay within specification.  ICs are designed to work on DC and many components have a low tolerance for high levels of AC ripple and noise on their DC supplies, which can lead to intermittent system instability.  But actually going hi out of spec is unacceptable, especially for a high-dollar power supply that advertises itself as being “guaranteed to outlast and outperform any other PSU in the market”.

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 ITZ1300 power supply uses Active PFC circuits so most of the readings should be close to 1.0. 

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While certainly well within the range of Active PFC, these numbers are a little lower than we are used to seeing with modern power supplies.

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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