Testing – AC Ripple (electrical noise) on DC Outputs
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The
amount of AC ripple present on the 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.
In each case, I adjusted the O-scope time base to look for AC ripple at
both low and high frequencies.
Interestingly enough, some PSUs exhibited predominantly one or the other
while a few had both present.
Oscilloscope
measuring AC ripple on DC output (each major division equals 50 mV)
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60 Hz AC Ripple KHz AC Ripple
The
ATX specification for DC output noise/ripple is defined in the ATX12V Power
Supply Design Guide.
Ideally
we would like to see no AC noise 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 each
power supply complied with the ATX standard.
The following table lists the ripple/noise results of our 300 watt load
tests. The four main output voltages of
interest (+3.3 V, +5.0 V, +12 V and +5 VSB) were recorded for each power supply
after the 24 hr burn-in period. Values
in red are out of tolerance.
Only
two of the power supplies tested showed AC ripple noise values out of
specification. The Antec TruePower 480 was only 5mV over spec and probably
within the limits of accuracy. The Raidmax 400 on the other hand appears to
have a problem on its 5VSB output.
Four
PSUs stood out as having very clean DC outputs, even while operating under a
relatively heavy load. Both the PC Power
& Cooling units showed low AC noise levels as did the Antec TrueControl 550
and the Zalman Noiseless 400.
It
is also interesting to note that two of the PSUs with the cleanest DC outputs
incorporate active Power Factor Correction (Turbo-Cool 510 and Zalman Noiseless
400).
