DC Load Regulation and AC Ripple

Testing Methodology

Establishing an accurate load is critical to testing and evaluating a PC power supply.  PCPerspective’s power supply test bench can place a precise DC load on the PSU under test.  Each power supply is tested under controlled, demanding conditions up to its maximum rated load (at 40ºC). Our current suite of tests includes:

•    DC Load Regulation
•    AC Ripple and Noise
•    Efficiency
•    Differential Temperature
•    Noise

The SilverStone Titanium ST1300-TI power supply was evaluated on both features and performance.  A full range of equipment was used to test the power supply under controlled load conditions.  

•    (2) CSI3710A Programmable DC load (+3.3V and +5V outputs)
•    (4) CSI3711A Programmable DC load (+12V outputs)
•    (3) 200W Precision resistor load bank (+12V outputs)
•    Switchable precision resistor load bank (-12V and +5VSB)
•    Agilent 34401A digital multimeter (Accuracy ±0.0035% vDC)
•    Extech 380803 Power Analyzer (Accuracy ±0.5% of full scale)
•    DS1M12 "StingRay" digital oscilloscope (20M S/s with 12 Bit ADC)
•    Powerstat Variable Autotransformer, 1.4 KVA, 0-140 VAC
•    Extech Model 407738 digital sound level meter (Accuracy ±1.5 dB)

The following cables/connectors were used to connect the power supply to the PCPerspective power supply test equipment.

•    (1) 20+4 pin ATX
•    (2) 8-pin EPS/ATX12V
•    (8) 6-pin PCI-E
•    (6) SATA
•    (5) Molex

DC Output Load Regulation

To simulate demanding and maximum loading conditions, the ST1300-TI power supply was connected to the load testers and supplied with a constant 120 VAC.  In this test we are interested in seeing how well a PSU can maintain the various output voltages while operating under different loads.  

The ATX12V V2.2 tolerance for voltages states how much each output (rail) is allowed to fluctuate and has tighter tolerances now for the +12V outputs.  We have included a second table of expanded tolerances (±1% to ±6%) for reference.

The following tables list the DC voltage results for the SilverStone Titanium Series 1300W PSU while operating on 120 VAC, 60 Hz.

The power supply produced very good voltage regulation on all of the DC outputs with the three main rails staying within ±2% of the recommended ATX guidelines; well within SilverStone’s claimed ±3%.

AC Ripple and Noise on the DC Outputs

The amount of AC ripple and noise present on the DC outputs was checked using a digital 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.  We adjust 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.

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.

On the surface the results of the AC ripple and noise tests all look pretty good, but at certain load combinations we saw some erratic behavior, specifically on the 12V output.  At various loads the AC ripple would jump up and then come back down again as the load was increased. This seems to be a characteristic of some new high efficiency designs as we have seen this before while testing other high efficiency power supplies, but not to this extent. At one point as we were adjusting the loads from 75% up to 100%, the AC ripple shot up to 182 mV p-p at about 85% load. This was a huge jump and far exceeds the recommended guidelines. And it is a constant AC component riding on the DC output, not just a transient blip. However increasing the load a little more (2~3%) caused the AC ripple to drop back down and get under control again. It’s like the power supply circuit resonates at particular loads. We have notified SilverStone about this and they are looking into the issue. Part of the problem is unless you are specifically looking for it you may not see it. It was by chance we caught that one load combination but we have also been able to retest and reproduce the problem.

42.60mV p-p (75% load)        182.2mV p-p (~85% load)        27.20mV p-p (100% load)
O-Scope screen shots of +12V output at three different loads

Overall, the ST1300-TI exhibited good AC ripple and noise suppression when measured at the standard 10%-20%-50%-75%-100% load points, but we can’t ignore the erratic behavior. Excess AC ripple and noise on a PSU’s DC outputs can stress your PCs components and potentially shorten its life.

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