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), using both 115 VAC and 240 VAC line voltage.  Our current suite of tests includes:

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

The SilverStone ST55F-PT and ST75F-PT power supplies were 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 (+12V1, +12V2, +12V3, and +12V4)
•    (2) 200W Precision resistor load bank (+12V5 and +12V6)
•    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 two SilverStone PSUs to the PCPerspective power supply test equipment.

SilverStone ST55F-PT 550W:
•    (1) 20+4 pin ATX
•    (1) 8-pin EPS/ATX12V
•    (2) 6-pin PCI-E
•    (2) SATA
•    (2) Molex

SilverStone ST75F-PT 750W:
•    (1) 20+4 pin ATX
•    (1) 8-pin EPS/ATX12V
•    (4) 6-pin PCI-E
•    (2) SATA
•    (2) Molex

DC Output Load Regulation

To simulate demanding and maximum loading conditions, the SilverStone ST55F-PT and ST75F-PT power supplies were connected to the load testers and supplied with a constant 115 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.  I have also included a second table of expanded tolerances (±1% to ±6%) for reference.

SilverStone ST55F-PT 550W: The following tables list the DC voltage results for the 550W PSU while operating on 115 VAC, 60 Hz.

The ST55F-PT produced very good voltage regulation on all of the DC outputs with the measured values falling within ±2% on the three major rails! This easily beats SilverStone’s own claims for tight ±3% voltage regulation.

SilverStone ST75F-PT 750W: The following tables list the DC voltage results for the ST75F-PT PSU while operating on 115 VAC, 60 Hz.

The 750W Platinum PSU also produced very good voltage regulation on all of the DC outputs; nearly identical to the results we saw on the 650W model and within ±2% for the three major rails.

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.

SilverStone ST55F-PT 550W:

The ST55F-PT power supply exhibited very good AC ripple suppression on all of the DC outputs, all the way up to full load.

SilverStone ST75F-PT 750W:

The results for the ST75F-PT power supply were also very good.

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