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 EVGA SuperNOVA 650 GM 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 (+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)
•    Extech Model 407738 digital sound level meter (Accuracy ±1.5 dB)

The following connectors were used to attach the SuperNOVA 650 GM PSU to the PCPerspective power supply test equipment.

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

DC Output Load Regulation

To simulate demanding and maximum loading conditions, the 650 GM power supply was connected to the load testers and supplied with 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.  I have also included a second table of expanded tolerances (±1% to ±6%) for reference.

The following tables list the DC voltage results for the 650 Supernova G3 PSU while operating on 120 VAC, 60 Hz.

The SuperNOVA 650 GM PSU produced good voltage regulation on all of the DC outputs, however not as good as we are used to seeing. The +3.3V output ran a little low while the +5V output ran a little high. But in all cases the outputs stayed well within the specified range of the ATX guidelines.

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.

The SuperNOVA 650 GM power supply did a good job of keeping the AC ripple and noise well under control, all the way up to the max 650W combined load.

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