DC Load Regulation and AC Ripple
PSU 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, real-world 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
• Differential Temperature
The be quiet! Dark Power Power Pro 11 1,000W CM 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)
• (3) 218W Precision resistor load bank (+12V)
• 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 SP 10 800W CM to the PCPerspective power supply test equipment. The power supply was tested in the multi-rail mode (Overclocking feature not engaged).
• (1) 20+4 pin ATX
• (2) 8-pin EPS/ATX12V
• (9) 6-pin PCI-E
• (2) SATA
• (2) Molex
DC Output Load Regulation
To simulate real world and maximum loading conditions, the be quiet! Dark Power Power Pro 11 1,000W CM power supply was 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.
The following tables list the DC voltage results for the PSU while operating on 115 VAC, 60 Hz.
The Dark Power Power Pro 11 1,000W CM power supply produced excellent voltage regulation on all of the DC outputs while delivering the full rated 1,000 watts total load. All of the critical rails stayed within ±2%, which easily meets or exceeds be quiets!’s claims for this PSU (±3%). Very good!
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 Dark Power Power Pro 11 1,000W CM power supply held AC ripple and noise well in check. There was one little blip at the 200W load mark where the +12V output went up to 32 mV but quickly dropped back down as the load increased during the next test. Overall, very good and well below the ATX recommended guidelines.