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 Thermaltake Toughpower DPS G 850W 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 Toughpower DPS G 850W PSU to the PCPerspective power supply test equipment.
• (1) 20+4 pin ATX
• (1) 8-pin EPS/ATX12V
• (6) 6-pin PCI-E
• (2) SATA
• (2) Molex
Note: As we mentioned earlier, noticeably missing is a second EPS cable. Not everyone needs two EPS cables but at this power level and price point, Thermaltake should have included it. It also makes testing a bit more difficult as we use that second EPS cable/connector to help spread out the +12V power across the load testers. Not having it means all the +12V load has to go through fewer cables/connectors, which results in more of a measured voltage drop as the +12V load increases.
DC Output Load Regulation
To simulate real world and maximum loading conditions, the Toughpower DPS G 850W 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 Toughpower DPS G 850W PSU produced very good voltage regulation on all of the DC outputs, with the three primary rails staying within ±2% of nominal. If a second +12V EPS cable/connector had been supplied with the power supply and used during testing, I suspect the +12V rail might have done even better.
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 Toughpower DPS G 850W power supply exhibited good AC ripple suppression as well. The +12V rail started off with a little more ripple than we would like to see but the PSU kept it under control as the load increased all the way up to a combined load of 850 watts. All values stayed well below half of the recommended ATX guidelines.