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, demanding conditions up to its maximum rated load. Our current suite of tests includes:
• DC Load Regulation
• AC Ripple and Noise
• Efficiency
• Differential Temperature
• Noise
The FSP Twins 500W redundant 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 power supply to the PCPerspective power supply test equipment.
• (1) 20+4 pin ATX
• (2) 8-pin EPS/ATX12V
• (2) 6-pin PCI-E
• (2) SATA
• (2) Molex
DC Output Load Regulation
To simulate demanding and maximum loading conditions, the power supply was connected to the load testers and supplied with 115 VAC for one set of tests and then 240 VAC for a second set. During these tests we are interested in seeing how well the 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, which is what most users in North America will be powering this unit with.
DC voltage regulation is good and well within the ATX guidelines, but not as good as what we have been seeing recently from some high-end desktop PC power supplies. We need to keep in mind that the Twins 500W redundant power supply has a server-style heritage, which places more emphasis on build quality and reliability and less on tight voltage regulation and low AC ripple (as we will see in the next section). However, with a $399.00 USD asking price I would like to see both high quality, reliability AND high performance (tight voltage regulation, low AC ripple, and high efficiency).
Failure of one Power Supply Module
To simulate what might happen if one of the internal power supply modules failed, we just unplugged the power cord to one module and let the other one pick up the full load. We wanted to see what affect this might have on the stability of the DC outputs.
During tests #1 and #2 (10% and 20% load) there was very little observable change in any of the DC outputs. Starting with test #3 (50% load) we started to measure a slight drop on the +12V output, ~0.02V. At 100% load we saw the +12V output drop 0.05~0.07V when the total load shifted to just one modular power supply. Even at 11.78V, this is still within 2% of the nominal 12V target. At the higher loads, the voltage drop on the minor rails was barely noticeable (0.01~0.02V).
Bottom line, yes the redundant feature of the Twins 500W PSU appears to be working quite well!
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 FSP Twins 500W redundant power supply exhibited acceptable AC ripple and noise suppression – well within the ATX guidelines, but more in line with what we expect from an average entry-level desktop power supply.
Do most redundant server PSUs
Do most redundant server PSUs operate in this fashion or keep one on standby?
Most of them can be
Most of them can be configured to act either as a 1+1 (failover) or a 2+0 (both live).
If this cant be configured to
If this cant be configured to be 1+1 then its kind of pointless.
I currently use a high end Seasonic with a APC UPS for reliability.
Whats the point of this?
Whats the point of this? Really high price? I dont see the purpose for desktops. Better get an 1000Watt PSU single/
Redundancy is the point of
Redundancy is the point of this.
Redundancy is exactly the
Redundancy is exactly the point of this. I will most likely be buying 15 of them. Have been waiting for something like this forever!
When your production machines are 10,000$ per hour per line, having a power supply go out can cost thousands.
So I’ve known that redundant
So I’ve known that redundant PSU’s have been standard in rack-mount cases for a long time, so what’s the use-case for using these in standard ATX machines?
Rendering boxes, or other such machines?
I just legitimately don’t know.
Anytime you have a PC running
Anytime you have a PC running a mission-critical app or controlling high-value process equipment, you want to maximize up time by minimizing any potential hardware failures, like the PSU. I used to work in Bio-tech R&D and we frequently used redundant PSUs along with large UPSs on mass spec instruments and PCs running High Throughput Screening robotic systems. Any down time could cost thousands of dollars per hour.