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.

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