DC Load Regulation and AC Ripple
DC Output Load Regulation
To simulate real world and maximum loading conditions, the Seasonic X-Series XM2-1250 PSU 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.
Seasonic claims the XM2-1250 PSU was upgraded to provide ultra-tight voltage regulation (+2% and -0% regulation on the +12V output with ±1% on the +3.3V and +5V rails). These are pretty lofty claims but the XM2-1250 PSU was able to meet them – outstanding! The +12V rail only dropped ~0.1 volt when going from a light load (9 amps) to a very heavy load (99 amps). And the 3.3V and 5V outputs stayed within ±1%. Even the two minor rails (-12V and +5VSB) stayed within ±2%.
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 XM2-1250 power supply exhibited excellent AC ripple suppression on all of the outputs and stayed well below the recommended values, even at full load.