DC Load Regulation, Line Regulation and Cross-Loading

DC Output Load Regulation

One of the first things we want to see is how well this little PSU can regulate the DC outputs and maintain stable voltages. To simulate real world and maximum loading conditions, the Seasonic 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.

As you can see, the little Seasonic SS-350M1U PSU produced very good load regulation on all of the outputs across a broad range of loads. The three primary outputs (+3.3V, +5.0V and +12V) stayed within ±3% of their target values.

DC Output Line Regulation

In this test we are interested in seeing how well a PSU can maintain the various output voltages while the AC input line voltage changes. In the previous Load Regulation test, the AC line voltage was held constant at 115 VAC. Now we will look at how much the DC outputs change as the load is held constant and the AC line voltage is changed from 120 VAC down to 90 VAC.

The SS-350M1U Line Regulation tests were performed with the combined DC loads set to 262.5W. The AC input voltage to the power supply (via the Extech power analyzer) was adjusted using a Powerstat variable autotransformer. We observed virtually no measurable change in the PSU’s DC outputs.

Cross-Loading Test

PC switching mode power supplies provide multiple DC output voltages. Ideally, the total load should be distributed across all the main outputs (+3.3V, +5V, +12V). This means that the combined +3.3V and +5V load should be proportional to the combined +12V load – as one increases, so should the other. Unfortunately, this is not always the case, especially in newer PCs that predominately use +12V and may put only minimal loads on the +3.3V and/or +5V rails.

Cross-loading refers to imbalanced loads. If a PC pulls 300W on the +12V outputs and only 20W (or less) on the combined 3.3V and +5V outputs, the resulting voltage regulation may suffer. However, this is usually not a problem for PSUs that incorporate DC-to-DC converters like the SS-350M1U does.

As expected, neither of the cross-load tests presented a problem for the Seasonic SS-350M1U PSU. All of the DC output voltages stayed well within spec.

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