Load Regulation, Line Regulation and Cross-Loading

DC Output Load Regulation

To simulate real world and maximum loading conditions, the Seasonic X-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.

As you can see, the Seasonic X-1250 PSU produced excellent 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 ±2% of their target values – very good!   

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 Line Regulation test was performed with the combined DC loads set to 937.5W.  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 DC outputs; again very good. 

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 has not always been the case, especially in 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.  It used to be if a PC pulled 400W on the +12V outputs and only 20W (or less) on the combined 3.3V and +5V outputs, the resulting voltage regulation might suffer.  However most newer PSUs that use DC-to-DC converters to produce the +3.3V and +5V outputs from the +12V rail are relatively immune to cross-loading problems.

The Seasonic X-1250 PSU handled our cross-loading tests without problems.  In the first test we put a heavy load on the +12V outputs and a light load on the remaining outputs.  Even with this large imbalance, the voltages all stayed well within spec. 

In the second test we reversed the cross-load and placed a heavy load on the +3.3V and +5V outputs with a light load on the +12V rail.  Once again, the PSU passed this test without issues.

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