Load Regulation, Line regulation and Cross-Loading
DC Output Load Regulation

One of the first things we want to see is how well this PSU can regulate the DC outputs and maintain stable voltages.  To simulate real world and maximum loading conditions, the HCG-900 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.

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The following tables list the DC voltage results for the PSU while operating on 115 VAC, 60 Hz.

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Overall the HCG-900 PSU produced good results for voltage regulation on the various DC outputs across a broad range of loads.  The +12V-2 and +12V-4 outputs dropped a little more than we would typically like to see but this was most likely due to pulling a relatively heavy load through a single 8-pin EPS or pair of PCI-E connectors.  The HCG-900 didn’t have enough cables to allow connecting the +12V outputs across my six +12V load testers as I would normally do with a 900W PSU.  With only one 8-pin EPS cable and four PCI-E cables I was not able to fully utilize my +12V-5 and +12V-6 loads, forcing a heavier load to be placed on the four available +12V loads, which results in a slightly greater voltage drop.  Even so, all of the outputs stayed within the recommended guidelines.

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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.

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The Line Regulation test was performed with the combined DC loads set to 675W.  The AC input voltage to the power supply (via the Extech power analyzer) was adjusted using a Powerstat variable autotransformer.  We observed very little measurable change in the DC outputs; 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 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 400W on the +12V outputs and only 20W (or less) on the combined 3.3V and +5V outputs, the resulting voltage regulation may suffer.

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The HCG-900 PSU had no problems handling our cross-loading tests.  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 problems with all the voltages looking good.


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