Load Regulation, Line Regulation and Cross-Loading

DC Output Load Regulation

Of course 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 two Corsair TX Modular PSUs were connected to the load testers and supplied with a constant 115 VAC. In these tests we are interested in seeing how well each 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 at the different loads for the two TX Modular PSUs while operating on 115 VAC, 60 Hz.

Corsair Enthusiast Series TX550M

The PSU produced good load regulation on all of the DC outputs across a broad range of loads; staying closer to ±3% instead of the recommended ±5%.

Corsair Enthusiast Series TX750M

The TX750M produced very good load regulation and kept all of the main outputs easily within ±3% of the nominal voltage instead of the recommended +/-5%, while delivering 750 watts of DC power.

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 tests were performed with the combined DC loads set to 413W and 563W respectively. The AC input voltage to each power supply (via the Extech power analyzer) was adjusted using a Powerstat variable autotransformer. Virtually no 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.

In the first set of tests we put a heavy load on the +12V output and a light load on the remaining outputs. Neither PSU had any problem delivering 36A or 52A on the +12V rail under these conditions. Even with this large imbalance, the voltages all stayed well within spec.

In the second set of tests 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, both PSUs passed this test without problems and all the voltages remained in spec.

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