Load Regulation, Line Regulation and Cross-Loading
DC Output Load RegulationOf 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 TX950 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 at the different loads for the TX950 PSU while operating on 115 VAC, 60 Hz.
The PSU produced excellent load regulation on all of the outputs across a broad range of loads. All of the outputs stayed within +/-2% of the nominal voltage instead of the recommended +/-5%, even while delivering 950 watts – very impressive.
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 712W. The AC input voltage to the 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 40W (or less) on the combined 3.3V and +5V outputs, the resulting voltage regulation may suffer.
In the first test we put a heavy load on the +12V output and a light load on the remaining outputs. The TX950 PSU had no problem delivering 72A on the +12V rail under these conditions. Even with this large imbalance, the voltages all stayed well within spec. This is pretty much what we have come to expect from newer PSUs that use DC-to-DC converters to produce the +3.3V and +5V outputs from the +12V rail, which makes them relatively immune to cross-loading problems.
In the second test we reversed the cross-load and placed a heavy load (143W) on the +3.3V and +5V outputs with a light load on the +12V rail. Once again, the PSU passed this test without problems and all the voltages remained in spec.
