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 rock-solid voltages. To simulate real world and maximum loading conditions, the SilverStone ST45NF fan-less 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. Note: For the final 450W load test, 240VAC line voltage was used.
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.
The following table lists the DC voltage results at the different loads for the SilverStone Nightjar St45NF fan-less PSU.
The PSU produced very good load regulation on all of the outputs across a broad range of loads; even when delivering the full rated 450 watts of DC power. The three primary rails (+3.3V, +5V and +12V) all stayed within 2% 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 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 250W. The AC input voltage to the power supply (via the Extech power analyzer) was adjusted using a Powerstat variable autotransformer. As expected, very little measurable change in the DC outputs was noted.
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 (30A/360W) on the +12V output and a light load on the remaining outputs. The SST-ST45NF PSU had no problem delivering the 30A on the single +12V rail. Even with this large imbalance, the voltages all stayed within spec.
In the second test we reversed the cross-load and placed a moderately heavy load (120W) on the +3.3V and +5V outputs with a light load (4A/48W) on the +12V rail. Once again, the PSU passed this test without problems.
