Wednesday, September 21, 2011

What is load effect and how does it affect my testing?

Load effect is a power supply specification (also known as load regulation) that describes how well the power supply can maintain its steady-state output setting when the load changes. More formally, it specifies the maximum change in steady-state DC output voltage (or current) resulting from a specified change in the load current (or voltage), with all other influence quantities maintained constant. So, when a power supply is regulating its output voltage in CV (constant voltage) mode, this specification tells you how much the voltage can change when the current changes. Here is an example:

Let’s say the voltage load effect specification for a 20 V, 5 A power supply is 2 mV and is specified for any load change. This means for any current change within the rating of the supply (in this case, up to 5 A), the output voltage will not change by more than 2 mV. For example, if the power supply is set to 10 V, the actual output may measure 9.999 V with no load (0 A). (Note that the difference between the setting and the actual output voltage is a different specification called programming accuracy.) If you then increase the current from 0 A to a full load condition of 5 A, the load effect specification guarantees that the output voltage will not change by more than 2 mV, so it will be somewhere between 9.997 V and 10.001 V. So if the actual output voltage started at 9.999 V with a 0 A load and measured 9.9982 V with a 5 A load, the load effect for this output when set for 10 V measures 0.8 mV (9.999 – 9.9982), well within the 2 mV specification. You must make the second voltage measurement immediately following the load current change to avoid capturing any short-term drift effects.

In the above example, the specified change in load current was “any load change”. Of course, it is implied that the load change is within the output ratings of the supply. You cannot change the output current from 0 A to 100 A on a 5 A power supply. Some load effect specifications state that the load change is a 50% change (e.g., 2.5 A to 5 A) while others may say 10% to 90% of full load (e.g., 0.5 A to 4.5 A).

And what does “with all other influence quantities maintained constant” mean? Things like temperature and the AC line input voltage can affect the output parameter, so these things must be held constant in order to see only the effect of the load change. The effects on the power supply output of changes in each of these influencing quantities (temperature, AC line input voltage) are described in different specifications.

Most performance power supplies have load effect specifications in the range of just a few hundred uV up to a few mV. A lower performance model may have a load effect specification of between 10 mV and 100 mV. Power supplies with higher maximum voltage ratings and higher maximum power ratings typically have higher load effect specifications.

If you have an application where maintaining an exact voltage at your DUT is critical and your DUT draws different amounts of current at different times, you will want to use a power supply with a low load effect specification. If changes in the voltage at your DUT with changes in DUT current are less critical to you, most power supplies will perform well for your application.

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