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Grasping linear power supply's data sheet (Part 2)

11 Jan 2013  | Robert Green

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To ensure confidence in a test's results and the repeatability of those results, a power supply that can accurately deliver the required power to the DUT is essential. A power supply that lacks sufficient accuracy or stability will have an effect on measurement results that will be indistinguishable from the effects due to the DUT's actual performance. Temperature drift, sudden load changes, and fluctuating AC line voltage are just some of the factors that can cause inaccurate test results. An accurate power supply that is designed to cope with these variations and to provide the voltage or current specified consistently and accurately makes it possible to be confident about the accuracy of test results.

Other important performance considerations
Instrument manufacturers typically don't attempt to specify every aspect of an instrument's performance; otherwise, instruments would never reach the market in a cost-effective and timely manner. Also, some aspects of an instrument's operation are best considered as "valuable features" rather than "performance specifications." Even though features are not specified, it's important to understand what advantages they can offer. Obviously, not all power supplies have the same feature sets, nor are all features implemented in the same way. Here are some power supply features to consider:

Multi-channel power supplies can have output channels that are tied to a common point on their low side, or they can have isolated outputs. When the channels are connected with the same common point, they cannot be used to power circuits that are isolated from each other. Medical monitoring devices, for example, have circuits that are in direct contact with a human. Those circuits use a reference point that is isolated from the earth ground-based circuitry on the power line side of the device. This is also true for a large number of products that use opto-isolators to create separate, independent common reference points for different analogue circuits or analogue and digital circuits. Testing the medical device circuits and other circuits with isolated references with a multi-channel power supply requires that the power supply channels be isolated. Before choosing a multi-channel power supply, first determine whether non-isolated or isolated channels are needed and how the channels of a given power supply are configured.

Some devices, such as battery-powered electronics, may contain circuits that can be turned off to minimise power consumption. A wireless phone, for example, will often turn off its RF power amplifier circuitry, its highest power consuming circuit, to extend battery life. If a multi-channel power supply is being used to power different circuits in a single device during testing, independent control, that is, the flexibility to turn off a power supply channel without turning off all channels, is essential when testing circuits that have their power supply controlled based on the state of the instrument. When investigating a multi-channel power supply, determine whether the capability to enable and disable channels individually is essential, and if such a feature is required, make sure the power supply includes it.

Triple-channel power supplies typically have two analogue channels (to power multiple circuits or to create bipolar power supplies for testing circuits that can output or measure both positive and negative signals) and a third channel intended to power a digital circuit. The voltage for this third channel is typically 10V or less (for testing digital circuits operating at 5V or less). Pay close attention to how the power supply's vendor specifies that channel. Some supplies have fixed (i.e., non-programmable) voltage output channels, which are often 5V channels. However, if the application requires testing digital circuitry that operates at 3.3V or 1.8V, a programmable third channel is essential.

Many tests may require sourcing more voltage or more current than an individual channel can provide. Some multi-channel power supplies allow combining channels in series or in parallel to extend the ranges of the output voltage and the output current. In some cases, a multi-channel supply might allow connecting two channels in series to increase voltage output, but combining channels in parallel will not be possible. This condition occurs if the two channels are not isolated and have a common low reference point. For full flexibility to get both increased voltage (typically double the voltage of one channel or double the current of one channel), select a multi-channel power supply with isolated output channels. For example, the Keithley Model 2220-30-1 Dual Channel and 2230-30-1 Triple Channel Power Supplies include operating display modes that configure the output display to show the combined output of the channels directly so that the exact output to the load is always displayed, regardless of whether the channels are configured in series or in parallel.

It is critically important to ensure that a circuit operates within its performance specifications over its defined voltage operating range. Supplies with tracking functionality offer a convenient way to test a bipolar circuit with a multi-channel power supply in which both channels, a positive-configured output and a negative-configured output, are linked so they change synchronously with each other. Some multi-channel power supplies can track only with both channels outputting voltages of the same magnitude. Other power supplies, such as the Keithley Model 2220-30-1 and 2230-30-1, allow tracking with a variable ratio between the two channels.

Conclusion
To be confident of the quality of a power supply's design and manufacturing, look for a clear explanation from the manufacturer of how it supports its published specifications. For example, the manufacturer should use test instruments calibrated to standards traceable to a recognised primary standards laboratory. Also, check to see if the supply has a safety certification from one of the internationally recognised agencies, such as CSA, UL, or VDE. This says that the manufacturer has had independent confirmation of compliance with international safety standards.

Regular performance verification is important to ensure that a power supply is operating within the manufacturer's specifications. Check the documentation that comes with the supply for information on performance verification procedures and recommended calibration cycles.

About the author
Robert Green is a senior market development manager at Keithley Instruments, which is part of the Tektronix test and measurement portfolio. During his 20+-year career at Keithley, Mr. Green has been involved in the definition and introduction of a wide range of products, including picoammeters, electrometers, digital multi-meters, and temperature measurement products. He received a B.S. in Electrical Engineering from Cornell University and an M.S. in Electrical Engineering from Washington University, St. Louis, Missouri.

To download the PDF version of this article, click here.


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