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How to Pick a Power Supply for the Test Bench



April 17, 2017

How to Pick a Power Supply for the Test Bench

 

Power is an essential ingredient for testing any active device under test (DUT), although electronic DUTs vary widely as do their power-supply requirements. Finding the right power supply for the laboratory or the production line starts with a budget and it ends hopefully; with finding a power source that meets all the test needs for present and future DUTs. Whether it is an AC power source or a DC power supply, or a single unit that can provide both types of power, a practical power supply will provide controlled current and voltage to enable testing a DUT under all the different operating conditions it is likely to see in actual use.

 

Performance requirements for a power supply start with the DUT. For example, a DUT may be a portable device such as a cell phone operating at low power, such as with a battery. A suitable power supply for that DUT should be capable of providing the DC power needed to simulate normal and even abnormal operating conditions (worst-case conditions). Surge voltages and currents for any DUT can be many times the normal operating power range, and a power supply should have the reserve power to simulate those short-term conditions. Since AC power may vary in voltage, frequency, and phase, a suitable AC power supply should be capable of controlling those parameters accurately and with the resolution needed for making fine tuning adjustments.

 

Reviewing AC Sources

 

Many of the factors to consider when selecting an AC power source are covered in an earlier Axiom Test blog, “How To: Selecting Suitable AC Sources & Loads,” available at:

http://www.axiomtest.com/blog/How-To:-Selecting-Suitable-AC-Sources-_and_-Loads/.

The amount of power needed for a test application is a key selection parameter, and the Axiom Test site organizes AC power sources into three categories: AC sources with 3 kVA or less power, AC sources with outputs from 3 to 10 kVA, and AC supplies capable of more than 10 kVA power. That earlier test blog reviews various other performance parameters for AC power sources, noting that both benchtop and rack-mount AC power supplies are available on the Axiom website from a variety of industry-leading suppliers, including California Instruments and Chroma.

 

The Chroma 61512, for example, is an AC power source that is really a test system in itself. It can program parameters such as voltage limit, current limit, and slew rate, and generate complex output power waveforms with controlled harmonic and distortion components for advanced measurements, including IEC 61000-4-13 compliance testing. The AC power source is rated for as much as 18 kVA, with selectable single- or three-phase output. It can deliver AC voltage as high as 300 V with adjustable frequency from 15 to 1500 Hz. When more power is needed, two Chroma 61512’s can be linked in parallel for as much as 36 kVA. The Chroma 61512 AC power source features a front-panel (LCD) for ease of programming and operation. It is also available with a number of different interfaces, such as GPIB, RS-232, USB, and Ethernet, for computer control.

 

Providing DC Power

 

Selecting a DC power supply for a test application also involves matching the capabilities of a power source to the test requirements for a particular DUT. The supply should provide enough power to perform testing at the maximum rated power limits of a DUT, with some amount of additional margin, such as 10% over maximum. DC power supplies are available with single polarity and dual polarity output ports. For applications that require testing multiple DUTs or channels, in such cases the maximum power should be specified for each of the channels.

 

Some DUTs, such as radio transmitters, may turn on and off with an almost instantaneous demand for current, and a DC power supply intended for testing such DUTs should have a fast transient response to handle the changing supply requirements. The load impedance presented by a DUT may also change quickly, requiring a DC power supply with good load regulation for high stability under changing DUT operating conditions.

 

A DUT may require constant current supply for proper testing. A DC power supply for such an application should provide the means of quickly and automatically adjusting voltage to maintain constant current to the DUT, even if the load impedance changes. On the other hand, for a DUT with rapidly changing current demands, a DC power supply should incorporate adequate load regulation to maintain voltage at a preset value even as the current to the DUT is changing. DC power supplies also incorporate line regulation, which contributes to stable DC output power even when the AC input power to the supply is fluctuating. Effective line regulation can help maintain output DC voltage regulated within ±0.5% or better, even when the AC input voltage varies by ±10% or more.

 

High stability and low ripple in the output of a DC power supply is often a function of the supply’s architecture, with linear power supplies typically providing better noise performance than switched-mode DC power supplies, although switch-mode power supplies are capable of greater power density and higher output power in smaller and lighter packages than linear supplies. A DC power supply with the filtering required for low ripple performance may sacrifice in terms of transient response speed, and this tradeoff between stability and transient response should be carefully weighed for any DC power supply being considered for measurements on a DUT with many dynamic test requirements.

 

Faster transient times in a DC power supply often come at the cost of higher noise and ripple, and these three performance parameters should be gauged together when considering any DC power supply for testing a DUT that requires low noise and fast transient response. A DC power supply’s slew rate is also an indication of its transient behavior, with faster slew rates indicating faster rise and fall times of the power supply’s output.

 

Additional functionality to consider when selecting a DC power supply for test applications is whether multiple units of a particular power supply can be run in series to achieve higher voltages than possible with a single unit, or whether multiple units can be operated in parallel when higher levels of current are needed than possible from a single unit.

 

 

Suppling AC with DC

 

Some DUTs may require both AC and DC power for testing, such as for bias, noise rejection, and simulation. Fortunately, integrated, multiple-function power supplies such as the California Instruments CSW Series are capable of providing both AC and DC voltage from the same unit. The California Instruments CSW5500 is a complete power supply system, featuring an arbitrary waveform generator (AWG), a harmonic waveform generator, and a digital power analyzer. It can provide AC or DC power up to 5500 VA, with excellent line and load regulation. In addition to generating harmonic signal components for testing, it includes a harmonic analyzer that can show harmonic content in graphical and tabular formats.

 

These are just a few of the AC, DC, and combination AC + DC power supplies available for rent or sale on axiomtest.com. Many benchtop and rack-mount supplies are in stock for rent from leading suppliers, with a wide range of power levels available to meet an assortment of DUT and measurement requirements.

 

Axiom Test Equipment is your source for used electronic test and measurement equipment rentals, sales, repair, and trade to support your specific test & measurement needs. If interested in help selecting test equipment for your project, contact Axiom Test Equipment’s sales department at sales@axiomtest.com, call 760-806-6600, or visit online at www.axiomtest.com.



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