Considerations when specifying a DC power supply

May 26, 2014 // By Bill Martin
Considerations when specifying a DC power supply
<p>Every automated test system that tests electronic circuit boards, modules or equipment needs one or more DC power supplies. DC supplies provide power to the device under test as well as test stimulus. In some cases, the supply used to provide power to the DUT also provides a test stimulus by simulating the operating environment. For example, while most automotive electronics run at a nominal 12 VDC, the maximum input voltage may be as high as 27 VDC. Because this is so, some automotive standards require margin testing up to 27 VDC on a 12 VDC device. Necessities such as these determine power supply requirements.</p>

Let's take a look at other common power supply specifications that you need to consider when selecting a power supply for an automated test system.

Linear or Switching Supply?

The first choice you must make when purchasing a DC power supply is whether to select a linear supply or switching supply. Linear power supplies offer low ripple and noise specifications and have fast transient behavior. They are, however, inefficient and generate a lot of heat. They are also quite heavy. As a result, most engineers find them desirable only at lower output power levels (typically less than 500 watts). Most linear DC power supplies are benchtop supplies.

Fig 1

One application for which a linear benchtop supply may be the best choice is when testing communication devices such as a radio or mobile phone or the demodulator module of a radar system. These devices have very sensitive discriminator or demodulator circuitry that work best with a low noise figure. To test the true performance of these units, we need to ensure that the DC power supply does not add any parasitic noise to the test setup, and because linear supplies have lower output ripple and noise than switching supplies, they are a better choice for this application.

Linear supplies are also a good choice when power requirements are low. The main benefits of a switching supply are only relevant at higher output power levels. It is, therefore, less expensive to use linear DC power supplies in applications that do not require more than 100 W to 200 W per DC output channel.

At this point, it is also important to consider the total power output of all the DC channels in your system. If your system has four channels or less, and the power requirements are relatively low, a good solution would be to use four linear supplies in a 19-in. rack-mount kit.

If your system requires more output channels,

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