Environmental Requirements for COTS Power Supplies

July 7, 2014
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This post, the third in the series about requirements for COTS power supplies, describes tests that ensure that the equipment is properly designed to meet the environmental conditions that it may face during its service. Many of these tests are very stringent: if these demanding requirements must be met, then often a custom solution is likely to be required.

A key concept of the standard is to establish chamber test methods that replicate the effect of the environment on the design, rather than replicating the environment itself. Mil-STD 810G is one of the applicable standards, and the most common requirements it defines are vibration and shock.

Vibration testing consists of many different types of tests that target very specific applications and environments such as helicopter, tracked vehicle, shipboard, missile, prop and jet aircraft. All of these have very different profiles, and obviously you cannot perform all the tests, nor can you know in advance how any unit may be mounted or located in a system.

The method does include some general basic integrity tests: Vicor’s Westcor division has selected General Integrity Category 24 as the most applicable. It’s a fairly demanding test, where the units are mounted on an exciter and vibrated with a swept frequency with a RMS acceleration level ranging from about 3 g (20Hz) to about 7 g (1000Hz) for one hour. This vibration is repeated for all three axis.

After all vibration is completed the units undergo a complete functional verification, as well as being opened up and examined for any physical signs of damage, such as loose components, chafed wiring, or cracking of plastic.

Shock is defined by method 516.6, and similarly to 514, there are a range of shock tests that also depend on the equipment and environment of the final application. Here Westcor chose Procedure 1: the 40G Functional Shock. During this test data loggers are connected to all outputs and performance is monitored during the shock events.

The shock consists of three positive and three negative saw tooth impulses with a peak acceleration value of 40G. The three positive and three negative pulses are applied to all three axis. No performance dropouts are allowed, and the units are then tested for full functional operation and, as with vibration, an internal visual inspection is performed.

In the next post, Jeff discusses the challenge of meeting holdup requirements to deal with variations in the input power supply.

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