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Building a burn-in chamber

Posted: 19 Jun 2015 ?? ?Print Version ?Bookmark and Share

Keywords:switch mode power supplies? SMPS? TS35? RS485? analogue to digital converter?

The probability of failure of any complex system follows the so-called bathtub curve where the likelihood of a system problem is relatively high early in its life and diminishes as it passes through infancy. The likelihood of failure then continues at a low rate until the system approaches the end of its life expectancy and then the likelihood of failure increases. Still dealing in generalities: The ageing of a system is accelerated in proportion to its operating temperature; the higher the temperature, the quicker the system ages.

The company I work for designs and manufactures several ranges of switch mode power supplies (SMPS) from 20W to 1KW. They typically have a universal AC input 120/240V and a 24VDC output, although there are some significant variations. The SMPS is a complex system and because of the competitive nature of the business and the criticality of their use it was decided to burn in all power supplies. I was not part of the decision to design our own burn-in chamber, but I presume that because we normally test a fairly large number of units and some can be quite bulky and there wasn't a suitable commercial unit.

There have been three iterations of the chamber: the first when it was built, the second when we couldn't find spares for our obsolete distributed I/O control system, and the third was when we moved location. In all we have invested many man-years of effort in the project.

Both chambers were built during building expansion or major renovation and are merely a sturdy insulated room (in fact the first room became the tornado shelter for the new building residents when we moved) with suitable ventilation plus a multi-kilowatt electricity supply. The rest gets added. The component parts of the chamber are part commercial products, part hand assembled prototype, and part custom designedand the system is designed to burn-in up to 200 power supplies simultaneously.

All of the SMPSs we make mount on TS35 rail. Most also have pluggable connectors as well and so the racks were fairly easy to make (as can be seen in figure 1). The power supply simply clips onto the rail and a quick connection is made using the pluggable connectors.

Figure 1: The racks are built around the TS35 rail and quick connect wires that are then connected to the load cables. A distribution panel is mounted on each rack to centralise the power source and protect the wiring.

Each SMPS is connected to an electronic load which is made up of a MOSFET as described in my earlier blog. Twelve MOSFETs and the associated power resistors are mounted on a large (550mm x 380mm x 15mm) heatsink. Several heatsinks can be slotted into a cart as you can see in figure 2.

Figure 2a: Heatsinks mounted in some racks.

Figure 2b: A close-up of one of the heatsink assemblies. The square object is the MOSFET and the gold devices are the power resistors connected to the MOSFET source. The op-amps are mounted on the PCB in the middle of the heatsink.

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