Temperature stabilization of electronics module

Abstract: Outdoor applications of electronics modules expose the systems to harsh
environmental conditions. When very high performance is required, it may be
necessary to actively stabilize the temperature in the module. This thesis
presents a systematic approach to the problem of designing a temperature
stabilized environment for medium size electronics modules.

The target system is the front-end electronics for the antennas in the
EISCAT_3D incoherent scatter radar system. This will be placed in northern
Scandinavia, with estimated outdoor temperature span from -40C to 40C. Very
high demands on precision and timing will most likely require a temperature
stable environment. The present state in high performance thermal
management of electronics focuses on single circuit design. Thus the need
of designing a temperature stable module which could hold the entire front-
end electronics was recognized. The electronics have an estimated constant
power dissipation of about 10 W.

The temperature stabilization system consists of a 250x250x100 mm large
aluminium box insulated on the outside. Two Peltier modules are used for
active cooling and heating. Inside the box a 10 mm thick aluminium heat
spreader is attached to the Peltier modules. The heat spreader is used both
as a mount for the most temperature critical components, as well as a means
to distribute heating and cooling inside the box. Also the aluminium casing
itself is used to distribute heat energy evenly. The current through the
Peltier modules is controlled using a PID controller acting on a linearly
controlled H-bridge.

The system was initially evaluated using FEM simulations. The simulations
verified the design approach and gave a clear picture of the heat
distribution in the box over the range of target temperatures. Measurements
were made on the prototype system using a climate chamber in which the
prototype box was exposed to temperatures from 40C down to -40C over a time
period of 5.5 hours. Inside the box two power resistors were used to
generate the estimated power dissipation of 10 W. The measurements show
that the center of the heat spreader is kept at 20C with minor deviations
of +-0.02C. The air inside the box measured 45 mm above the aluminium heat
spreader shows temperature variations of +-5C.

Both simulations and measurements clearly show the feasibility of the
proposed design, with temperatures kept to close tolerances. Critical parts
can be attached to the aluminium heat spreader while less critical can be
positioned above. The use of a completely enclosed box without rotating
parts should provide long life expectations.