Electronics warning and optimization study subject to low temperature environments

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URI: http://hdl.handle.net/2042/6556
Title: Electronics warning and optimization study subject to low temperature environments
Author: Khalkhali, K.; Gombosev, M.
Abstract: Electronics warming has broad applications in the aerospace industry, where electronics assemblies are subject to lower than recommended temperature environments. Prior to operation it is necessary to raise the temperature of the electronics to the minimum safe operating level, recommended by the manufacturer. This is managed by adding an electrical heater inside the electronics assembly enclosure. A very important characteristic of the heater is how quickly the desired temperature needs to be reached. Of course, this characteristic determines the size of the heater needed for the warm up process. Since relatively low failure mode temperatures are often required by the aerospace industry, there is an additional level of complexity involved in designing the optimum size of a heater. In a failure mode condition, the analysis requires a heater to be sized to produce a small amount of heat even under hot environmental conditions. The two requirements of fast warm up time and low failure mode temperatures seem contradictory. The art of the Thermal Management in this type of design is to come up with a specific optimum heater size that can satisfy a rapid warm up requirement and yet not exceed the maximum allowable temperature during a failure mode scenario. Often a thermal engineer needs to work with an electrical engineer to manipulate the control algorithm for the heater in order to satisfy these two requirements. This study will investigate how to approach such a problem and how one can size such heater. Also, optimization techniques are considered in order to choose the proper design among all available solutions. There are two different ways to achieve a desired warm up time of a cold soaked electronics assembly. The first approach may require some manipulation of the location of the heater inside the box or the addition of insulating materials. This approach captures the heat generated by the heater inside the enclosure in order to achieve quicker warm up period. However, the presence of an insulator increases the risk of dangerously high temperatures during a failure mode scenario. To alleviate the insulator effect, the external enclosure heat dissipation mechanism must be altered. This kind of modification is costly and requires additional design steps. This is where the benefit of the second approach comes in. Namely, the desired result of a fast warm up time and lower failure mode temperatures can be achieved by manipulating the heater controller and controlling the amount of power to the heater. A Pulse Width Modulation (PWM) Controller can be used to control not only the amount of heat generated by a heater, but also the percentage of heater "on” time within one pulse width and therefore the warm up time. Having the ability to control the frequency of releasing heat into the cold enclosure allows us to indicate two new parameters : the amount of heat load and the heat load frequency of inserting heat. A thermal management scheme can be developed to optimize the most efficient pair of values that will meet warm up time and failure mode temperature criteria. An analytical model is developed to study the thermal management of such a problem. The analytical model is used to perform an optimization study and to recommend a heater controller design for a given required specification.
Subject: Electronics warning, low temperature environments
Publisher: TIMA Editions , Grenoble, France
Date: 2006

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