Publisher Summary Thermal control of electronic components has one principal objective, to maintain relatively constant component temperature equal to or below the manufacturer's maximum specified service temperature, typically between 85 and 100°C. It is noted that even a single component operating 10°C beyond this temperature can reduce the reliability of certain systems by as much as 50%. Therefore, it is important for the new thermal control schemes to be capable of eliminating hot spots within the electronic devices, removing heat from these devices and dissipating this heat to the surrounding environment. Several strategies have developed over the years for controlling and removing the heat generated in multichip modules, which include advanced air-cooling schemes, direct cooling, and miniature thermosyphons or free-falling liquid films. The chapter summarizes analytical, numerical, and experimental work in literature, in order to facilitate the improvement of existing schemes and provide a basis for the development of new ones. The chapter focuses on investigations performed over the past decade and includes information on the thermal control of semiconductor devices, modules, and total systems.

Thermal Control of Electronic Equipment and Devices

Test data indicates that hottest spot allowances used in IEEE standards for ventilated dry type transformers above 500 kVA are too low. A mathematical model to predict hottest spot temperature rises in ventilated dry type transformers was developed. Data from six layer type test windings and a 2500 kVA prototype was used to refine the model. A correlation for the local heat transfer coefficient in the cooling ducts was developed. The model was used to study the effect of various parameters on the ratio of hottest spot to average winding temperature rise. The number of conductor layers, insulation thickness, and conductor strand size were found to have only a minor effect on the ratio. Winding height was found to be the main parameter influencing. The ratio of hottest spot to average winding temperature rise. The study based on the mathematical model confirmed previous conclusions based on test data that the hottest spot allowances used in IEEE standards for ventilated dry type transformers above 500 kVA should be revised. >

Predicting hottest spot temperatures in ventilated dry type transformer windings

Design criteria for free convection cooling c,f electronic systems are considered, The primary emphasis is on electronic equipment in which vertically oriented circuit cards are aligned to form vertical channels. Results of an experimental study utilizing a simulated electronic cabinet are discussed and compared with theoretical solutions obtained by numerical integration of the partial differential equations governing the heat transfer. Good agreement is shown between predicted and measured card temperatures for several different card spacings. Parametric effects on the maximum card temperature are given for card spacing, heat dissipation, and channel height. Nomograms are developed for rapid estimation of card temperatures and for choosing optimum card spacings. The effects of channel flow restrictions, staggered cards, and baffles are also discussed.

Free Convection Cooling of Electronic Systems

This paper describes a practical thermal design approach to natural air-cooled electronic equipment casings. A set of simplified equations for the thermal design of natural air-cooled electronic equipment casings has been proposed. The proposed set of equations satisfied the demand of practical air-cooling systems, since it takes account of factors such as the stack effect, the air flow resistance and the heat transfer due to natural convection. The effects of the outlet area and the location of the main power supply unit on the natural cooling capability of electronic equipment casings were studied using a set of equations. The results have shown that a uniform temperature distribution could be achieved when the main power supply unit was placed at the bottom of the casing. It has also been suggested that the value of the heat removed from the casing surface could be more significant than that from the outlet vent in the thermal design of natural air-cooled electronic equipment casings.

The effects of the outlet area and the location of the main power supply unit on the cooling capability through naturally air-cooled electronic equipment casings

An experimental heat-transfer study was performed to obtain design criteria for totally enclosed electronic equipment. Tests were performed on an experimental cabinet containing nine sources simulating electronic devices, and results were obtained for different distributions of source locations and heat-flux levels. Results indicate that 1) improving exterior wall emissivity from 0.2 to 0.98 drops the temperature for both cabinet wall and internal ambient air by 25°F: 2) the difference between internal ambient-air and cabinet-wall temperatures at a given power level remains nearly constant despite variations in cabinet-wall temperature; and 3) concentrating sources near the cabinet top produces extreme gradients in the local ambient while the air below remains considerably cooler whereas dissipation only in the lower levels produces near isothermal ambient temperatures throughout.

Free Convection Heat Transfer from a Totally Enclosed Cabinet Containing Simulated Electronic Equipment

A theory has been developed based on the accepted heat transfer theories of Fishenden, Saunders, Weise and others, together with the results of the author's experiments. From this theory it is possible to evolve a design sheet which can be used to predict ambient temperatures within cabinets for predetermined areas of ventilation. Hence, design curves of temperature, in relation to areas of ventilation, can be obtained for specific cabinets and methods of cooling. From these curves, it should be possible to select the method of ventilation and the optimum cooling area and hence ensure that the maximum tolerable ambient temperature in which the components are required to operate shall not be exceeded.The effects on the hottest component, when the methods of ventilation are altered, the ventilating area is varied and the main heat source is progressively raised, are studied. The results of these experiments and the comparisons between predicted and observed ambient temperatures are discussed. Recommendation...

R. I. Noronha

Papers

Free Convective Cooling of Cabinets Containing Heat Dissipating Components