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Unformatted text preview: Chapter 15 Cooling of Electronic Equipment Chapter 15 COOLING OF ELECTRONIC EQUIPMENT Introduction and History 15-1C The invention of vacuum diode started the electronic age. The invention of the transistor marked the beginning of a revolution in that age since the transistors performed the functions of the vacuum tubes with greater reliability while occupying negligible space and consuming negligible power compared to the vacuum tubes. 15-2C Integrated circuits are semiconductor devices in which several components such as diodes, transistors, resistors and capacitors are housed together. The initials MSI, LSI, and VLSI stand for medium scale integration, large scale integration, and very large scale integration, respectively. 15-3C The electrical resistance R is a measure of resistance against current flow, and the friction between the electrons and the material causes heating. The amount of the heat generated can be determined from Ohm’s law, W = I R 2 . 15-4C The electrical energy consumed by the TV is eventually converted to heat, and the blanket wrapped around the TV prevents the heat from escaping. Then the temperature of the TV set will have to start rising as a result of heat build up. The TV set will have to burn up if operated this way for a long time. However, for short time periods, the temperature rise will not reach destructive levels. 15-5C Since the heat generated in the incandescent light bulb which is completely wrapped can not escape, the temperature of the light bulb will increase, and will possibly start a fire by igniting the towel. 15-6C When the air flow to the radiator is blocked, the hot water coming off the engine cannot be cooled, and thus the engine will overheat and fail, and possible catch fire. 15-7C A car is much more likely to break since it has more moving parts than a TV. 15-8C Diffusion in semi-conductor materials, chemical reactions and creep in the bending materials cause electronic components to fail under prolonged use at high temperatures. 15-1 Chapter 15 Cooling of Electronic Equipment 15-9 The case temperature of a power transistor and the junction-to-case resistance are given. The junction temperature is to be determined. Assumptions Steady operating conditions exist. Analysis The rate of heat transfer between the junction and the case in steady operation is case junction case junction case junction R T T R T Q--- = ∆ = Then the junction temperature is determined to be C 120 ° ° ° = + =- = C/W) 5 W)( 12 ( + C 60 case junction case junction R Q T T 15-10 The power dissipated by an electronic component as well as the junction and case temperatures are measured. The junction-to-case resistance is to be determined....
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This note was uploaded on 08/25/2009 for the course AET AET432 taught by Professor Rajadas during the Spring '06 term at ASU.
- Spring '06