Chapter 15: Thermal Considerations

Many years ago, the author was involved in the design of a small transistor inverter for an early space application. The interesting problem with this unit was that, in the vacuum of space, there is no convection or conduction cooling. All heat must be removed by radiation. Accordingly, the aluminum case was milled with rounded ridges to present the largest possible surface area and radiation normal to the surface at all angles. Since that time, there has been a succession of air and water cooled systems, but that was the only one that relied solely on radiation for cooling.

15.1 Heat and Heat Transfer

Some thermal relationships may help put cooling systems in perspective. Table 15.1 covers some of the basics.

Table 15.1: Thermal Constants

Specific heat is the ratio of the thermal capacity of a material to that of the same weight of water at 15 C. ? T = Q/ms where Q is the applied heat, m is the mass, and s is the specific heat.
Thermal conductivity is the time rate of heat transfer by conduction through a unit thickness, across a unit area for a unit difference of temperature. ? T = Qd/ka where Q is the applied heat, d is the material thickness, k is the thermal conductivity, and a is the area.
One calorie will raise the temperature of 1 g of water 1 C in 1 sec. One calorie is equivalent to 4.18 Joule.

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Power Electronics Design: A Practitioner's Guide

TABLE OF CONTENTS

Chapter 15: Thermal Considerations

15.1 Heat and Heat Transfer

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