Introduction

The focus of this chapter will be transient heat-transfer calculations of an entire electronics-cooling system. It will essentially work from the small to the large aspects in transient calculations, first starting with the heat-dissipating parts, then with how the parts are packaged and tied to a heat sink, continuing with how to handle a coldplate transient, and then finishing up with a total system transient response. Topics to be discussed will include when to apply lumped capacitance to a transient calculation; how to use a detailed steady-state analysis to perform a simple transient analysis; and finally when and how to approach transient heat-transfer calculations, how to calculate a time constant, and how to properly apply a time constant to obtain a temperature response of a thermal system. Simple approximations will be made to solve complex transients without the aid of a finite-difference solver. Practical solutions to transient thermal problems will be discussed in detail.

A typical electronics-cooling system consists of electrical components generating heat, cooled by conduction across a board to a rack. The rack in turn is cooled by fluid flowing through it. It is desirable to know not only whether the components will remain cool enough to operate continuously under normal conditions but also if they can operate during transient conditions. Transient thermal events include power surges, coolant flow interruptions, and spikes in coolant temperature.

Problem Definition