Teaching Thermal Design of Power Electronic Systems with Web-Based Interactive Educational Software Uwe DROFENIK and Johann W. KOLAR Power Electronic Systems Laboratory Swiss Federal Institute of Technology (ETH) Zurich ETH-Zentrum / ETL, CH-8092 Zurich, Switzerland Phone: +41-1-632-4267, Fax: +41-1-632-1212 E-mail: drofenik@lem.ee.ethz.ch , kolar@lem.ee.ethz.ch Abstract. For designing reliable power electronic systems it is essential to understand basic thermal issues like the stationary and transient relation of the power semiconductor losses the junction temperature and the application of thermal equivalent circuits. Also, thermal properties are of special importance in connection with further increasing the compactness of power converter systems. The aim of this paper is to give an introduction into the basic theory of heat energy conduction and thermal design which should serve as an addition to the Java applets compiled in a the iPES-Thermal, a module of the interactive educational software iPES which is freely available at www.ipes.ethz.ch and employed at the ETH Zurich for supporting an introductory course on power electronics. 1 Introduction 1.1 Thermal Problems in Power Electronics In lectures on fundamentals of power electronics thermal design is often neglected because the focus is traditionally on circuits, topologies and control. Therefore, students often do not have a clear understanding of the importance of a proper thermal design of power electronic systems. This paper should compile lecture material providing an introduction into the basics of thermal issues relevant to power electronics. The paper is complimentary to Java applets of the interactive educational software iPES (I nteractive P ower E lectronics S eminar) which also will be discussed in this paper. 1.2 Interactive Power Electronics Seminar (iPES) The Interactive Power Electronics Seminar (iPES) [1], [2] available at www.ipes.ethz.ch at no costs is an effort to make educational software interactive by employing Java applets. Different power electronic circuits are shown with animated current-flow in dependency of operating parameters like load- resistance or input-voltage. The user can click with the mouse directly into the circuit elements and signal for changing parameters, and the corresponding changes of the time- behavior or other diagrams are shown immediately. The focus of iPES is not on simulation but on interactive animation. Currently available educational modules are “iPES-Cicuits”, “iPES-Advanced Circuits” and “iPES-Electromag-netics” and “iPES-Thermal” which will be in the focus in the following. 2 Heat Conduction 2.1 Heat Conduction in Power Electronic Systems In general there are three basic heat transfer mechanisms: heat conduction, convection and radiation [3]. In a medium the heat transfer is by lattice oscillations and electrons (for conductive materials). This mechanism, denominated as heat conduction is of paramount importance in power electronic applications, and will be discussed in detail in this paper. Power losses occurring inside a power semiconductor are finally conducted to a heat sink surface, i.e. a convective interface. In natural convection the heat is conducted to the surrounding medium via a thin boundary layer. There, the heat energy does change the adjacent medium density where the created buoyancy causes the medium to flow. For significantly increasing this mechanism of heat transfer the mass flow could be created by fans, i.e. forced convection could be employed. According to the Stefan-Boltzmann law an object does emit heat power in the form of electromagnetic waves dependent on its temperature, the ambient temperature and the surface emissivity. Heat transfer via radiation also works if there is no media surrounding the heat sink and therefore is the only relevant heat transfer mechanism e.g. for space applications. 2.2 Mathematical Model In this section a mathematical model of heat conduction for calculating stationary and dynamic temperature distributions inside power semi-conductors and heat sinks will be derived. For simplification a one-dimensional structure (e.g. an isolated rectangular rod, cf. Fig.1, top-left) of homogenous material will be analyzed. The results gained could be expanded easily to three dimensions (see section 2.5). According to the one-dimensional consideration the time- dependent temperature T is equally distributed over a cross- section A (called Isotherm) of the rod at a position x considered and the heat energy flow density q [W/m 2 ] is proportional to the negative local temperature gradient x T q th ∂ ∂ - = λ . (1) The proportional factor called thermal conductivity λ th [W/(K·m)] is a material property. Generally, heat flow density 0-7803-7768-0/03/$17.00 (C) 2003 IEEE 1029