Application of Phase Change Materials for Low Duty Cycle High Peak Load Power Supplies Andrija STUPAR, Uwe DROFENIK, and Johann W. KOLAR Power Electronic Systems Laboratory, ETH Zurich ETH-Zentrum / ETL H18, CH-8092 Zurich, Switzerland Phone: +41-44-632-7447, Fax: +41-44-632-1212, E-mail: stupar@lem.ee.ethz.ch Abstract A power electronic device’s lifetime depends on its maximum operating temperature and the temperature swings it is subjected to. Heat sinks employing phase change materials (PCMs) can be employed to achieve a temperature reduction, but only for a limited du- ration. This makes such heat sinks appropriate for use in applications with high peak loads but with low duty cycles. The heat sink is modelled using the thermal RC network approach, and an optimization procedure for designing a hybrid air-cooled heat sink contain- ing PCM is developed, yielding a maximum possible temperature reduction for a given application. 1 Introduction Power supplies having high peak loads yet low duty cycles are to be found in many up and coming applications. These are typically power electronics converters for systems which are inactive for relatively long periods of time and then need to suddenly burst into activity, such as electro- mechanical actuators in aircraft, namely retractors extend- ers for landing gear [1]. Such actuators are operated for several seconds during take-off and landing, and are then inactive for the duration of the flight, which can last sev- eral hours. Another application for which high peak low duty cycle power electronics can be used are novel ultra- capacitor-powered electric buses [2], which recharge their batteries on certain stops, for 5 minutes, with 20 minutes in between charges. A generalized power profile representing applications as the above is shown in Fig. 1. The reliability of such power converters, especially for transportation applications such as aerospace, is of great importance. Typically these power supplies are built using IGBT modules. It has been shown [3], [4] that the peak operating temperature and the temperature cycle amplitude affect the lifetime of an IGBT module. It follows that the method of cooling, that is, the thermal management of power electronic devices, is of significant importance from the reliability standpoint. One of the usual conventional approaches to cooling power electronic devices is to place them on an air-cooled finned metal heat sink with an attached fan. An alternative are hybrid heat sinks which employ phase change materi- als (PCM) [6-9], as depicted in Fig. 2. In such configura- tions the PCM absorbs heat as it changes from solid to liq- uid or liquid to gas, temporarily slowing the temperature rise of the device and resulting in a lower operating tem- perature over a certain period. PCM heat sinks are well suited for high peak load low duty cycle applications: the PCM absorbs the heat, lowering the device temperature, and then follows a long period of inactivity during which this absorbed heat can be released to the ambient. How- ever, there are tradeoffs involved with this approach: while adding PCM increases the peak thermal capacity of the heat sink, it also significantly increases its thermal resis- tance. This paper presents an optimization procedure for design- ing a hybrid PCM-metal heat sink so as to arrive at a maximum reduction of the peak operating temperature compared to a conventional heat sink of equivalent vol- ume. To achieve this, a thermal network model is devel- oped, allowing for quick simulations and comparisons of different designs. Section 2 of this paper briefly explains previous heat sink optimization work that it builds on. Section 3 discusses how the PCM is modelled, different possible configura- tions of the heat sink as well as material properties, and presents the optimization procedure, while Section 4 gives Figure 1 – A generalized high peak load low duty cycle power profile, with peak power P and period T much greater than on time DT. Figure 2 – A hybrid heat sink, with a portion of the chan- nels filled with phase change material (gray), and the rest free for air flow. CIPS 2010, March, 16 – 18, 2010, Nuremberg/Germany Paper 11.4 ISBN 978-3-8007-3212-8 © VDE VERLAG GMBH ∙ Berlin ∙ Offenbach