Compact thermal model of a D2Pak case with convection

Abstract

The paper presents a compact thermal model of an electronic component enclosed in a D2Pak case. The model includes convection cooling and is compared to a linear model with isothermal boundary condition. The model presented is a static model. 1. Introduction A compact thermal model is a simple network of about ten to twenty nodes. Thanks to the low number of nodes, the time required to compute the output variable – the junction temperature, is considerably reduced. The model consists of any finite numer of branches connecting the external (surface) and internal nodes. The model considered up to now was a linear model, being only a rough approximation of reality. To build a complete model it is essential to include the heat transfer phenomena between the component and its environment, like convection and radiation. At the present stage of the study, the convection is considered. To calculate the parameters of a compact thermal model, the temperature distribution in the investigated component must be simulated or measured. A thermal camera is an excellent tool to determine the external temperatures of the component without influencing its structure, which is very difficult to do using contact temperature measurement techniques. The thermal images provide information about the temperature of the characteristic points of the component, chosen to be the nodes of the compact thermal model. These temperature values are then used to calculate the junction temperature. 2. Thermal model of the D2Pak component A finite element model of a D2Pak case power transistor together with its dedicated printed circuit board (PCB) was created using the ANSYS 5.7 simulation environment and based on the component parameters taken from the manufacturer’s data sheets. Figure 1a shows the actual position of the active element (the junction) within the structure of the component. Figure 1b is the general view of the modelled transistor placed on the PCB. Figure 1c presents the base structure for the compact thermal model. The seven nodes are: junction (in the middle), case, connections, paths, soldering pad, substrate and PCB. The nodes are joined together with the use of model parameters Rthz, ai, qk, Ri,k. The additional node is introduced for the ambient temperature. This node is connected to the remaining part of the model by means of the convection coefficient αk.