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Switch Mode Power Supply used for Medical Applications – Packaging and Thermal Analysis Lucian Man, Cristian Fărcaş Applied Electronics Department The Faculty of Electronics, Telecommunications and Information Technology Technical University of Cluj-Napoca, Romania [email protected] Abstract — Placed, in the structure of the paper, in the center of the general PCB integration process, the Intermediate Data Format (IDF) is a specification designed to provide a neutral representation for exchanging printed circuit assembly (PCA) data among mechanical design (MCAD), PCA layout (ECAD), and physical design analysis (MCAE) applications. The paper presents a combined analysis – starting from electrical design, then the 3D model, thermal and mechanical approach of the PCB Layout for a power supply unit for medical applications. Design verification tools are considered invaluable in studying structural problems as deflections, deformations, stresses, or natural frequencies. The structural performance of a new product is only one of many challenges facing design engineers. Other common problems are thermally related, including overheating, the lack of dimensional stability, excessive thermal stresses, and other challenges related to heat flow and the thermal characteristics of their products. Keywords – thermal analysis, CAD, MCAD, IDF I. INTRODUCTION To reduce product development cost and time, traditional prototyping and testing has largely been replaced in the last decade by a simulation-driven design process. Figure 2. Integrated product design The need for the thermal design of electronic systems is well established but rarely given the attention it deserves. This is partly attributable to the mystique that has built up around thermal predictions and the tradition that thermal calculations are only for the academic mathematician. Figure 1. Evolution of design process A process like this, reducing the need for expensive and time-consuming physical prototypes, allows engineers to successfully predict product performance with computer models, which are easy to modify and to simulate. Integrated Product Design (IPD) is the key to shortening electronic product design schedules and getting to the market faster than the competition. One of the difficulties is linking together the different MCAD, ECAD and CAE (analysis) software packages used by design engineers. To deal with thermally caused failures it is necessary to know where the large temperature differences are and also which of these can be economically reduced. This paper concentrates on the part of an electronic system which generally contributes most to the overall temperature. Designing cooling fans and heat sinks must balance the need for small size with an adequate heat removal; tight component packaging must still ensure sufficient air flow so that the components and the printed circuit boards do not deform or crack under excessive thermal stress. II. CAD TO SIMULATION; SIMULATION TO CAD A. The Intermediate Data Format (IDF) Excellent data exchange standards exist for communicating raw 3D geometric data such as IGES, DXF, STL and more rich data formats such as STEP. However, with the exception of STEP, none of these standards go much beyond geometric data and are unsuitable for the application specific data exchange required for PCA design. For example, thermal data and component attributes are not part in any of these data standards. STEP AP210 for PCA data exchange is an exception to this. The difficulties with AP210 are that it is a very complex and data rich specification and, secondly, it is currently supported by very few MCAD or ECAD vendors. In addition, AP210 does not define a means of including compact R-C thermal models of components. The Intermediate Data Format (IDF) is a specification designed to provide a neutral representation for exchanging printed circuit assembly (PCA) data among mechanical design (MCAD), PCA layout (ECAD), and physical design analysis (MCAE) applications, bridging the existing gap. An overview of the IDF file format versions (2.0, 3.0, 4.0), with the included features like file format, component shapes, component instances, graphics, holes, conductors, footprints, sublayouts, thermal properties, and others, was presented in [1] and [4]. B. The Power Supply Unit The approached problem of this paper is a switch mode power supply for medical equipment. The simplified block diagram of power supply unit is presented in Figure 3. Figure 4. The power supply unit The analysis presented in the paper starts from the actual design of the power supply. We want to determine the optimal position of the fan, of the opens and of the heat sinks. Heat transfer simulation results for the structure with natural and forced convection, for different configurations and different structures are presented. C. The CAD model of the Power Supply Unit Using the block diagram and the layouts of the power supply unit, the complete 3D CAD model was built. The use of the IDF file in order to obtain the basic 3D model was described, in details, in [1]. Figure 3. The block diagram of the power supply unit Figure 5. The 3D model of the power supply unit The power supply has the mains voltage between 100V and 230V, and line frequency 50Hz or 60Hz. The maximum power consumption is 2025W at 230V. The output voltages are +24V/12A, -5V/4A, +12V/1A, -12V/1A and +12V stand-by voltage. There is another AC output (230V/6A). One can control the DC output voltages, with a remote control input. If remote control input is high level (+12V) the outputs are enable and if remote control input is low level (0V) the outputs are disable. A backup battery will provide power for 30 minutes in the absence of the main power supply. In the model, a series of components were ignored, due to the size and the complexity of the model (a large matrix is generated by the solver, in order to apply the Finite Element Method and each component is analyzed and take into consideration separately – each boundary must be taken into account in order to built the matrix and in order to solve the problem), and, not less, because they don’t have a major influence on the conducted and completed thermal analysis. Each component was created according to the specifications and the model chosen for the simulations accurately follows the prototype. D. Heat sources for the CAD simulations An important step in the analysis was to determine the heat sources. They can be noticed in the Figure 6 and they have been calculated according to the producer’s specifications. Figure 7. Natural convection case study Figure 6. The heat sources and the boundary conditions The boundary conditions used for the simulations are determined by the position and size of the fan and by the position and size of the openings – environment pressure. All the simulations with forced convection were completed using the environmental temperature 10 C , the device being considered in an air-conditioned room, with a constant temperature. E. Natural convection simulations There are 5 structures to be taken into consideration for the thermal analysis, in the case of natural convection. The 8 transistors dissipating 0.5W (on the left part of the power supply unit in Figure 6) were ignored in the analysis. The simulation’s results are presented in Figure 7. The environmental temperature was considered 20 C and the results are represented on each heat sink with the corresponding components. For the simulations, the convection coefficient was approximated at 6 W/(m2 K) , for each surface. This approximation introduces errors, calculated at 5% , an unacceptable error. But the purpose of these simulations is to determine if a forced convection is needed, so the conclusion can be drawn: the need of forced convection (a fan). F. Forced convection simulations In order to complete the forced convection thermal simulation, the model was simplified even more. These simplifications were necessary due to a complex geometry which gives a very large matrix and that takes more time to complete and solve the simulation. For example, on a 4GB RAM memory, Intel Pentium 4 at 3GHz PC, the simulation time for a simple geometry was approximately 9 hours. The added fan and openings, representing the boundary conditions, are shown in Figure 8. Figure 8. The fan and the openings Figure 9 presents the air flow in the enclosure. The air temperature is presented in Figure 10. These results are important for the calculus of the convection coefficient, hC or , a complex function, depending on the fluid speed, density, temperature, specific heat, flow type (laminar, transient, turbulent) and others (almost all being variable in time): f ( w, , t , c p , l , ....) . Figure 9. The air flow – velocity Figure 10. The air flow – temperature Figure 13. The solid bodies’ temperatures - details III. Figure 11. The temperature in the enclosure – cross section The temperatures on the components that were considered for the simulations are presented in Figure 12 and, with more details, in Figure 13. CONCLUSIONS The paper presents 3D model for thermal and mechanical analysis of a power supply unit for medical applications. The power supply was analyzed in the case of natural convection and in the case of forced convection. The forced convection is absolutely necessary to the full load. A good position of the fan is on the front panel of the power supply. The simulation results are closed to the real thermal measurements realized on the prototype. REFERENCES [1] [2] [3] [4] Figure 12. The solid bodies’ temperatures L. Man, C. Farcas, R. Fizesan, “Packaging and Thermal Analysis of Power Electronics Modules”, ISSE 2010 – 33rd International Spring Seminar on Electronics Technology, 12-16 May 2010, Warsaw, Poland R. Remsburg, Thermal Design of Electronic Equipment, CRC Press, Norten Networks, Boca Raton, Florida, 2001 M. Bourbel, “Electronic Information Exchange”, Maya Heat Transfer Technologies Ltd. L. Man, D. Pitica, M. Zolog, "Electrical / Mechanical / Thermal Design Integration", SIITME 2009 – International Symposium for Design and Technology of Electronic Packages, 15th Edition, 17-20 September 2009, Gyula, Hungary, 978-1-4244-50330309 ©2009 IEEE, pg. 157-161