Microelectronics Reliability

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1 Microelectronics Reliability 52 (2012) Contents lists available at SciVerse ScienceDirect Microelectronics Reliability journal homepage: Thermal modeling and design of power converters with tight thermal constraints P. Cova, N. Delmonte Dipartimento di Ingegneria dell Informazione, University of Parma, viale G.P. Usberti, 181/a, Parma, Italy INFN Pavia, via Agostino Bassi, 6, Pavia, Italy article info abstract Article history: Received 31 May 2012 Received in revised form 25 June 2012 Accepted 25 June 2012 Available online 20 July 2012 The aim of this paper is to show and discuss results of 3D finite-element simulations for thermal management design with tight constraints taking care of reliability aspects of hybrid power converters. A procedure to obtain simplified but accurate device models has been shown together with experimental validation. The simplified models have been used for converter module modeling. The same procedure has been applied to analyze the thermo-fluid dynamic problem of a whole converter comprising of three modules, inner air and enclosure. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Electronic systems operating in hostile environments, such as space or High Energy Physics Experiments (HEPEs), require the highest levels of reliability and performance. In these applications high-density power supplies with tight thermal specifications and constraints are needed and their thermal management design is critical. Indeed efficient heat removal is crucial for reliability and to ensure, even in case of high dissipated power, negligible heat exchange between the power converters and surrounding electronics. In HEPEs applications the main thermal design targets are twice: (a) for reliability purposes the maximum temperature of converter components should be as low as possible; (b) almost all of the generated heat must be extracted by liquid cooled heat-sink, and only a negligible amount of it by air convection [1]. For this reasons the thermal design must consider the 3D conduction problem, together with air convection in (almost) sealed enclosure. Accurate numerical studies, e.g. Finite Element (FE) analysis, can be useful to evaluate heat exchange in the environment and the steady state maximum temperatures reached in the system components, but detailed geometries representation is needed. This implies very high Degrees Of Freedom (DOF) for the FE model of the whole system, and consequent big computational effort. One way to circumvent this problem is to use simplified geometries or simplified models of the single components embedded in the system. It is possible to build a library of component models following the procedure described in [2]. Corresponding author at: Dipartimento di Ingegneria dell Informazione, University of Parma, viale G.P. Usberti, 181/a, Parma, Italy. Tel.: ; fax: address: paolo.cova@unipr.it (P. Cova). In this paper, as a case study, FE based thermal design of a boxed DC power supply for HEPEs application is shown, referring to the 3 kw, V DC/DC, converter for the next generation of ATLAS experiment [3], which is composed by three paralleled power modules, each cooled by its own water cold plate [4]. Every module implements a 100 khz Switch In Line Converter (SILC) [5] able to supply up to 125 A output current. Aim of the paper is to show a simplified, but accurate FE modeling for thermal design of power converters subjected to tight thermal constraints. In the ATLAS experiment, the converter is neither subjected to power cycling, nor to thermal cycling, except in case of system shut down (seldom), then only steady state thermal analysis is significant. In Section 2 the system thermal requirements are given; in Section 3 the accurate and simplified models of the main heating components are described; Section 4 shows the module thermal simulation and characterization; in Section 5 the thermal model of the whole system, including the box and the air inside it, is presented and discussed. 2. System thermal constraints The three-modules power converter considered has to be closed in a mm ideally adiabatic box to avoid heat flow towards other electronics of ATLAS experiment. It will work at 18 C ambient temperature. The electronic inside the box can be cooled by mean of aluminum cold plate. The cooling liquid is water with T inlet =18 C and maximum delivery of 1.9 l/min. The chosen modules redundancy leads to two possible configurations, each with different module output power rating and heat exchange distribution: (i) all modules working, each delivering 1 kw; (ii) two modules each delivering 1.5 kw, when a module has failed [3] /$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.

2 2392 P. Cova, N. Delmonte / Microelectronics Reliability 52 (2012) Thus, together with the control of the chosen DC/DC converter topology, the choice of the devices to embed in the module is crucial to ensure good heat removal and high electrical power efficiency. 3. Components thermal library Each component embedded in the converter was modeled by a simplified geometry, in order to obtain a DOF of the entire model as low as possible, even though maintaining sufficient accuracy of the simulation results. The simplified device models can be collected from previous works [2,6,7] or added for the current case study. In this section, numerical models (and their validation) of the devices which have to be used in the power module simulation, are presented TO247 thermal modeling and characterization The designed modules include some TO247 packaged MOSFETs, assembled as SMD to take advantage of Insulated Metal Substrate (IMS) boards (instead of conventional FR4), that can be well connected to the cold plate [8]. More expensive ceramic materials could be possibly tested in next prototypes. A device in TO247 package has been analyzed in geometry and materials by using technical sheets and direct inspection. Starting from this, the model for FE analysis was drawn, as in Fig. 1, which refers to an assembly on IMS board. In this geometry only the bonding wires were simplified and drawn with simpler and larger shapes than the real ones; the thermal conductivities in this subdomains were set in order to obtain the same thermal resistances of the real bonding wires between silicon die and pins. The unavoidable small elements of the mesh (the higher total number of elements, the greater the DOF) derive from the presence of thin layers (125 lm silicon die, 70 lm copper traces and 75 lm resin insulator underneath) modeled with their actual thickness. In the simplified model they can be drawn as 2D geometries. In COM- SOL 4.2, for instance, although modeled as 2D, a thermal conductivity and a thickness can be set for copper traces, in order to apply the Fourier model of heat transmission in this highly conductive layers, neglecting the temperature gradient along the thickness. To setup a simplified model of a TO247 packaged device, a MOS- FET biased in order to dissipate 1 W was thermally characterized by Infrared (IR) thermography. It was biased without mounting it on a board and heat-sink. The obtained thermal map is shown in Fig. 2. The detailed FEM simulation of the MOSFET dissipating 1 W, gives the thermal map of Fig. 3 (left). At the boundaries it was imposed the Newton s law of cooling q = h (T T ref ), where q is heat flux, h is the heat transfer coefficient and T ref is the temperature Fig. 2. IR thermal map of a TO247 device dissipating 1 W at room temperature of 25 C; labeled by letters, the reference points used to compare experimental with simulations. Fig. 3. FEM thermal map of the TO247 MOSFET dissipating 1 W at room temperature of 25 C. Left: detailed model; right: simplified model. Temperature color range in C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.). suitably far from the boundary (i.e. the ambient temperature). The FEM was tuned with experimental results by using the heat transfer coefficient at boundaries as fitting parameters. This fitting is needed because the actual convection conditions during experiments were not well controlled: the transistor was kept in quasi natural air convection due to air blown by the fans of the near test bench electronic equipment. In this operating condition, to have good matching of FEM and experimental, h has to be set slightly greater than the value computed from handbook formula for natural air convection coefficient. Fig. 1. TO247 detailed 3D model geometry TO247 simplified FE model To lower the DOF, the device was modeled with a simplified geometry. To keep the number of elements as low as possible, the MOSFET geometry was drawn with a 2D silicon die, and without holes. As done for the detailed model, the heat transfer coefficient at the boundaries was used as fitting parameter. The result after tuning is shown in Fig. 3 (right). To compare simulation and experimental results some reference points have been taken (see Fig. 2) whose temperature increase over room temperature are given in Table 1. The deviation is always below 10%, so the matching between measurement and simulation of simplified model can be considered good.

3 P. Cova, N. Delmonte / Microelectronics Reliability 52 (2012) Table 1 Temperature increase in points of Fig. 2 for measurement and simulation (simplified model). e is the absolute error to DT meas. Point DT meas ( C) DT sim ( C) e (%) A B C D E F G ISOTOP thermal modeling and characterization As done for TO247, a double diode in ISOTOP package has been analyzed by inspections and technical sheets [9]. Four of these components are used as output rectifiers and each of them is loaded by half of the output current. They are mounted with the thermal flange on the circuit baseplate. Fig. 4 shows the ISOTOP 3D geometry used for detailed FE thermal analysis. Fig. 5 shows the IR picture of a couple of ISOTOP diodes powered to dissipate 2.58 W. The diodes were biased in series, using two 1 m long cables from power supply to ISOTOP with 8 mm 2 section area to avoid heating from them (I d 3.5 A). The short cable (around 5 cm long) used to put in series the diodes can have smaller section (1.5 mm 2 ) than the power supply cables. The ISOTOP was painted black in order to have an emission coefficient equal to 1. The ambient temperature was 27 C. The ISOTOP FEM is composed by copper flange and screwed external contacts, silicon die, copper electrical contacts between anode and cathode on die and resin lid. The heat generation has been placed in the two silicon die equal to the one of the IR thermal measurement. As boundary conditions as been considered only the convective cooling, neglecting the heat-sink behavior of the electrical cables used to bias the diodes (see in Fig. 5 the lowest temperatures recorded on the contacts connected to the larger cables). In Fig. 6 (left) is shown the result of the FE simulation. It is in good agreement with the measurement. Fig. 5. IR picture (top view) of the ISOTOP diodes in horizontal position (flange down) dissipating 2.58 W at room temperature of 27 C; all the boundaries are on air, except for the contacts. Excluding the coldest contacts, the maximum and minimum temperatures are 58.6 C and 48.5 C, respectively. Fig. 6. Thermal map by detailed (left) and simplified (right) FEM of the ISOTOP diodes dissipating 2.58 W at T amb =27 C. Fig. 7. Simplified ISOTOP 3D geometry for FE analysis ISOTOP simplified FE model The simplified ISOTOP was modeled by drawing (Fig. 7) all subdomains as hexahedral, except for the silicon die, modeled as 2D geometry. The electrical connections are drawn as vertical hexahedral with sections wider than those typical of bonding wires. In these subdomains the thermal conductivity was set in order to obtain the same thermal resistance as that of the actual internal connections. Again, using convective heat transfer coefficients as fitting parameters, it was possible to obtain a FE model whose simulations are in good agreement with experimental. Fig. 6 (right) shows the simulated thermal map with dissipated power of 2.58 W and T a =27 C Planar transformer simplified FE model Fig. 4. ISOTOP detailed 3D geometry: the inner components, such as dies and electrical connections, are depicted in green. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) This simplified FE model was built as done in [6] assuming the windings as homogeneous material, with uniform heat sources, and starting from the detailed model described in [7]. The simplified transformer, together with aluminum bars and iron screws used for clamping it to the heat-sink, is depicted in Fig. 8. This model was used for simulating the whole converter, and it was tuned by the measurements described in Section 4. The tuning was done by mean of three parameters: thermal conductivity of

4 2394 P. Cova, N. Delmonte / Microelectronics Reliability 52 (2012) Iron screw Core Aluminum bar Windings Fig. 8. FE thermal map of the simplified planar transformer model dissipating 50 W on core and 70 W on windings. h bottom = 180 W/m 2 K, h air = 20 W/m 2 K. Fig. 9. Module prototype. the windings domain, heat transfer coefficient of the boundaries exposed to the air, and heat transfer coefficient of the aluminum bar surfaces connected to the heat-sink. Fig. 8 represents a thermal map obtained from simulations in the case of a dissipated power of 120 W. It is in good agreement with experimental results. 4. Power module thermal modeling and measurements The converter power module taken as case study has a complex geometry, since it is composed by many devices assembled in stacked boards made with different technologies (Fig. 9). Four zones compose the module: the primary, the transformer, the secondary and the auxiliary power supply of the control. The control is placed at the primary top board. The TO247 power MOSFETs in the primary zone, which need to dissipate a lot of power, are placed in the bottom IMS board. The FR4 boards in the middle layer house both devices with significant and less dissipating power. However, the more dissipating power devices (e.g. the ISOTOP diodes in Fig. 10) in middle boards are thermally connected to the aluminum baseplate (by means of their specific flange), which acts as module substrate and thermal bridge between the devices and the cold plate. The devices in the FR4 boards at the top (auxiliary circuits) are those that dissipate less (ideally no) power. The two bigger magnetic components, the planar transformer and a toroidal inductor are thermally and structurally connected to the baseplate by metallic screws and aluminum bars and plates. The planar transformer (Fig. 11) has the core bottom thermally connected to the baseplate. Starting from the analysis in [7], this transformer was wrapped by aluminum bars and plates, to carry the heat generated in the upper zones towards the cold plate. Between aluminum thermal bridges and everywhere a part is near the baseplate, a thermal conductive gap filler (paste or pad) is placed. The module was thermal characterized by mean of thermocouples and IR thermography at different output power levels. The thermography at the maximum power rating is shown in Fig. 12. The inlet water temperature and the delivery of the cold plate were fixed in agreement with constraints in Section 2. To achieve a high and uniform emission coefficient the more reflective parts of interest were painted by an antireflective coating. The geometry in Fig. 10 was drawn to simulate the module operating at the same conditions of measurements, by taking into account only the devices where heat generation was significant. The dissipated power in the parts was evaluated by electrical measurements or by calculations, for a total amount of 378 W at the maximum output power rating of 1.5 kw. These values are listed in Table 2. The devices are modeled as the simplified ones. To take into account the thermal interaction between the devices assembled on FR4 boards, the copper traces have also been drawn, but they were modeled as 2D highly conductive layer, in order to keep low the DOF. The cold plate has been modeled as 2 mm thick aluminum plate in contact with the baseplate, setting the external Fig. 10. Module geometry drawn for FE analysis (partially exploded view).

5 P. Cova, N. Delmonte / Microelectronics Reliability 52 (2012) Fig. 13. FE thermal map (in C) of the module prototype delivering 1.5 kw at T amb =24 C. The cold plate temperature is 19.5 C. Primary section to the right. Fig. 11. Planar transformer: thermal bridging to cold plate. Fig. 14. Thermal map of the simplified module delivering 1.5 kw at room temperature of 24 C. Fig. 12. IR thermal map of the module prototype delivering 1.5 kw at 24 C room temperature. Primary region to the right. Table 2 Module devices: description and dissipated power at delivered power of 1.5 kw. No. Device P d (W) 1 Primary MOSFETs (TO247) 30 2 Planar transformer core Planar transformer windings Diodes (ISOTOP) 12 5 Inductor 12 6 Copper traces at secondary 10 7 Auxiliary MOSFET (TO247) <1 8 Auxiliary transformer core <1 9 Auxiliary transformer windings <1 10 Auxiliary MOSFET (D2PAK) <1 11 Capacitors and other devices 5 Total power dissipation 378 surface temperature (the mean temperature measured in experiments between inlet and outlet water) to 19.5 C. Natural air convection was set over all the other boundaries of the module, by using the heat transfer coefficient as fitting parameter. The result, shown in Fig. 13, is in good agreement with measurements. 5. Thermal modeling of the whole system Fig. 15. Thermal map of the middle section of the converter with all modules operating at nominal power (1 kw). A key issue in the present work was to develop a numerical model of the whole converter box, able to get accurate indications about the insulation of the box walls, in order to keep the heat release towards surrounding electronics as low as possible. At the same time the maximum temperatures reached by the internal components of the converter must be kept under control. Inside the converter box (containing three modules as the one described in the previous section) there is air, which can move because of buoyancy, resulting in internal heat flow by convection. Therefore, the equations of thermal and fluid dynamic problems must be self consistently coupled in the model. The problems related to the needed of high computational capability for fluid dynamic FE simulations are well known. Typically, to manage thermal problems of electronic circuits, the Finite Volume Method (FVM) is used. Although this method is suitable for simulating fluid dynamic problems, when analyzing 3D thermal problems such as those presented in Section 4 where, for accuracy, the copper thermal connections between the devices cannot be modeled as 2D, the DOF turns out to be rather high. Analogue observation can be done for the die of semiconductor components. Thus, with FVM, especially when using FR4 boards, one tends to neglect or roughly approximate the influence of a device on its neighbors, both because of the heat transport through the PCB, and the way to generate heat within the devices. Therefore, even if with models graphically suggestive and accurate fluid dynamic results, also application of FVM leads to thermal results with errors between 5% and 10%. The purpose here promoted is to use the FEM with simple models, running on standard PC, giving errors comparable to those of FVM models.

6 2396 P. Cova, N. Delmonte / Microelectronics Reliability 52 (2012) To evaluate the effect of an additional insulation in the upper surface of the three modules on their maximum temperatures, a simple approach, which does not require drawing additional layers, consists in lowering the heat transfer coefficient h at the surfaces which should be covered by insulation. The coefficient h can be calculated once the thermal conductivity and thickness of the insulator material are known. Fig. 17 shows the simulated effect of the thermal insulation on the maximum temperature of the most critical components, indicating that further investigation is needed to improve the heat removal from the planar transformer. Fig. 16. External boundaries thermal map of the converter with all modules operating at nominal power. 6. Conclusions We developed and experimentally validated a 3D FEM model for the thermal analysis of a power converter to use in HEPEs. The model was built using simplified models of the more heating devices embedded in the converter. Thermal measurements and simulations showed good agreement both for single devices and the whole module. Finally, we reported about results of thermo-fluid dynamic simulations of the converter in enclosure, aimed at evaluating maximum temperatures inside, and heat flux from the box walls to the ambient. The analysis showed that more efficient heat removal at the transformer is needed to satisfy the quasi-adiabatic constraints on the whole converter. Acknowledgements Fig. 17. Temperatures from simulation of some critical devices of the module operating at the maximum delivering power for different heat transfer coefficient. A box with three modules, simplified with respect to the one in Fig. 10, was drawn to analyze the thermal problem of the whole converter. Following the same idea of the simplified model device library, a simplified model of the converter was built: the four (primary, planar transformer, secondary, auxiliary) zones were modeled as hexahedral of homogeneous materials, whose dimensions give approximately the external shape of the real module. To fit this model with experimental results, the thermal conductivities of the four zones, and their thermal contact resistances with the baseplate, were used as fitting parameters. The thermal map of Fig. 14 was obtained after fitting. Referring to the simulation result of detailed module, the absolute error in the mean temperature increase for every zone is below 5%. By this model, the stationary state of the converter operating at room temperature of 18 C, with the three modules delivering the nominal power, and setting natural air convection condition at the external boundaries of the box, was simulated (Figs. 15 and 16). The results showed that more internal insulation will be required to increase the amount of heat extracted by the water heat-sink and reduce the walls temperature. The research presented in this paper was conducted in the frame of the APOLLO experiment and financially supported by the Italian Istituto Nazionale di Fisica Nucleare (INFN). The authors are grateful to researchers who contributed to this activity, namely, M. Bernardoni, M. Citterio, A. Lanza, R. Menozzi, and M. Riva. References [1] Bohm J, Stastny J, Vacek V. Cooling performance test of the SCT LV&HV power supply rack. ATL-INDET-PUB ; November [2] Cova P, Delmonte N, Menozzi R. Thermal characterization and modeling of power hybrid converters. Microelectron Reliab 2006;46: [3] Aad G et al. The ATLAS experiment at the CERN LHC. J Instrum 2008;3:S [4] Baccaro S, Busatto G, Citterio M, Cova P, Delmonte N, Iannuzzo F, et al. Reliability oriented design of power supplies for high energy physics applications. Microelectron Reliab 2012;52, this issue. [5] Legnani M, Maranesi P, Naummi G. SILC: a novel phase-shifted PWM converter. In: Proc 5th IET Eur conf power electr appl, vol. 3; p [6] Cova P, Delmonte N, Menozzi R. Thermal modeling of high frequency DC DC switching modules: electromagnetic and thermal simulation of magnetic components. Microelectron Reliab 2008;48: [7] Bernardoni M, Cova P, Delmonte N, Menozzi R. Thermal modeling of planar transformer for switching power converters. Microelectron Reliab 2010;50: [8] Jord X, Perpi X, Vellvehi M, Milln J, Ferriz A. Thermal characterization of insulated metal substrates with a power test chip. In: Proc ISPSD, Barcelona, Spain; p [9] SOT-227B ISOTOP package. Fairchild Semic. < products/discrete/pdf/sot227_dim.pdf>.