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ABSTRACT - The trend in Power Electronics continues towards greater packaging density, speed and high power dissipation. If these trends are not matched with the development and application of improved cooling techniques, the operating temperature will increase significantly and cause damage to the device. These higher operating temperatures in turn lead to an increase of the failure rate and a reduction of the reliability. Thus, thermal simulations are playing an essential role in the design of high power applications. The increasing usages of multi-chip modules, surface mount technology and reduced package dimensions also add to the thermal problems faced by the designer. So, the thermal analysis of power electronic devices became vital. In this paper, thermal analysis for a Power Bipolar Junction Transistor (2N3055) and IGBT (IRGBC30U) component layers model with case have been done for different Heat Sink's (surface area) under natural convection. The Junction temperature of the Power Electronic Device versus Heat Sink surface area and thermal behavior of the Power Electronic Devices were investigated and analyzed for constant power dissipation through Finite Element Method (FEM) in ANSYS v7.1. Practical set-up has been developed to conduct experiments to verify the simulation results. Thermal characteristics of Power Electronic Devices were measured under constant power dissipation condition using the practical set-up. These measurements were compared with the simulation results of the Power Electronic Devices through Finite Element Method and found to have perfect match in the variation of temperature with respect to time.
Index Terms- Electro - Thermal modeling, Component-Level thermal model, FEM, IGBT, Power BJT, Power Electronics.
The most reliable and well-designed electronic device can malfunction if it overheats. Good thermal design is often paramount in achieving high reliability, low manufacturing cost, small size and a predictable development time.
Power electronics devices refer to control and conversion of electrical power by semiconductor devices wherein these devices operate as switches . It is an enabling technology and widely used in computers, automobiles, telecommunications, space systems and satellites, motors, lighting and alternative energy. Power electronics is essential for the development of extremely dynamic, low-voltage applications such as high performance microprocessor computer systems and it is increasingly important to the automotive industry. For instance, power electronics is an enabling technology for the development of cleaner and more efficient vehicles. As a result, reliability, durability and cost become very crucial issues.
1.3 Paper Outline
The remainder of this paper is organized as follows: consists of seven Sections including the objectives and background in Section 1. Section 2 provides some related information about need for thermal modeling of electronic devices. Section 3 discusses previous research done by researchers in the thermal modeling area. It includes a review of recent literature of heat transfer in electronics.
Section 4 covers the various types of power semiconductor with their characteristics. The design of heat sink and thermal resistance are covered in Section 5.
A valid and reasonable modeling method for discrete power electronics components and an experiment set up and results from experiments are also discussed and compared to the simulation results in order to validate the modeling method on power electronics components are presented in Section 6. Finally, last Section provides conclusion of the paper as well as the recommendations for future research interest.
MAJOR CAUSES OF THE ELECTRONICS FAILURE
Thermal failures such as mechanical stresses, thermal de-bonding and thermal fracture become many of the possible breakdowns of electronics components. Mismatch of the thermal coefficient expansion between two different materials, especially at the interface conditions, could result in the separation of interfaces and bonds between different parts in a module at higher temperature. In addition, fatigue in the solder connections and cracking in substrate is common failure in electronics components when operating at off-limit temperatures. Fig. 2.4 shows that the failure in electronics during operation due to temperature is as high as 55%.
The probability of failure in electronic systems is strongly dependent on the operating temperature as shown in Fig. 2.4. Thermal overstress is the most common failure cause in modern electronic systems. Therefore, it is really important to understand static and dynamic thermal characteristics of the electronics, especially regarding the junction temperature.
Fig 2.4 Failures in Electronics Components
The failure rate of plastic encapsulated power transistors as a function of the junction temperature is illustrated in Fig. 2.5 . It shows that the failure rate increases approximately ten times for a 50oC rise of the junction temperature. Thus, long-term reliability consideration requires that junction temperature should be maintained at a low level. Therefore, thermal performance should be considered carefully for electronics design.
Fig. 2.5 The Failure Rate of Plastic Encapsulated Power Transistors as a Function of the Junction Temperature
MOTIVATION TO THERMAL MODELING
There are many other reasons why thermal management is of ever increasing importance. Lasance  mentioned three typical reasons. Firstly, at the component level, designers try to minimize package dimensions while increasing power density, which makes the problem of minimizing the thermal resistance from junction to case a crucial part of the package design. Secondly, in most of the electronic industries, thermal design tends to be an afterthought of the design process only if the prototype raises any thermal issues. Thus, it results in a longer and ineffective design time. So, thermal management should be an important and necessary part of any concurrent design environment, and must be accepted by all the people involved. Thirdly, the limit of pushing the use of air-cooling with a heat sink and fan is expected to be reached in the coming years. As a consequence of such a physical limit, design rules become very important in achieving reliable temperature predictions and better thermal solutions without costly redesign or over design.
thermal Modeling of Discrete Components and validation
Current commercial software provides a very powerful method for carrying out thermal simulation. However, it is very important that the users be aware of the mathematical foundation behind the software. Conservation of energy, conservation of mass and momentum, heat transfer coefficients, and calculation of pressure drop and flow resistance of flow field are the foundational concepts of heat transfer behind the software. In addition, it is also important to know how reliable the simulation results are. Therefore, verification by measurements is desired whenever possible. This Section presents a modeling method for power electronics systems with the validation of results from experimental data. To validate the thermal modeling method for modeling a generic power electronics component, two discrete power electronics components were modeled using the ANSYS 7.1.
The FEM thermal modeling procedure used in this work is presented. This procedure is implemented on a Power BJT 2N3055 and an IGBT IRGBC30U. Software used in this paper is ANSYS©. The validation approach is to perform a comparison of simulation results to experimental data thermal transducer IC LM35 measurements.
6.1 Modeling Approach
Typical computational process includes modeling, meshing, defining boundary conditions and analysis options, solving, post processing and evaluation. Fig. 6.1 presents an overview for ANSYS simulation.
Fig. 6.1 An Overview for ANSYS Thermal Simulation
6.2 Computational Modeling of Power BJT and IGBT
a.) Power BJT 2N3055
b.) IGBT IRGBC30U
Fig. 6.2 2D Cross-Section Component Layers
A discrete power electronics module such as a Power BJT or An IGBT consists of many elements.
Silicon - power device material.
Copper - high specific heat conductor.
Substrate - HV isolation.
Solder - eutectic soft solder.
Ext, as per the device specification.
a.) Power BJT 2N3055
b.) IGBT IRGBC30U
Fig. 6.3 Modeled and Meshed 2D Cross-Section Component Layers
The 2D- cross-section component layers of Power BJT and IGBT are shown in Fig. 6.2, modeled and meshed 2D cross-section Component layers of power BJT and IGBT are shown in Fig. 6.3. A commercial package of a Power BJT or An IGBT is an assembly of parts bonded together by solder, adhesive, molding compound, and mechanical parts such as bolts and springs.
a.)TO-3 Pack Power BJT 2N3055
b.)TO-220 Pack IGBT IRGBC30U
Fig. 6.4 Typical Commercial Package
In this study, commercial packages as shown in Fig. 6.4, TO-3 Power BJT 2N3055 mounted on a 2.5 inch x 2.5 inch heat sink was used and modeled in simulation for dynamic and steady state thermal behavior. TO-220AB IGBT IRGBC30U mounted on three different (2.5 inch x 2.5 inch, 2 inch x 2 inch and, 1.5 inch x 1.5 inch) heat sink were used and modeled in simulation for steady state thermal behavior.
The layer dimensions of the Power BJT 2N3055  and IGBT IRGBC30U  were listed in Table 6.1 and Table 6.2. The layers are assembled as shown in Fig. 6.2
Table 6.1 Dimensions of Layers in 2N3055
Table 6.2 Dimensions of Layers in IRGBC30U
Pb-Sn - I
Pb-Sn - II
The heat sink was used to increase the heat dissipation to the air. The heat sink was modeled. The Heat sink 2D model was shown in Fig. 6.5 the extrusion length of it was 63.5mm. Fig. 6.6 and Fig. 6.7 shows the modeled Power BJT and IGBT with heat sink.
Table 6.3 Typical Heat Sink Thermal Resistance and Their Equivalent Heat Transfer Coefficient for a 2.5 Inch x 2.5 Inch Module
Heat sink thermal resistance
Equivalent heat transfer
Natural convection (air)
Forced convection (air)
Fig 6.5 Dimensions of 2.5 inch x 2.5-inch Heat Sink Model
a.) Power BJT 2N3055
b.) IGBT IRGBC30U
Fig. 6.6 Meshed Model with 2.5 inch X 2.5 inch Heat Sink
a.) 2 inch X 2 inch heat sink
b.) 1.5 inch X 1.5 inch heat sink
Fig. 6.7 IGBT Model with Different Heat Sink
6.2.1 Material Properties
Typical packaging materials and their thermal properties used in this modeling are Listed in Table All the modeled elements were put in same window of ANSYS and the assembly was created by combining them with one another. After Glue of different layers the material property of different elements were given as stated in the Table-2. Although the thermal conductivity of most materials is usually temperature dependent, to some extent only, the thermal conductivity of silicon varies sufficiently over the temperature range of interest to require such temperature dependent values. The silicon layer is thus modeled using isotropic nonlinear thermal properties, whereas the remaining materials use constant linear properties. The temperature dependent thermal conductivity of silicon used in simulation is shown in Table 6.5. And the model was meshed to finite elements.
Table 6.4 Thermal Properties of Packaging Materials
Table 6.5 Temperature Dependent Thermal Conductivity of Silicon Used For Simulation
6.2.2 Determination of the Boundary Conditions
In most practical cases, to cool a module, an aluminum heat sink is attached to the bottom heat spreader. This makes heat sink modeling and the determination of boundary conditions very difficult without using a CFD code. As mentioned Table 6.3, a rough assumption could be made by applying a uniform temperature distribution across the whole area of the heat spreader. This usually implies an ideal cooling condition because this means an infinite heat sink heat capacitance.
A better approach is to apply an equivalent convective heat transfer coefficient to the bottom side of the module, following this equation:
Where Q is the amount of heat dissipated through the bottom side of the module, and Rsa is the sink-to-ambient thermal resistance. Geometry parameter A is the module footprint of each conductive layer. And hequ is the equivalent convective heat transfer coefficient used for thermal boundary conditions. Note that this equation is assuming a single heat source, and that the footprint of the silicon chip is equal to the heat sink area. It is usually used for a quick estimate of average junction temperature.
The equivalent convective heat transfer coefficient (hequ) 2.5 inch X 2.5 inch heat sink is  80 (W/°Cm2), 2 inch X 2 inch heat sink hequ is 100 (W/°Cm2) and 1.5 inch X 1.5 inch heat sink hequ is 120 (W/°Cm2) as the surface area of heat sink decreases the hequ increases.
6.2.3 Loading Conditions
After the modeling the simulation was done as per the Fig. 6.1. FE model used 8-noded 3D thermal solid elements (SOLID 70) [ANSYS-Help] with temperature being the only degree of freedom analyzed. The room temperature was applied to the entire setup. Chip volumetric power dissipation in W/m3 is applied in the active volume of the device i.e. the silicon volume and convection was applied to all the exterior areas.
The constant power applied for Power BJT 2N3055 are 3W and, 10W for steady state thermal analysis and, with 10W power for dynamic thermal analysis. The IGBT IRGBC30U with three different heat sinks was loaded 2W and 10W power for steady state thermal analysis.
6.3 EXPERIMENTAL SETUP
To validate the thermal model discussed, a well-designed experiment was conducted. The experimental setups used to validate the thermal model were shown in Fig.6.8 and Fig. 6.9. The power dissipation of the device was made equal to the heat generation power applied for the Finite Element Model in the simulation; this was achieved by taking control over the VCE and IC of 2N3055 or IRGBC30U as constant power dissipation mode.
The thermal transducer IC LM35 mounted on the case of Power BJT or Base Plate of IGBT. The thermal IC LM35 as shown in Fig. 6.10 gives 10mV/1°C with accuracy of 0.1°C from +2°C to +150°C. The temperature was noted up to steady state for constant power dissipation of the device, which was compared with the simulation results. The experiment was conducted all the power rating for what the simulation were done.
Per day one set of reading only observed from the device, after taking reading the device is subjected to ambient for the entire day to cool. The experiments were conducted many times till it governs constant sets of reading.
Fig 6.8 Power BJT 2N3055 Experimental Setup
Fig 6.9 IGBT IRGBC30U Experimental Setup
Fig. 6.10 Basic Centigrade Temperature Sensor LM35
6.4 RESULTS AND DISCUSSIONS
The simulation results were compared with the experimental results.
6.4.1 Power BJT 2N3055
Table 6.6 Comparison of Steady State Case-Temperature (°K) of Power BJT 2N3055 with 2.5 inch X 2.5 inch Heat Sink
The Table 6.6 shows the steady state case temperature of the simulation and experimental for different power dissipation condition. The case temperature of Power BJT 2N3055 with 2.5 inch X 2.5 inch heat sink simulation results is shown in Fig.6.11 for 10W. The dynamic temperature condition of the Power BJT 2N3055 with 2.5 inch X 2.5 inch heat-sink also studied for 10W power, the Time (sec) Vs Temperature (°K) is drawn in Fig. 6.12
Fig. 6.11 Case Temperature (°K) of Simulation Model of Power BJT 2N3055 with 2.5inch X 2.5inch Heat Sink for 10W Power
Fig. 6.12 Comparison of Simulation and Experimental of Dynamic Case-Temperature (°K) of Power BJT 2N3055 with 2.5 inch X 2.5 inch Heat Sink for 10W Power
6.4.2 IGBT IRGBC30U
The results of simulated model and experimental results were compared different heat sink basis for 2W and 10W power.
Table 6.7 Comparison of Steady State Base Plate-Temperature (°K) of IGBT IRGBC30U with 1.5 inch X 1.5 inch Heat Sink
Fig. 6.13 Base Plate Temperature (°K) of Simulation Model of IGBT IRGBC30U
with 1.5inch X 1.5inch Heat Sink for 2W Power
Fig. 6.14 Base Plate Temperature (°K) of Simulation Model of IGBT IRGBC30U
with 1.5inch X 1.5inch Heat Sink for 10W Power
Table 6.8 Comparison of Steady State Base Plate-Temperature (°K) of IGBT IRGBC30U with 2 inch X 2 inch Heat Sink
Fig. 6.15 Base Plate Temperature (°K) of Simulation Model of IGBT IRGBC30U
with 2 inch X 2 inch Heat Sink for 2W Power
Fig. 6.16 Base Plate Temperature (°K) of Simulation Model of IGBT IRGBC30U
with 2 inch X 2 inch Heat Sink for 10W Power
Table 6.9 Comparison Of Steady State Base Plate-Temperature (°K) of IGBT IRGBC30U with 2.5 inch X 2.5 inch Heat Sink
Fig. 6.17 Base Plate Temperature (°K) of Simulation Model of IGBT IRGBC30U
with 2.5 inch X 2.5 inch Heat Sink for 2W Power
Fig. 6.18 Base Plate Temperature (°K) of Simulation Model of IGBT IRGBC30U
with 2.5 inch X 2.5 inch Heat Sink for 10W Power
The table 6.7, 6.8 and 6.9 shows the comparison of simulation results with experimental results and the Fig. 6.13, 6.14, 6.15, 6.16, 6.17 and 6.18 shows the simulation results of IGBT URGBC30U with different heat sinks for 2W and 10W power.
The component level simulation results good correlate with experimental results for the both device.
Conclusion and future extension
The component level thermal modeling of Power BJT 2N3055 and IGBT IRGBC30U were done and analyzed through ANSYS V7.1. The comparison between simulation results and experimental results were found to have good correlation. By having the thermal modeling the case or base plate temperature was predicted and, to control the case or base plate temperature the heat sink can be redesigned in model level itself.
The reliability and longevity of any semiconductor device is inversely proportional to the square of the junction temperature change. Thus halving the junction temperature will result in approximately 4 times the expected life of the component. The converse is also true. A worthwhile increase in reliability and component life can be achieved by a relatively small reduction in operating temperature, since these parameters increase exponentially as temperature is reduced.
Good thermal design is often required to achieve high reliability, low manufacturing cost to avoid expensive cooling solutions, small size, and a predictable development time (to avoid redesigns and reduce time-to-market). With the available of powerful computational tools, the design cycle time and the cost of designing new power electronics can be greatly reduced. In addition, the efforts of improving current computational tools as design tools have proved the future potential of computational analysis in various heat transfer analysis.
7.2 FUTURE WORK
The maximum temperatures of Power Electronic Devices do not exceed the limit of the specified junction temperature. However, with the continual achievement of Moore's prediction and the continuous increase in power densities of the future semiconductor devices as discussed in section 2.1 and 2.2, the power density of Power Electronic Devices can also increase over the years where the junction temperature of the Power Electronic Devices could be over 150°C. Therefore, other possible methods of improving the thermal performance of the Power Electronic Device should be investigated in the future. These include using better thermal conductivity materials for the silicon die and the interface materials, implementing different cooling strategies such as improved heat sink design, active cooling techniques, and parallel operation of devices for improving current handling capacity or series operation of devices due to operating voltage, also to be considered.
Power Electronic Devices were used as power semiconductor switches, the constant temperature effects and the dynamic thermal effects should be known in design stage itself, so the future electronic simulators should include the thermal effect and become electro-thermal simulators as shown in Fig 7.1.