Thermal Design Tool for Vehicle Electric Parts
by Keiji Mashimo * , Hiroshi Nagase * , Yukihiro Saita * , Ken Koizumi * and Shin-ichi Jingama * 2This paper proposes a software tool utilizing CAE, and a method for parts
design. Progress in vehicle electronics has been remarkable, and electronic
circuit design now has a significant influence on car performance. Hence, it is desirable to link
analysis and circuit design directly. Commercial design software allows approximate calculationof heat transfer in circuit boards, but the distribution of heat generation and differences of heattransfer coefficients are not taken into account. This paper addresses these issues and reports
on an advantageous circuitry technique. The main feature of this development is the speciallymade component library that can be used with commercial analysis software. Information stored
in the component library includes the shape and heat transfer coefficient of each surface, theheat generated, and the terminal connecting structure. Even if the shape and heat transfer
coefficients can be defined with commercial software, residual parameters are hard to define
due to the limitations of the software. A CAD-based geometry file is combined with component
data, and the combined file is applicable for computer analysis. As a result of utilizing the abovementioned technique, the cost of designing a new circuit board or a junction block, etc. can behalved compared to conventional techniques.

ABSTRACT1.

INTRODUCTION

Vehicle electric parts made up of several circuit boards
and electrical joints are the target of this development.
One of these is the junction block (JB), which typicallyhas relays, fuses and circuit patterns as components. Its
main function is to distribute signals and power to theproper devices. However, preparation for analysis can be
an arduous and time-consuming process, and due to theincrease in the scale of circuits, the situation is becoming
even more serious.
This paper introduces a solution to this problem, andthe software tool developed here proves effective in practical application with reduced labor.

2.

TOOL CONSTRUCTION

Figure 1 shows the data flow in the software developed
here. Its basic concept is to use a customized preprocessor with a commercial solver and post-processor.
The customized pre-processor, which is main focus of thispaper, includes three principle elements. The first element
is a CAD process, which constructs geometric data foranalysis. Library editors are included in this process. The
second element is an automatic mesh generator. The lastelement is a boundary condition generator.
* Automotive Technology Center, R&D Div.
* 2 Harness Dept., Automotive Products Div., Electronics andAutomotive Systems Co.

In conventional methods, the most time-consuming
process is defining boundary conditions, generally followed by automatic mesh generation. The heat generated
and the heat transfer coefficient are the two most important aspects of boundary conditions in this type of analysis.
In this development, the heat generated from a circuit
pattern is calculated with a Joule heat calculator, while theheat generated from the components is calculated withthe component's boundary condition generator, which
can also calculate the heat transfer coefficient.
The automatic mesh generation process is based on
the original CAD tool which contains library editors. The
library editors enable us to construct an outline structure
for the automatic mesh generator, and, using component
data created by the library editors, the original line drawing tool creates an outline drawing for analysis.
Finally as a pre-process, the analysis data generator
provides data for sending to the solver program. Theanalysis data generator requires geometrical data andboundary conditions. These are delivered from theabove-mentioned tools.

3.

UTILIZATION OF THE CAD SOFTWARE

Only with difficulty can commercial CAD software beapplied directly to this type of analysis. For example,
automatic recognition of a structure with a narrow crevice
or a laminated structure is very hard to do. If there isneither a narrow crevice nor an overlap structure (asFurukawa Review, No. 27

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when a lid is put on a box), we can use commercial
software capable of automatic generation of a meshfrom a quite complicated CAD drawing, which hascome on to the market recently. However, this is notentirely satisfactory, in that commercial automatic meshgeneration software fundamentally deals with tetrahedralmesh generation. There are, however, disadvantages
with a finite element method using a tetrahedral mesh.
At least, a hexahedral mesh enables us to make amodel with fewer nodes, and in some types of analysis
a tetrahedral mesh cannot provide sufficient accuracy.
Automatic generation of a hexahedral mesh is still being
developed by several groups and has not yet been
completed.
In this development, geometrical data and boundary conditions are given from libraries. That means theboundary conditions for each position are prepared automatically and precisely. Data from commercial softwareare only for circuit pattern drawing. Other geometrical
data are also from the libraries. There are numerous typesof library editors, while their basic functions are the same.
They provide geometrical data and the value of heat generated. Typically, editors are applied for circuit boards,relays and terminals. The geometrical data provided bylibrary editors become the basis of mesh generation.
Information on only a few coordinates is enough to create line drawings, and no complicated data is necessary.
Figure 2 is a typical line drawing generated by the tool
developed here. It has a layered structure as if sliced intoshells. The extracted data are treated as in the case of CT
scan, and a three-dimensional figure is then reconstructed.

Data flow.

Figure 24.

Layered line drawing.

AUTOMATIC MESH GENERATOR

Figure 3 shows how an actual model is represented as aline drawing by the automatic mesh generator.
A two-dimensional line drawing is provided from eachlayer. This data set of a line drawing is defined here asa “CAD,” and from the CAD file a filtering tool can provide the minimum geometrical information, which can bedefined using “corner points.” Mesh generation software
can then generate a calculation mesh from the cornerpoints. The filtering and mesh generation software areof a proprietary design. The role of filtering software is topurge unused information and to provide useful geometrical information from each layer of the line drawing. The
automatic mesh generation software reconstructs a threedimensional structure from the vertices and generates
a hexahedral mesh using a structured method, which issuitable for a rectangle-based object. In spite of limited
applicable objects, this method is suitable for many typesof JB since the components used in them are mostly rectangular.

Furukawa Review, No. 27

2005

59

Thermal Design Tool for Vehicle Electric Parts

sions of each component are required, and the dimension data are also obtained from libraries.

6.

RESULTS AND DISCUSSION

6.1 Heat Transfer Analysis
The temperatures at several points located on the board
surface are compared between measured and calculatedvalues. Figure 4 takes calculated values along thehorizontal axis and actual measured values along thevertical axis, and plots various points on the board. Alsoin the figure, ∆ T expresses the temperature rise in theKelvin scale. In this example, the maximum error between
measurement and calculation is approximately ±2K,
which is sufficient for practical purposes.

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Automatic mesh generation.

BOUNDARY CONDITION GENERATOR

The boundary condition generator can output two typesof data: heat generated from components and heattransfer coefficients for each portion of the surface.
Source data for both types of boundary conditions aresupplied from the libraries.
Heat from the circuit is calculated with a Joule heat calculator. Hence, the objectives are relays and terminals.
The heat generation data are stored in libraries as volumeflux. The boundary condition editor acquires these dataand assigns each of them to corresponding coordinatesof a fully constructed analysis model.
The Joule heat calculator requires drawing data of a circuit pattern. Also, it requires the input of electrical current
data. In the first step, Joule heat distribution on the circuit board is calculated, and in next step, these data are
combined with data for another boundary condition andexported to the solver.
Heat transfer coefficients for each portion of the surface of the components should be calculated using thisboundary condition generator. In this case, the dimen-Calculated and measured data.

6.2 Calculation Time
Analysis time can roughly be divided into model creation
time and calculation time. Among commercial solvers,
calculation times do not differ by much in normalcases. Preparing boundary conditions is the most timeconsuming process. The developed boundary condition
generator can dramatically reduce the preparation time.
As a result, total simulation time is less than 50 % of theconventional method.

7.

CONCLUSION

In this paper, we have presented a new design method
for vehicle parts considering thermal properties. This
simulation tool is constructed of a library editor, automaticmesh generator and boundary condition generator. Thisdevelopment induces the enough accuracy for design
and the reduction of simulation time by 50 % or less.
In the future, more generally used software tool should
be developed which realizes virtual trial production.

Furukawa Review, No. 27

200560Thermal Design Tool for Vehicle Electric Parts

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