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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