AUTOMOTIVEElectromagnetic and Thermal Design Technology forReactor Development
Takuji KANTOU*, Shinichiro YAMAMOTO, Masayuki KATO, Tomohiro KEISHIand Takanori SAWAI

Sumitomo Electric Industries, Ltd. is working to develop low-cost and compact reactors, which contribute to the improvement of the converter. We have developed an original tool for reactor electromagnetic/thermal design based onlogic equations and Computer Aided Engineering (CAE). In our study, the predicted values calculated using the tool
were found to be highly consistent with observed values and the development time of the reactor was successfully
shortened. This paper reports the details of our development.
Keywords: reactor, electromagnetic design, thermal design, eco-friendly vehicle

1. Introduction
Auxiliary
battery

Auxiliaries(ex.wiper,audio)
Motor

InverterConverter

EngineFor auxiliaries

Converter
Battery fordrive motor

ConverterAC
100/200V

For drive motor

For power supply

Fig. 1. HEV, PHEV, EV Systems

Power module

650V

ReactorInverter

Recently, global warming has become a serious social
issue. To tackle this issue, the automobile companies areconcentrating on the development of environmentallyfriendly automobiles, such as the hybrid electric vehicle(HEV), plug-in hybrid electric vehicle (PHEV), electricvehicle (EV) and fuel cell vehicle (FCV). To promote thedissemination of these eco-friendly vehicles, it is vital to
improve their driving performance and power acceleration as well as energy efficiency to the level of the gasoline
vehicle performance.
In this regard, the converter plays an important role,
specifically to increase the battery voltage for the highpower motor and recharge the EV and PHEV batteries
from household outlets. In response to these varied demands of the converter, automakers are trying to develop
small and cost-efficient converters.
Sumitomo Electric Industries, Ltd. is working to develop low-cost and compact reactors, one of the core components for the converter. In our study, we developed an
original tool for reactor electromagnetic/thermal design
based on logic equations and Computer Aided Engineering (CAE). The evaluation result showed that the predicted values calculated using our tool were highly consistent
with observed values and the development time of reactors was successfully shortened. This paper reports the details of the development.

200V

Drive circuit

2. Reactor Structure

Fig. 2. Example of the boost converter

Figure 1 shows the possible locations where the boost
converter is applicable in a battery converter for the HEV,
PHEV, EV systems, etc. An example of the inner structureof the boost converter is shown in Fig. 2. The structure
consists of a reactor, power semiconductor, condenser
and the circuits which drive these components. As shownin Photo 1, a reactor is an iron core with a copper wire
coil wound around it. The reactor is the core componentof voltage conversion, which stores and emits energy inalternation by running a current through the coil.
The specifications for the reactor developed at ourcompany are shown in Table 1.

SEI TECHNICAL REVIEW · NUMBER 70 · APRIL 2010 · 41

Basic electromagnetic design
Specification Material Design parameter

Theoretical calculation
Developed software
for a reactor design
Determine the rough electromagnetic properties
Detailed
electromagnetic
designCreate CAE model

CoilCore

Electromagnetic field analysis

Photo 1. Developed reactor

Determine the detailed electromagnetic properties

Table 1. Specifications of the developed reactor

Inductance~500µHCurrent~200A

Drive frequency~100kHz

Thermal design
Create detailed model
Heat analysis
Determine the detailed thermal properties
Complete the reactor design

3. Design
Fig. 3. Process of designing a reactor

When designing a reactor, the most important factors
to consider are inductance, which is a necessary parameter for voltage conversion, and loss that affects fuel consumption. There are methods to measure inductanceaccurately; however, loss cannot be measured accurately
because of the large phase error between current andvoltage caused by the low phase factor of the reactor.
Since loss is converted to heat when a current is applied
to the reactor, we examined the consistency between thedesigned values and measured values of the reactor temperature to assess the validity of the loss design value as
an alternate method to measure loss.
Figure 3 displays the process of designing a reactor.
It can be largely divided into electromagnetic design andthermal design.
In electromagnetic design, inductance and size isfound in 2 steps: basic design and detailed design. By
using our original development tool to obtain the roughsize during the basic design step, this design method enables us to shorten the design period.
The main feature of thermal design is the ability to
predict the reactor temperature within ±10% accuracy bycreating a model that is almost identical to the originalstructure. Heat resistant parts are selected based on thepredicted temperatures, then the finalized structure andsize is determined.
Ultimately, these analysis results should be comparedwith observed values to confirm the validity of the design.
By using the above process to design the reactor, wewere able to perform high precision electromagnetic design and thermal design in a short time, and also develop
a reactor that satisfies all the specifications on our first attempt. The details are discussed in the following.

3-1 Basic electromagnetic design
For basic electromagnetic design, our objective wasto develop a tool that could efficiently determine therough reactor structure satisfying the requested specifications (inductance, loss, size). Thus, there would be noneed to repeatedly perform detailed design and the overall design period would be shortened.
The representative equations (1), (2) based on magnetic theory were used to calculate inductance during tooldevelopment.

LI maxB max = —NS(・・・・・・・・・・（1）lglc
L= N2 —+—
µr µ0 S µ0 S
Bmax : Maximum flux density
L : Inductance
Imax : Maximum Current
N : Coil turns
S : Core cross-section area)-1・・・・・・・・・・（2）lc : Flux path lengthlg : Gap lengthµ0 : Space permeabilityµr : Relative permeability of core

Loss can be classified as shown in Fig. 4. Alternatingcurrent (AC) resistance loss cannot be calculated using
simple equations, so detailed design (magnetic flux analysis) was used. Coil loss was calculated using the generallogic equations (3), (4).

42 · Electromagnetic and Thermal Design Technology for Reactor Development

＝Reactor loss
Coil loss (copper loss)
DC resistance loss (by DC resistance of coil)+AC resistance loss (by skin effect and proximity effect of coil)+
Loss by the flux leakage＋
Core loss (iron loss)
Hysteresis loss7+
Eddy current loss

Fig. 5. Contour figure of electromagnetic field analysis

Fig. 4. Classification of the loss

Top crosssectional viewlR dc = ρ0 — {1+ α 0 ( T – 20 )}SR dc : Direct Resistanceρ0 : Resistivity (at 20C)
l : Coil length

CoilGap

Flux leakage

・・・・・・・・・・（3）
CoreT : Temperature
S : Coil cross-section area
α 0 : Temperature coefficient of resistivity

CoilCore Gap

Fig. 6. Flux leakage from the gap・・・・・・・・・・（4）Pcoil = I R dc
2

Pcoil : Coil loss

I : Current
BCore loss was calculated using the Steinmetz equation (5).

Pcore = K h Bm

1.6 ~ 2f + K e Bm f22・・・・・・・・・・（5）Pcore : Core loss
Bm : Amplitude of magnetic flux density
K h : Hysteresis loss coefficient f : Frequency
K e : Eddy-current loss coefficient

The core physical values (hysteresis loss coefficient,
eddy-current loss coefficient, and amplitude of magnetic
flux density) required for equation (5) differs dependingon the material. Generally, the reactor core materials aremagnetic steel plate, ferrite, and amorphous, but in thisreport we will discuss the results based on our study which
utilized our original powder core (pure iron or Fe-Si
based powder coated with insulation then press formed)
and coil wire material. We were able to enhance the design accuracy by using the known detailed physical valuesof these main materials
In past designs, the optimal design parameters were
found by a trial and error process, repeatedly altering theparameters then testing them, which was a time consumingtask. For that reason, we developed our original software
to automatically compute all combinations of the design
parameters in a given range and determine the optimal reactor structure. As a result, we were able to significantly re-Blinear

Basic design

Hnonlinear

H
Detailed design

Fig. 7. Nonlinear magnetic properties

duce the design time compared to past design methods.
3-2 Detailed electromagnetic design
Following the basic electromagnetic design step todetermine the reactor’s rough size, detailed electromagnetic design was performed to calculate inductance andloss (AC resistance loss, iron loss).
The details of the flux expansion flowing inside the core
were analyzed using CAE analysis (Fig. 5), the effects of theflux leakage from the gap are also analyzed (Fig.6) and premeasured nonlinear magnetic properties of the powder core(Fig. 7) were reflected to increase the precision of the inductance direct current (DC) superposition property.
Figure 8 is a contour figure showing the flux density
calculated using the above mentioned contents with themagnetic flux analysis software (CAE), and the leakage flux
from the gap shown in Fig. 6 can be predicted.
The detailed inductance superposition property canbe calculated with equation (6) using magnetic energy obtained from the magnetic field analysis, and current value.

SEI TECHNICAL REVIEW · NUMBER 70 · APRIL 2010 · 43

CoilFluxleakage

Expansion
CoreCoilCoreCore
GapFig. 8. Contour figure of electromagnetic field analysis

12
E = — LI
2・・・・・・・・・・（6）temperature change over time, were entered as properties
for the model. In order to increase the accuracy of theseparameters, physical values were confirmed with actual
measurements for each of the parts before configuring tothe model.
In addition, we increased the accuracy of the thermal
analysis results by giving the model specific values for thereactor heat release factors: DC resistance loss (Fig. 10:
configure the entire coil with a function that considers
the temperature dependency of the copper resistor), AC
resistance loss (Fig. 11: configure the skin effects, proximity effects, leakage flux effects to the inner surface ofthe coil), and iron loss (Fig. 12: configure hysteresis andeddy current to the entire coil), calculated through detailed electromagnetic design.

E : Magnetic field energy L : Inductance I : Current

Simultaneously, each loss is calculated from the magnetic field analysis. DC resistance loss was calculated fromthe current distribution flowing through the coil. AC resistance loss was calculated from the current distribution
caused by skin effect, proximity effect, and leakage flux.
Iron loss was calculated for every distribution of the core
flux density using the Steinmetz equation (5).
The detailed loss value found from above was included as a heat generating condition for thermal design.
3-3 Thermal design
For thermal design, the validity of the loss value has
to be verified and also, the design must allow for a sufficient temperature margin for each of the parts.
Based on the data calculated from detailed electromagnetic design, a heat analysis model (Fig. 9) including
the peripheral parts (case, etc.) was created. Our company
is proficient at producing a model that is structurally accurate. We are able to do this, for example, by creating
separate models for the coil center wire and coating, anda detailed model for the places that effect heat dissipation,such as the core-case contact heat resistor and adhesivematerial between the gap and core, etc. As a result, theprecision of the transient property was greatly improved.
The basic parameters of thermal conductivity and thephysical values relating to the thermal properties for eachpart, as well as specific heat and density, used to identify

Fig. 10. Setting the DC resistance loss

Fig. 11. Setting the AC resistance loss

REACTOR
CASE

Fig. 9. CAE model for heat analysis

44 · Electromagnetic and Thermal Design Technology for Reactor Development

Fig. 12. Setting the iron loss

Figure 13 shows the results of the thermal analysis conducted with the above described conditions. The upper surface of the coil has very high temperatures. Figure 14 is thecross sectional view represented by the white box in Fig. 13;
the area where the white arrow points (the mid section of
the coil’s upper surface) has the highest temperature. Fromthese results, we were able to compare the maximum temperature limit of each material and select the structure thathas an adequate temperature margin.

①②⑥③④⑤Fig. 13. Contour figure of heat analysis

Point of the highest temperature

Fig. 15. Measurement point of the reactor temperature

5. Conclusion

Fig. 14. Contour figure of heat analysis (cross-section view)

We developed an original tool for reactor electromagnetic/thermal design, which performs basic design
based on logic equations and detailed design based onCAE. The predicted values of inductance and transientphenomenon of elevated temperature calculated using
our tool were found to be highly consistent with observed
values. With these results, we hope that our design tool
will be useful in reactor development and serve for product development in the future.

4. Evaluation
In order to verify the validity of the above design results,
we manufactured the reactor, then compared the designvalues and observed values of inductance. The design values
and observed values of 5 different current conditions set in
between the minimum current and maximum current werecompared. All results came within ±10% of each other.
In addition, 2 types of power distribution were used
to test the thermal design results. In the transient temperature mode, the reactor was placed under high stress conditions and temperature was gradually increased from thestart point as time elapsed, while in the other mode, thetemperature remained constant. After evaluating the 6measurement points shown in Fig. 15, we found that allvalues came out to be within ±10% accuracy when compared to the design values. In conclusion, both electromagnetic and thermal design was able to perform highprecision design, and we were also able to confirm the capacity of the design tool.

(1)
(2)
(3)

References
Hideho Yamamura: “The toroidal core handbook” CQ Publishing
Naoki Kunimine: “A Complete Introduction to Electronics for
Thermal design” Nikkan Kogyo Shimbun, Ltd.
Technical Information Institute Publication: “Technique for providing heat radiation material for electronic device/component
with high thermal conductivity and measuring/evaluating thermalconductivity”

SEI TECHNICAL REVIEW · NUMBER 70 · APRIL 2010 · 45

Contributors (The lead author is indicated by an asterisk (*)).

T. KANTOU*• Assistant Manager, Automotive Technology R&D Laboratories
He is engaged in the development anddesign of reactors for environmentallyfriendly motor vehicles.

S. YAMAMOTO• Automotive Technology R&D Laboratories

M. KATOU• Manager, Automotive Technology R&D Laboratories

T. KEISHI• Senior Specialist,
Dr. Engineering,Senior Assistant Manager, Analysis Technology Research Center

T. SAWAI• General Manager, Automotive Technology R&D Laboratories

46 · Electromagnetic and Thermal Design Technology for Reactor Development