Simulation of Silicon Carbide Diode Heating 

and Natural Convection 

by Gregory K. Ovrebo 

ARL-MR-631 November 2005 

Approved for public release; distribution unlimited. 
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Army Research Laboratory 

Adelphi, MD 20783-1145 

ARL-MR-631 November 2005 

Simulation of Silicon Carbide Diode Heating 

and Natural Convection 

Gregory K. Ovrebo 

Sensors and Electron Devices Directorate, ARL 

Approved for public release; distribution unlimited.
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Simulation of Silicon Carbide Diode Heating and Natural Convection 

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Gregory K. Ovrebo 

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U.S. Army Research Laboratory 

Sensors & Electron Devices Directorate 

(ATTN: AMSRD-ARL-SE-DP) govrebo@arl.army.mil 

Adelphi, MD 20783-1145 

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13. SUPPLEMENTARY NOTES 

14. ABSTRACT 

I modeled two different electronic modules with SiC diodes to be used in pulsed power applications. Cooling in this 

high power application is provided by natural convection in a bath of oil. SolidWorks was used to construct a 

computer model and Cosmos FloWorks was used to simulate heating and natural convection in the oil. I applied two 

80 W pulses to the modules and calculated temperature increases of approximately 60 °C or less. I also simulated 

modules heated with a periodic signal over a 60 second period. Results were inconsistent with expectations, casting 

doubt on the merit of this simulation approach with Cosmos FloWorks. 

15. SUBJECT TERMS Simulation, fluid dynamics, silicon carbide.

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Gregory K. Ovrebo 

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ii 
Contents 

1. Introduction 1 

2. Device Models and Simulation Setup 1 

3. Pulsed Power Simulation Results 5 

4. Simulated Heating with Periodic Functions 9 

5. Conclusions 11 

Distribution List 12

Figures 

Figure 1. The diode assembly with 4 diodes.............................................................................................2

Figure 2. The diode assembly with 2 diodes.............................................................................................3

Figure 3. A diode assembly shown in its thermal simulation setting. The aluminum box 

containing the diodes has been made transparent in this view.........................................................4

Figure 4. Pulsed heating of the SiC diodes as a function of time. Peak power is total 

combined power in all diodes...............................................................................................................5

Figure 5. Contour plot of surface temperatures of the 4-diode assembly at t = 2 s..............................6

Figure 6. Contour plot of surface temperatures of the 2-diode assembly at t = 2 s..............................7

Figure 7. Cut plot showing the temperature distribution in the device and surrounding 

oil at t = 2 s.............................................................................................................................................8

Figure 8. Trajectories of convective oil flow near the SiC devices during heating...............................9

Figure 9. Sample of the periodic function used to simulate heating of diode assemblies...................10

Figure 10. Maximum device temperature in degrees Celsius calculated for the 2-diode 

model during heating by a periodic function over 60 seconds........................................................10

iii 
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iv 
1. Introduction 

The Army intends to use high power, high temperature silicon carbide (SiC) devices now 

under development in a number of applications, including power conversion and electric 

motors. The devices considered here are SiC diodes in series, which would carry wide, 

high power pulses in quick succession. I used computer simulation to investigate heating 

of these diode pairs while immersed in oil. Because there is no flow of oil, this is a 

problem in natural convection. Two different circuit layouts are simulated, with different 

levels of heating power per diode. I also considered the problem of simulating periodic 

power waveforms which heat devices over extended periods. 

2. Device Models and Simulation Setup 

Solid models of the SiC devices were prepared with SolidWorks modeling software. SiC 

chips 6mm square were placed on aluminum nitride (AlN) substrates. Kovar tabs were 

placed on top of the SiC chips for electrical contact. The diode assembly was modeled 

inside an aluminum box which contained one quart of BP oil. The box was a cube 

roughly 100 mm on a side with walls 2.5 mm thick. Figure 1 shows a layout with 

4 diodes, two pairs in parallel. In figure 2, we show a layout with a single diode pair. 

1 
Figure 1. The diode assembly with 4 diodes. 

2 
Figure 2. The diode assembly with 2 diodes. 

Simulation of the heating and convection in the model was performed with Cosmos 

FloWorks, a computational fluid dynamics code which can perform finite element 

calculations of fluid flow and heat transfer on SolidWorks models. I specified materials 

for each part of the model, as well as heat sources, heat sinks, and other boundary 

conditions. The physical properties of the aluminum, Kovar, AlN, and SiC were entered 

into the Floworks model, along with the properties of the oil. The oil’s physical 

properties were defined with values for specific heat, density, and viscosity which varied 

with temperature. Figure 3 shows a diode assembly in the center of the aluminum box, as 

it was modeled in the thermal simulation. The space inside the box surrounding the diode 

assembly is defined to be BP oil. 

3 
Figure 3. A diode assembly shown in its thermal simulation setting. The aluminum box containing 

the diodes has been made transparent in this view. 

Figure 4 is a diagram of the heating power applied to the diodes collectively in all of the 

pulsed thermal simulations. Each pulse is 0.8 s wide, with a maximum total heating 

power of 80 W. In a 4-diode device, each diode is heated with 20 W per pulse. In a 

2-diode device, each diode is heated with 40 W per pulse. This function was 

programmed into the simulation as the heat source. 

4 
0

20

40

60

80

100

0 0.511.522

Time (s)

Power (W)

.5

Figure 4. Pulsed heating of the SiC diodes as a function of time. Peak 

power is total combined power in all diodes. 

I defined the initial temperature of the entire simulation body—diodes, aluminum box, 

and oil—to be 80 °C, an ambient temperature common in automotive environments. The 

temperature of the outside surface of the box is specified to be a constant 80 °C, serving 

as a heat sink for the simulation. Maximum temperatures on the SiC diodes were 

recorded at the end of each pulse, at 1 second and 2 seconds, respectively. 

3. Pulsed Power Simulation Results 

When I simulated pulsed heating of the 4-diode setup, I recorded a maximum temperature 

on the diode assembly of 110 °C at t = 1s, the end of the first pulse, and 119.4 °C at 

t = 2s, the end of the second pulse. Figure 5 is a contour plot of surface temperatures on 

the diode assembly at t = 2s. We see a maximum temperature increase of approximately 

40 °C during pulsed heating. 

5 
Figure 5. Contour plot of surface temperatures of the 4-diode assembly at t = 2 s. 

When I simulated pulsed heating of the 2-diode assembly, I recorded a maximum 

temperature on the diode assembly of 120 °C at t = 1s, the end of the first pulse, and 

142 °C at t = 2s, the end of the second pulse. Figure 6 is a contour plot of surface 

temperatures on the diode assembly at t = 2s. Thus, maximum temperature increase 

during heating was calculated to be 62 °C. The temperature increase here is greater than 

that for the 4-diode assembly, but it did not scale linearly with power—doubling the 

power per device did not lead to a doubling of maximum temperature. The temperature 

also remained in a range which should be safe for operation; temperatures much above 

200 °C could threaten component integrity. 

6 
Figure 6. Contour plot of surface temperatures of the 2-diode assembly at t = 2 s. 

Figure 7 is a cut plot of temperature contours in and around the diode assembly at the end 

of the second pulse, the point of maximum temperature rise. The contour plot is 

projected on a plane cutting through the diode assembly. It shows the limited extent of 

temperature increase in the oil around the SiC devices during heating. Although I chose 

to use a volume of 1 quart of oil for this simulation, it appears that our results will not 

change significantly with changes in oil volume because so little of the oil is being heated 

by the diodes. 

7 
Figure 7. Cut plot showing the temperature distribution in the device and surrounding oil at t = 2 s. 

Figure 8 is a plot showing the direction of convective flow of the oil around the diode 

assembly calculated by FloWorks during the heating simulation. This plot should be 

taken as a qualitative plot, rather than a quantitative measure of fluid velocity or mass 

flow. 

8 
Figure 8. Trajectories of convective oil flow near the SiC devices during heating. 

4. Simulated Heating with Periodic Functions 

I attempted to simulate the heating of the diode assemblies with a periodic function, 

roughly mimicking the heating performed in laboratory evaluations with rectified wave 

signals. The function I devised to simulate heating power in the diodes was a sawtooth 

wave with a period of 16 ms and a frequency of 62.5 Hz. The peak power was a 

combined 80 W for all the diodes. A diagram of a portion of the signal is shown in 

Figure 9. 

9 
0

20

40

60

80

100

0 20 40 60 80 100

Time (ms)

Power (W)

Figure 9. Sample of the periodic function used to simulate heating of 

diode assemblies. 

The signal was applied for a total of 60 seconds in this simulation, which I assumed to be 

enough time to reach a steady state temperature in the model. The results of the 

simulation show the limits of FloWorks as a tool for predicting changes in temperature in 

a system as it is heated by a steady signal, rather than a pulsed signal. Figure 10 is a plot 

created during the FloWorks calculation which shows maximum temperature, in degrees 

Celsius, in the diodes over the 60 second simulation time. 

Figure 10. Maximum device temperature in degrees Celsius calculated for the 2-diode model during 

heating by a periodic function over 60 seconds. 

10 
The 60 seconds of simulated physical time was performed with 210 iterations. The 

problem with FloWorks appears to be that the simulation does not break up the period of 

simulation evenly. The first 10 seconds of the simulation was performed in only 10 

iterations; the remaining 50 seconds was done in 200 iterations. Thus, the first few 

seconds of temperature calculations show a series of jagged peaks rather than a smooth 

increase in temperature. Even after the first few seconds, the simulation does not rise 

smoothly to a temperature plateau; instead, maximum temperature swings wildly within a 

range of more than 40 °C. This result is not physical. 

The last 30 seconds of the simulation show a mean temperature of approximately 230 °C. 

Compare that result to a steady state simulation I performed, in which the diodes were 

heated with a constant 40 W total power. This is the power, averaged over time, put into 

the diodes by the sawtooth wave in the time dependent simulation. This steady state 

simulation, with all other factors the same as the time dependent simulation, calculated a 

maximum temperature of 240 °C. The steady state calculation of heating power into the 

diodes compares well with calculation of heating by a periodic signal over an extended 

time, and requires much less time to perform. 

5. Conclusions 

I performed thermal simulations of SiC diodes heated by twin pulses 800 ms wide with 

total peak heating losses of 80 W. When submerged in one quart of BP oil, heat was 

transferred by convection away from the heated diodes. Under these conditions, the 

simulation showed moderate temperature increases. A module with 2 diodes rose in 

temperature approximately 60 °C and a module with 4 diodes rose in temperature only 

40 °C. With a starting temperature of 80 °C, both of these results keep the peak 

temperature below 150 °C and well within safety margins. 

Thermal simulations using long periodic functions as heat sources, mimicking steady 

state AC signals, do not show reliable convergence to an equilibrium temperature value. 

The temperature instead rises and falls randomly within a wide range over the simulation 

period. A steady state simulation using the average power of the AC signal as a constant 

heat input may yield as much useful information as a time dependent simulation using the 

actual AC signal. 

11 
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