1. ECE2799 - Engineering Design Thermal Considerations
Prof. Mazumder
Prof. Bitar
Updated
11/16/2017

2. Heat Flow Heat (Power) flows from an area of
higher temperature To lower temperature
3 Heat flow mechanisms
Conduction  transferring heat through a solid body
Power easily flows thru high thermal conductivity material (e.g. copper, aluminum)
Convection  heat is carried away by a moving fluid
Free convection
Forced convection  uses fan or pump
Radiation
Power is radiated away by electromagnetic radiation

3. Thermal Modeling, Marc Thompson, 2000
Conduction
Heat is transferred through a solid from an area of higher temperature to lower temperature
Good heat conduction  need large area, short length and high thermal conductivity
Example: aluminum plate,
l = 10 cm, A=1 cm2, T2 = 25C (298K), T1 = 75C (348K), k = 230 W/(m-K)
Thermal Modeling, Marc Thompson, 2000

4. Thermal Modeling, Marc Thompson, 2000
IC Mounted to Heat Sink
Thermal Modeling, Marc Thompson, 2000

5. New Japan Radio
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7. Heat flow model by electrical circuit
Heat flow  Current ;
Temperature  Voltage ;
Heat source  Current Source
Thermal resistance  Resistor ;
Thermal capacitance  Capacitor

8. Heat flow model by electrical circuit
Heat flow  Current ; Temperature  Voltage ; Heat source  Current Source
Thermal resistance  Resistor ; Thermal capacitance  Capacitor
: Thermal resistance of the heatsink TJ TC TH
TAMB
Rtheta-J-C
Rtheta-C-H
Rtheta-H-A Q IS
: Power dissipated by device  IS
: Junction Temp of Device  Voltage at node Tj
: Case temp  Voltage at node Tc
: Temp where heatsink is attached  Node voltage at TH
: Ambient temp  Node voltage at Tamb.
: Thermal resistance from junction to case
: Thermal resistance from case to heatsink
New Japan Radio
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9. Thermal Resistance of Electronic Components
Junction Temperature:
TJ = TA + (ΘJA × P)
Where:
TJ = junction temperature
TA = ambient temperature, and
P = power dissipation in Watts
Maximum Allowable
Power Dissipation:
Pmax = (TJ-max - TA) / ΘJA
Maxim Deration Function:
describes how much the power dissipation must be reduced for each °C of ambient
temperature over +70°C. The deration function is expressed in mW/°C.
Deration function = P / (TJ - TA) Where:
TA is typically +70°C (commercial) and
TJ is the maximum allowable junction temperature, typically +150°C.
To find the maximum allowable power when the ambient temperature is above +70°C (for example,
+85°C in the extended temperature range), proceed as follows:
Pmax85C = Pmax70C - (Deration Function × ( ))

10. New Japan Radio
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11. New Japan Radio
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New Japan Radio
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15. Convection Heat Transfer Coefficient
Convection can be described by a heat transfer coefficient h and Newton’s Law of Cooling:
Heat transfer coefficient (h) depends on properties of the fluid, flow rate of the fluid, and the shape and size of the surfaces involved, and is nonlinear
Equivalent thermal resistance:
Reference: B. V. Karlekar and R. M. Desmond, Engineering Heat Transfer, pp. 14, West Publishing, 1977

16. Thermal Modeling, Marc Thompson, 2000
Free Convection
Heat is drawn away from a surface by a free gas or fluid
Buoyancy of fluid creates movement
For vertical fin:
Example: square aluminum plate, A=1 cm2, Ta = 25C (298K), Ts = 75C (348K)
Thermal Modeling, Marc Thompson, 2000

17. Thermal Modeling, Marc Thompson, 2000
Forced Convection
Need to use when heat sinks can not dissipate sufficient power by natural convection and radiation
In forced convection, heat is carried away by a forced fluid (moving air from a fan, or pumped water or coolant etc.)
Forced air cooling can provide typically 3-5 increase in heat transfer and 3-5 reduction in heat sink volume
In extreme cases you can do 10x better by using big fans, convoluted heat sink fin patterns, etc.
Thermal Modeling, Marc Thompson, 2000

18. Thermal Performance Graphs for Heatsinks
Curve #1: natural convection (P vs. DTsa)
Curve #2: forced convection curve (Rsa vs. airflow)
Thermal Modeling, Marc Thompson, 2000

19. Thermal Modeling, Marc Thompson, 2000
Radiation
Energy is lost to the universe through electromagnetic radiation
 = emissivity (0 for ideal reflector, 1 for ideal radiator “blackbody”); s = Stefan-Boltzmann constant
= 5.6810-8 W/(m2K4)
Example: anodized aluminum plate,  = 0.8, A=1 cm2, Ta = 25C (298K), Ts = 75C (348K)
Thermal Modeling, Marc Thompson, 2000

20. Thermal Modeling, Marc Thompson, 2000
Comments on Radiation
In multiple-fin heat sinks with modest temperature rise, radiation usually isn’t an important effect
Ignoring radiation results in a more conservative design
Effective heat transfer coefficient due to radiation for ideal blackbody ( = 1) at 300K is hrad = 6.1 W/(m2K), which is comparable to free convection heat transfer coefficient
However, radiation between fins is usually negligible (generally they are very close in temperature)
Thermal Modeling, Marc Thompson, 2000

21. Cost for Various Heat Sink Systems
Note: heat pipe and liquid systems require eventual heat sink

22. Comparison of Heat Sinks
STAMPED
“CONVOLUTED”
EXTRUDED
FAN

23. Liquid Cooling Advantages Best performance per unit volume
Typical thermal resistance C/W
Disadvantages
Need a pump
Heat exchanger
Possibility of leaks
Cost

24. Thermal Modeling, Marc Thompson, 2000
Heat Pipe
Heat pipe consists of a sealed container whose inner surfaces have a capillary wicking material
Boiling heat transfer moves heat from the input to the output end of the heat pipe
Heat pipes have an effective thermal conductivity much higher than that of copper
Thermal Modeling, Marc Thompson, 2000

25. Thermoelectric (TE) Cooler
“Cooler” is a misnomer; a TE cooler is a heat pump
Peltier effect: uses current flow to pump heat from cold side to warm side
Pumping is typically 25% efficient; to pump 2 Watts of waste heat takes 8 Watts or more of electrical power
However, device cooled device can be at a lower temperature than ambient
Thermal Modeling, Marc Thompson, 2000

26. Other Important Thermal Design Issues
Contact resistance
How to estimate it
How to reduce it
Thermal pads, thermal grease, etc.
Geometry effects
Vertical vs. horizontal fins
Fin efficiency (how close together can you put heat sink fins ?)
Thermal Modeling, Marc Thompson, 2000