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Bridging Theory in PracticeTransferring Technical Knowledgeto Practical Applications
Introduction to Power Dissipation and Thermal Resistance
Introduction to Power Dissipation and Thermal Resistance
Intended Audience:• Engineers interested in the basics of power dissipation and thermal design
calculations• A basic knowledge of resistive circuits is required
Topics Covered:• What is power, temperature, and thermal resistance?• What are the basic thermal parameters and how are they specified?• How do heatsinks affect thermal designs?• DC thermal calculations
Expected Time: • Approximately 90 Minutes
Introduction to Power Dissipation and Thermal Resistance
• What is Power?• What is Junction Temperature?• What is Thermal Resistance?• Electrical Parameters vs. Thermal Parameters• Thermal Specifications• Heatsinks• DC Thermal Calculations
Introduction to Power Dissipation and Thermal Resistance
• What is Power?• What is Junction Temperature?• What is Thermal Resistance?• Electrical Parameters vs. Thermal Parameters• Thermal Specifications• Heatsinks• DC Thermal Calculations
Introduction to Power Dissipation and Thermal Resistance
What is Power?
Work is the result of a power applied for a given amount of time
Work = Power * Time
What is Power?• Electrically, power is a product of a voltage and a
current:
• For example, a battery that can deliver 10A at 12V can supply 120W of power:
Power = Voltage * Current
P = V * I
P = 12V * 10A = 120W
• If a battery can provide 120W of power, the battery load must consume 120W of power
• Some of the power put into the battery load is absorbed and dissipated as heat
• From Ohm’s Law (V=IR), the power dissipated as heat in a load is given by:
What is Power?
120WSupplied
120WConsumed
P = V * I = (IR)*I = I2R
• If a battery can provide 120W of power, the battery load must consume 120W of power
• Some of the power put into the battery load is absorbed and dissipated as heat
• From Ohm’s Law (V=IR), the power dissipated as heat in a load is given by:
What is Power?
120WSupplied
120WConsumed
P = V * I = (IR)*I = I2R
Electrical Power
P = VI
P = I2R
• The important things you must remember here:
• What is Power?• What is Junction Temperature?• What is Thermal Resistance?• Electrical Parameters vs. Thermal Parameters• Thermal Specifications• Heatsinks• DC Thermal Calculations
Introduction to Power Dissipation and Thermal Resistance
Junction Temperature
• Junction temperature is the temperature of the silicon die in an integrated circuit
PC Board
Sili
con
die
JunctionTemperature
Lead
fram
e
• This is not the same as the case (or package) temperature or the ambient (or air) temperature
PC Board
Sili
con
die
JunctionTemperature
CaseTemperature
AmbientTemperature
Lead
fram
e
Ambient & Case Temperature
Junction, Case, and Ambient Temperatures
• First, the system is off (no power is being dissipated)• The ambient, package case, and silicon die junction
temperatures are in thermal equilibriumTambient = Tcase = Tjunction
PC Board
Lead
fram
e
Sili
con
die
JunctionTemperature
CaseTemperature
AmbientTemperature
• Next, the system is turned on• The silicon die heats up due to the absorbed power being
dissipated as heatTambient = Tcase < Tjunction
PC Board
Lead
fram
e
Sili
con
die
JunctionTemperature
CaseTemperature
AmbientTemperature
Junction, Case, and Ambient Temperatures
• Some of the heat is transferred to the package (case)• The case heats up, but not as much as the silicon die
Tambient < Tcase < Tjunction
PC Board
Lead
fram
e
Sili
con
die
JunctionTemperature
CaseTemperature
AmbientTemperature
Junction, Case, and Ambient Temperatures
• From the package (case), some of the heat is transferred to the ambient air
• The air heats up, but not as much as the caseTambient,original < Tambient < Tcase < Tjunction
PC Board
Lead
fram
e
Sili
con
die
JunctionTemperature
CaseTemperature
AmbientTemperature
Junction, Case, and Ambient Temperatures
• Therefore, under almost all conditions:
Tambient,original < Tambient < Tcase < Tjunction
PC Board
Lead
fram
e
Sili
con
die
JunctionTemperature
CaseTemperature
AmbientTemperature
Junction, Case, and Ambient Temperatures
Why Is Junction Temperature Important?
• Semiconductor devices are specified by their manufacturers at a maximum temperature range:
• Above this temperature (150C in the example), the device may not work as well, or it may stop working completely
• Therefore, it is necessary to keep the junction temperature below the maximum rated operating temperature
Why Is Junction Temperature Important?
• Semiconductor devices are specified by their manufacturers at a maximum temperature range:
• Above this temperature (150C in the example), the device may not work as well, or it may stop working completely
• Therefore, it is necessary to keep the junction temperature below the maximum rated operating temperature
• What is Power?• What is Junction Temperature?• What is Thermal Resistance?• Electrical Parameters vs. Thermal Parameters• Thermal Specifications• Heatsinks• DC Thermal Calculations
Introduction to Power Dissipation and Thermal Resistance
What Is Thermal Resistance?• Thermal resistance is a measure of a materials ability to
conduct heat
• Materials that are good conductors of heat (metal) have a low thermal resistance
• Materials that are poor conductors of heat (plastics) have a high thermal resistance
• The total thermal resistance determines how well an integrated circuit can cool itself
Why Is Thermal Resistance Important?
• If the thermal resistance is LOW, heat flows easily from an integrated circuit to the ambient air
Tambient Tjunction
PC Board
Sili
con
die Junction
TemperatureAmbientTemperature
Lead
fram
e
Why Is Thermal Resistance Important?• If the thermal resistance is HIGH, heat does not flow well
from an integrated circuit to the ambient air
Tambient << Tjunction
PC Board
Lead
fram
e
Sili
con
die Junction
TemperatureAmbientTemperature
Why Is Thermal Resistance Important?
In summary, a “good” thermal resistance will:
• Lower the integrated circuit’s junction temperature
• Keep the integrated circuit functioning at a specified (guaranteed) operating temperature
• Minimize the semiconductor long term failure rate
• Minimize problems associated with the glassification of plastic epoxy packages
• What is Power?• What is Junction Temperature?• What is Thermal Resistance?• Electrical Parameters vs. Thermal Parameters• Thermal Specifications• Heatsinks• DC Thermal Calculations
Introduction to Power Dissipation and Thermal Resistance
Electrical & Thermal Parameters
Electrical Parameters
IRV
+
-
V = I R
R = Resistance ()
V = Potential Difference (V)
I = Current (A)
Thermal Parameters
+
-
Electrical Parameters Thermal Parameters
IRV
+
-
V = I R
R = Resistance ()
V = Potential Difference (V)
I = Current (A)
Rth = Thermal Resistance (C/W)
+
-
Rth
Electrical & Thermal Parameters
Electrical Parameters Thermal Parameters
IRV
+
-
V = I R
R = Resistance ()
V = Potential Difference (V)
I = Current (A)
Rth = Thermal Resistance (C/W)
T = Temperature Difference (C)
+
-
RthT
Electrical & Thermal Parameters
Electrical Parameters Thermal Parameters
IRV
+
-
V = I R
R = Resistance ()
V = Potential Difference (V)
I = Current (A)
Rth = Thermal Resistance (C/W)
T = Temperature Difference (C)
PD = Power Dissipated (W)
PD
RthT
+
-
Electrical & Thermal Parameters
Electrical Parameters Thermal Parameters
IRV
+
-
V = I R
R = Resistance ()
V = Potential Difference (V)
I = Current (A)
T = PD Rth
Rth = Thermal Resistance (K/W)
T = Temperature Difference (K)
PD = Power Dissipated (W)
PD
RthT
+
-
Electrical & Thermal Parameters
Electrical Resistance vs. Thermal Resistance
Electrical Resistance Thermal Resistance
I
R
V
+
-
Electrical Resistance vs. Thermal Resistance
Electrical Resistance Thermal Resistance
V = VoltageI = CurrentA = Area
d = Thickness = Electrical Conductivity
R = Resistance ()
I A
} d
R
AdR
V
+
-
Electrical Resistance vs. Thermal Resistance
Electrical Resistance Thermal Resistance
V = VoltageI = CurrentA = Area
d = Thickness = Electrical Conductivity
R = Resistance ()
I A
} d
R
AdR
V
+
-
Electrical Resistance vs. Thermal Resistance
Electrical Resistance Thermal Resistance
I
R
V
+
-
PD
Rth
T
+
-
V = VoltageI = CurrentA = Area
d = Thickness = Electrical Conductivity
R = Resistance ()
AdR
Electrical Resistance vs. Thermal Resistance
Electrical Resistance Thermal Resistance
I A
} d
R
V
+
-
PD A
} d
th Rth
T
+
-
T = Temperature DifferencePD = Power Dissipated
A = Aread = Thickness
th = Thermal Conductivity
V = Voltage DifferenceI = CurrentA = Area
d = Thickness = Electrical Conductivity
R = Resistance ()
AdR
Electrical Resistance vs. Thermal Resistance
Electrical Resistance Thermal Resistance
I A
} d
R
T = Temperature DifferencePD = Power Dissipated
A = Aread = Thickness
th = Thermal ConductivityRth = Thermal Resistance (C/W)
thth A
dR
V
+
-
PD A
} d
th Rth
T
+
-
V = Voltage DifferenceI = CurrentA = Area
d = Thickness = Electrical Conductivity
R = Resistance ()
AdR
Electrical Circuits Thermal Circuits
IRV
+
-PD
RthT
+
-
I = 10AR = 1
V = IR
V = (10A)(1) = 10V10V Potential Difference
Electrical Circuits vs. Thermal Circuits
Electrical Circuits Thermal Circuits
IRV
+
-PD
RthT
+
-
I = 10AR = 1
V = IR
V = (10A)(1) = 10V10V Potential Difference
PD = 10WRth = 1C/W
Electrical Circuits vs. Thermal Circuits
Electrical Circuits Thermal Circuits
IRV
+
-PD
RthT
+
-
I = 10AR = 1
V = IR
V = (10A)(1) = 10V10V Potential Difference
PD = 10WRth = 1C/W
T = PDRth
T = (10W)(1C/W) = 10C10C Temperature Difference
Electrical Circuits vs. Thermal Circuits
Electrical Circuits Thermal Circuits
IRV
+
-PD
RthT
+
-
I = 10AR = 1
V = IR
V = (10A)(1) = 10V10V Potential Difference
PD = 10WRth = 1C/W
T = PDRth
T = (10W)(1C/W) = 10C10C Temperature Difference
Electrical Circuits vs. Thermal Circuits
• What is Power?• What is Junction Temperature?• What is Thermal Resistance?• Electrical Parameters vs. Thermal Parameters• Thermal Specifications• Heatsinks• DC Thermal Calculations
Introduction to Power Dissipation and Thermal Resistance
Thermal SpecificationsDatasheet Parameters
Maximum Junction TemperatureTj,max = 150C
Thermal SpecificationsDatasheet Parameters
Thermal Resistance Junction to AmbientRthJA = 80K/W = 80C/W
Thermal SpecificationsDatasheet Parameters
Thermal Resistance Junction to AmbientRthJA = 80K/W = 80C/W
Thermal SpecificationsDatasheet Parameters
Thermal Resistance Junction to CaseRthJC = 1.1K/W = 1.1C/W
Thermal SpecificationsDatasheet Parameters
Why is RthJC << RthJA?
RthJC vs. RthJA
What is the package case?• In a integrated circuit package, the silicon die is attached to
a “lead frame” which is usually electrically grounded
• The die attach material and lead frame (often copper) are both low thermal resistance materials, and conduct heat very well
Silicon Die
Die Attach Material
Lead frame (Case)
RthJC vs. RthJA
What is the package case?• The “case” is the most thermally conductive point of the integrated
circuit package – where the lead frame is exposed:
RthJC vs. RthJA
Case Temperature Difference
Silicon Die (Junction)
Die Attach Material
Lead frame (Case)
T
• Recall: T = PDRth
PD = 1.5W
RthJC
1.1C/W
T = PDRthJC = (1.5W)(1.1C/W)
T = Tjunction – Tcase = 1.65C
• Unlike metal, air is a relatively poor conductor of heat
• Imagine a pot is being heated on the stove• If you are very close to the pot, you can tell it is hot• If you touch the pot, you get burned
• There is a large temperature difference from the pot to the air immediately next to the pot
• Therefore, there is a large thermal resistance involved in heat leaving metal and going into the air
RthJC vs. RthJA
RthCA = RthJA – RthJC
RthJC vs. RthJA
Silicon Die (Junction)
Die Attach Material
Lead frame (Case)
T
• Recall: T = PDRth PD = 1.5W
RthJC
1.1C/W
RthCA = RthJA – RthJC
RthCA = 80C/W – 1.1C/WRthCA = 78.9C/W
T = PDRthCA = (1.5W)(78.9C/W) = 118.35C
RthJC vs. RthJA
• In Summary:
TJunction-Case = 1.65C
TCase-Ambient = 118.35C
TJunction-Ambient = 1.65C + 118.35C = 120C
• In practice, a 120C temperature difference is unrealistic
• A heatsink can be used to reduce the case-to-ambient thermal resistance and the temperature difference
• What is Power?• What is Junction Temperature?• What is Thermal Resistance?• Electrical Parameters vs. Thermal Parameters• Thermal Specifications• Heatsinks• DC Thermal Calculations
Introduction to Power Dissipation and Thermal Resistance
Heatsinks• Since heat escapes from the surface of the case, increasing
the case surface area will reduce RthCA
• To a first order, this is similar to using parallel electrical resistors
Original Case AreaRthCA ~ 80C/W
2 x Case AreaRthCA ~ 40C/W
4 x Case AreaRthCA ~ 20C/W
• In General:
The larger the surface area,the lower the RthCA of a
heatsink
Heatsinks
Surface Mount Heatsinks (TO-252 DPAK)
RthJA
FR-4 PCB1 oz Copper
• What is Power?• What is Junction Temperature?• What is Thermal Resistance?• Electrical Parameters vs. Thermal Parameters• Thermal Specifications• Heatsinks• DC Thermal Calculations
Introduction to Power Dissipation and Thermal Resistance
DC Thermal CalculationMOSFET or Driver
• Conditions: Tambient = 85C, Iload = 5A
• Power DissipationPD = I2R = (5A)2(24m) = 0.6W
• Thermal Resistance (with 6cm2 Copper)RthJA = 55C/W
• Junction TemperatureTjunction = Tambient + PDRthJA
Tjunction = 85C + (0.6W)(55C/W) = 118C
DC Thermal CalculationMOSFET or Driver
• Conditions: Tambient = 85C, Iload = 5A• Conditions: Tambient = 85C, Iload = 5A
• Power DissipationPD = I2R = (5A)2(24m) = 0.6W
• Conditions: Tambient = 85C, Iload = 5A
• Power DissipationPD = I2R = (5A)2(24m) = 0.6W
• Thermal Resistance (with 6cm2 Copper)RthJA = 55C/W
DC Thermal CalculationVoltage Regulator
DC Thermal CalculationVoltage Regulator
• Conditions: Tambient = 85C, VIN = 14V, VOUT = 5V, IOUT = 100mA
• Power DissipationPD = VI = (14V – 5V)(100mA) = 0.9W
• Thermal Resistance (with 6cm2 Copper)RthJA = 55C/W
• Junction TemperatureTjunction = Tambient + PDRthJA
Tjunction = 85C + (0.9W)(55C/W) = 134.5C
• What is Power?• What is Junction Temperature?• What is Thermal Resistance?• Electrical Parameters vs. Thermal Parameters• Thermal Specifications• Heatsinks• DC Thermal Calculations
Introduction to Power Dissipation and Thermal Resistance
Introduction to Power Dissipation and Thermal Resistance
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