intro power diss thermal res
TRANSCRIPT
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Bridging Theory in PracticeTransferring Technical Knowledgeto Practical Applications
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Introduction to Power Dissipation and Thermal Resistance
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Introduction to Power Dissipation and Thermal Resistance
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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
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• 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
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• 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
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What is Power?
Work is the result of a power applied for a given amount of time
Work = Power * Time
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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
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• 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
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• 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
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Electrical Power
P = VI
P = I2R
• The important things you must remember here:
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• 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
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Junction Temperature
• Junction temperature is the temperature of the silicon die in an integrated circuit
PC Board
Sili
con
die
JunctionTemperature
Lead
fram
e
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• 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
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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
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• 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
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• 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
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• 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
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• 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
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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
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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
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• 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
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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
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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
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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
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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
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• 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
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Electrical & Thermal Parameters
Electrical Parameters
IRV
+
-
V = I R
R = Resistance ()
V = Potential Difference (V)
I = Current (A)
Thermal Parameters
+
-
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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
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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
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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
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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
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Electrical Resistance vs. Thermal Resistance
Electrical Resistance Thermal Resistance
I
R
V
+
-
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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
+
-
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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
+
-
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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
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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
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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
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Electrical Circuits Thermal Circuits
IRV
+
-PD
RthT
+
-
I = 10AR = 1
V = IR
V = (10A)(1) = 10V10V Potential Difference
Electrical Circuits vs. Thermal Circuits
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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
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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
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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
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• 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
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Thermal SpecificationsDatasheet Parameters
Maximum Junction TemperatureTj,max = 150C
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Thermal SpecificationsDatasheet Parameters
Thermal Resistance Junction to AmbientRthJA = 80K/W = 80C/W
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Thermal SpecificationsDatasheet Parameters
Thermal Resistance Junction to AmbientRthJA = 80K/W = 80C/W
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Thermal SpecificationsDatasheet Parameters
Thermal Resistance Junction to CaseRthJC = 1.1K/W = 1.1C/W
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Thermal SpecificationsDatasheet Parameters
Why is RthJC << RthJA?
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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)
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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:
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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
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• 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
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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
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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
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• 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
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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
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• In General:
The larger the surface area,the lower the RthCA of a
heatsink
Heatsinks
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Surface Mount Heatsinks (TO-252 DPAK)
RthJA
FR-4 PCB1 oz Copper
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• 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
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DC Thermal CalculationMOSFET or Driver
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• 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
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DC Thermal CalculationVoltage Regulator
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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
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• 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
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Introduction to Power Dissipation and Thermal Resistance
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