Bridging Theory in Practice. Transferring Technical Knowledge to Practical Applications. Introduction to Power Dissipation and Thermal Resistance. Introduction to Power Dissipation and Thermal Resistance. Introduction to Power Dissipation and Thermal Resistance. Intended Audience:
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Introduction to Power Dissipation and Thermal Resistance
Intended Audience:
Topics Covered:
Expected Time:
Introduction to Power Dissipation and Thermal Resistance
Introduction to Power Dissipation and Thermal Resistance
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?120W
Supplied
120W
Consumed
P = V * I = (IR)*I = I2R
What is Power? must consume 120W of power
120W
Supplied
120W
Consumed
P = V * I = (IR)*I = I2R
The important things you must remember here:
P = VI
P = I2R
Introduction to Power Dissipation and Thermal Resistance must consume 120W of power
Junction Temperature must consume 120W of power
Lead frame
Silicon die
Junction
Temperature
PC Board
Ambient & Case Temperature must consume 120W of power
Ambient
Temperature
Case
Temperature
Lead frame
Silicon die
Junction
Temperature
PC Board
Junction, Case, and Ambient Temperatures must consume 120W of power
Tambient = Tcase = Tjunction
Ambient
Temperature
Case
Temperature
Silicon die
Lead frame
Junction
Temperature
PC Board
Junction, Case, and Ambient Temperatures must consume 120W of power
Tambient = Tcase< Tjunction
Ambient
Temperature
Case
Temperature
Lead frame
Silicon die
Junction
Temperature
PC Board
Junction, Case, and Ambient Temperatures must consume 120W of power
Tambient < Tcase < Tjunction
Ambient
Temperature
Case
Temperature
Lead frame
Silicon die
Junction
Temperature
PC Board
Junction, Case, and Ambient Temperatures must consume 120W of power
Tambient,original < Tambient < Tcase< Tjunction
Ambient
Temperature
Case
Temperature
Lead frame
Silicon die
Junction
Temperature
PC Board
Junction, Case, and Ambient Temperatures must consume 120W of power
Tambient,original < Tambient < Tcase < Tjunction
Ambient
Temperature
Case
Temperature
Lead frame
Silicon die
Junction
Temperature
PC Board
Why Is Junction Temperature Important? must consume 120W of power
Why Is Junction Temperature Important? must consume 120W of power
Introduction to Power Dissipation and Thermal Resistance must consume 120W of power
What Is Thermal Resistance? must consume 120W of power
Why Is Thermal Resistance Important? must consume 120W of power
TambientTjunction
Junction
Temperature
Ambient
Temperature
Lead frame
Silicon die
PC Board
Why Is Thermal Resistance Important? must consume 120W of power
Tambient << Tjunction
Junction
Temperature
Ambient
Temperature
Lead frame
Silicon die
PC Board
Why Is Thermal Resistance Important? must consume 120W of power
In summary, a “good” thermal resistance will:
Introduction to Power Dissipation and Thermal Resistance must consume 120W of power
+ must consume 120W of power
V
R
I

Electrical & Thermal Parameters
Electrical Parameters
Thermal Parameters
+

V = I R
R = Resistance ()
V = Potential Difference (V)
I = Current (A)
+ must consume 120W of power
V
R
I

Electrical & Thermal Parameters
Electrical Parameters
Thermal Parameters
+
Rth

V = I R
R = Resistance ()
V = Potential Difference (V)
I = Current (A)
Rth = Thermal Resistance (C/W)
+ must consume 120W of power
V
R
I

Electrical & Thermal Parameters
Electrical Parameters
Thermal Parameters
+
T
Rth

V = I R
R = Resistance ()
V = Potential Difference (V)
I = Current (A)
Rth = Thermal Resistance (C/W)
T = Temperature Difference (C)
+ must consume 120W of power
V
R
I

Electrical & Thermal Parameters
Electrical Parameters
Thermal Parameters
+
T
Rth
PD

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)
Electrical & Thermal Parameters must consume 120W of power
Electrical Parameters
Thermal Parameters
+
+
T
V
R
Rth
I
PD


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)
Electrical Resistance vs. must consume 120W of powerThermal Resistance
Electrical Resistance
Thermal Resistance
I
+
V

R
Electrical Resistance vs. must consume 120W of powerThermal Resistance
Electrical Resistance
Thermal Resistance
I
A
+
} d
V

R
V = Voltage
I = Current
A = Area
d = Thickness
= Electrical Conductivity
R = Resistance ()
Electrical Resistance vs. must consume 120W of powerThermal Resistance
Electrical Resistance
Thermal Resistance
I
A
+
} d
V

R
V = Voltage
I = Current
A = Area
d = Thickness
= Electrical Conductivity
R = Resistance ()
Electrical Resistance vs. must consume 120W of powerThermal Resistance
Electrical Resistance
Thermal Resistance
PD
I
+
+
T
V


R
Rth
V = Voltage
I = Current
A = Area
d = Thickness
= Electrical Conductivity
R = Resistance ()
Electrical Resistance vs. must consume 120W of powerThermal Resistance
Electrical Resistance
Thermal Resistance
PD
I
A
A
+
+
} d
} d
T
V


R
Rth
th
V = Voltage Difference
I = Current
A = Area
d = Thickness
= Electrical Conductivity
R = Resistance ()
T = Temperature Difference
PD = Power Dissipated
A = Area
d = Thickness
th = Thermal Conductivity
Electrical Resistance vs. must consume 120W of powerThermal Resistance
Electrical Resistance
Thermal Resistance
PD
I
A
A
+
+
} d
} d
T
V


R
Rth
th
V = Voltage Difference
I = Current
A = Area
d = Thickness
= Electrical Conductivity
R = Resistance ()
T = Temperature Difference
PD = Power Dissipated
A = Area
d = Thickness
th = Thermal Conductivity
Rth = Thermal Resistance (C/W)
Electrical Circuits vs. must consume 120W of powerThermal Circuits
Electrical Circuits
Thermal Circuits
+
+
V
T
R
Rth
I
PD


I = 10A
R = 1
V = IR
V = (10A)(1) = 10V
10V Potential Difference
Electrical Circuits vs. must consume 120W of powerThermal Circuits
Electrical Circuits
Thermal Circuits
+
+
V
T
R
Rth
I
PD


I = 10A
R = 1
V = IR
V = (10A)(1) = 10V
10V Potential Difference
PD = 10W
Rth = 1C/W
Electrical Circuits vs. must consume 120W of powerThermal Circuits
Electrical Circuits
Thermal Circuits
+
+
V
T
R
Rth
I
PD


I = 10A
R = 1
V = IR
V = (10A)(1) = 10V
10V Potential Difference
PD = 10W
Rth = 1C/W
T = PDRth
T = (10W)(1C/W) = 10C
10C Temperature Difference
Electrical Circuits vs. must consume 120W of powerThermal Circuits
Electrical Circuits
Thermal Circuits
+
+
V
T
R
Rth
I
PD


I = 10A
R = 1
V = IR
V = (10A)(1) = 10V
10V Potential Difference
PD = 10W
Rth = 1C/W
T = PDRth
T = (10W)(1C/W) = 10C
10C Temperature Difference
Introduction to Power Dissipation and Thermal Resistance must consume 120W of power
Thermal Specifications must consume 120W of powerDatasheet Parameters
Maximum Junction Temperature
Tj,max = 150C
Thermal Specifications must consume 120W of powerDatasheet Parameters
Thermal Resistance Junction to Ambient
RthJA = 80K/W = 80C/W
Thermal Specifications must consume 120W of powerDatasheet Parameters
Thermal Resistance Junction to Ambient
RthJA = 80K/W = 80C/W
Thermal Specifications must consume 120W of powerDatasheet Parameters
Thermal Resistance Junction to Case
RthJC = 1.1K/W = 1.1C/W
Thermal Specifications must consume 120W of powerDatasheet Parameters
Why is RthJC << RthJA?
R must consume 120W of powerthJC vs. RthJAWhat is the package case?
Silicon Die
Die Attach Material
Lead frame (Case)
R must consume 120W of powerthJC vs. RthJAWhat is the package case?
R must consume 120W of powerthJC vs. RthJACase Temperature Difference
PD = 1.5W
Silicon Die (Junction)
RthJC
1.1C/W
T
Die Attach Material
Lead frame (Case)
T = PDRthJC = (1.5W)(1.1C/W)
T = Tjunction – Tcase = 1.65C
R must consume 120W of powerthJC vs. RthJA
R must consume 120W of powerthJC vs. RthJA
PD = 1.5W
Silicon Die (Junction)
RthJC
1.1C/W
T
Die Attach Material
Lead frame (Case)
RthCA = RthJA – RthJC
RthCA = RthJA – RthJC
RthCA = 80C/W – 1.1C/W
RthCA = 78.9C/W
T = PDRthCA = (1.5W)(78.9C/W) = 118.35C
R must consume 120W of powerthJC vs. RthJA
TJunctionCase = 1.65C
TCaseAmbient = 118.35C
TJunctionAmbient = 1.65C + 118.35C = 120C
Introduction to Power Dissipation and Thermal Resistance must consume 120W of power
Heatsinks must consume 120W of power
Original Case Area
RthCA ~ 80C/W
2 x Case Area
RthCA ~ 40C/W
4 x Case Area
RthCA ~ 20C/W
In General: must consume 120W of power
Heatsinks
The larger the surface area,
the lower the RthCA of a
heatsink
Introduction to Power Dissipation and Thermal Resistance must consume 120W of power
DC Thermal Calculation must consume 120W of powerMOSFET or Driver
DC Thermal Calculation must consume 120W of powerMOSFET or Driver
PD = I2R = (5A)2(24m) = 0.6W
RthJA = 55C/W
Tjunction = Tambient + PDRthJA
Tjunction = 85C + (0.6W)(55C/W) = 118C
PD = I2R = (5A)2(24m) = 0.6W
PD = I2R = (5A)2(24m) = 0.6W
RthJA = 55C/W
DC Thermal Calculation must consume 120W of powerVoltage Regulator
DC Thermal Calculation must consume 120W of powerVoltage Regulator
PD = VI = (14V – 5V)(100mA) = 0.9W
RthJA = 55C/W
Tjunction = Tambient + PDRthJA
Tjunction = 85C + (0.9W)(55C/W) = 134.5C
Introduction to Power Dissipation and Thermal Resistance must consume 120W of power
Introduction to Power Dissipation and Thermal Resistance must consume 120W of power
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