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:
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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
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
120W
Supplied
120W
Consumed
P = V * I = (IR)*I = I2R
Introduction to Power Dissipation and Thermal Resistance
Lead frame
Silicon die
Junction
Temperature
PC Board
Ambient
Temperature
Case
Temperature
Lead frame
Silicon die
Junction
Temperature
PC Board
Junction, Case, and Ambient Temperatures
Tambient = Tcase = Tjunction
Ambient
Temperature
Case
Temperature
Silicon die
Lead frame
Junction
Temperature
PC Board
Junction, Case, and Ambient Temperatures
Tambient = Tcase< Tjunction
Ambient
Temperature
Case
Temperature
Lead frame
Silicon die
Junction
Temperature
PC Board
Junction, Case, and Ambient Temperatures
Tambient < Tcase < Tjunction
Ambient
Temperature
Case
Temperature
Lead frame
Silicon die
Junction
Temperature
PC Board
Junction, Case, and Ambient Temperatures
Tambient,original < Tambient < Tcase< Tjunction
Ambient
Temperature
Case
Temperature
Lead frame
Silicon die
Junction
Temperature
PC Board
Junction, Case, and Ambient Temperatures
Tambient,original < Tambient < Tcase < Tjunction
Ambient
Temperature
Case
Temperature
Lead frame
Silicon die
Junction
Temperature
PC Board
Why Is Junction Temperature Important?
Why Is Junction Temperature Important?
Introduction to Power Dissipation and Thermal Resistance
Why Is Thermal Resistance Important?
TambientTjunction
Junction
Temperature
Ambient
Temperature
Lead frame
Silicon die
PC Board
Why Is Thermal Resistance Important?
Tambient << Tjunction
Junction
Temperature
Ambient
Temperature
Lead frame
Silicon die
PC Board
Why Is Thermal Resistance Important?
In summary, a “good” thermal resistance will:
Introduction to Power Dissipation and Thermal Resistance
V
R
I

Electrical & Thermal Parameters
Electrical Parameters
Thermal Parameters
+

V = I R
R = Resistance ()
V = Potential Difference (V)
I = Current (A)
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)
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)
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
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. Thermal 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. Thermal 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. Thermal 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. Thermal 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. Thermal 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. Thermal 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. Thermal 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. Thermal 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. Thermal 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
Thermal SpecificationsDatasheet Parameters
Thermal Resistance Junction to Ambient
RthJA = 80K/W = 80C/W
Thermal SpecificationsDatasheet Parameters
Thermal Resistance Junction to Ambient
RthJA = 80K/W = 80C/W
Thermal SpecificationsDatasheet Parameters
Thermal Resistance Junction to Case
RthJC = 1.1K/W = 1.1C/W
Thermal SpecificationsDatasheet Parameters
Why is RthJC << RthJA?
RthJC vs. RthJAWhat is the package case?
Silicon Die
Die Attach Material
Lead frame (Case)
RthJC vs. RthJAWhat is the package case?
RthJC 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
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
TJunctionCase = 1.65C
TCaseAmbient = 118.35C
TJunctionAmbient = 1.65C + 118.35C = 120C
Introduction to Power Dissipation and Thermal Resistance
Original Case Area
RthCA ~ 80C/W
2 x Case Area
RthCA ~ 40C/W
4 x Case Area
RthCA ~ 20C/W
Introduction to Power Dissipation and Thermal Resistance
DC Thermal CalculationMOSFET 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 CalculationVoltage 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
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