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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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Bridging Theory in Practice

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## Bridging Theory in Practice

Transferring Technical Knowledge

to 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

### 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?

120W

Supplied

120W

Consumed

P = V * I = (IR)*I = I2R

What is Power?

• 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:

120W

Supplied

120W

Consumed

P = V * I = (IR)*I = I2R

### Electrical Power

The important things you must remember here:

P = VI

P = I2R

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

Junction Temperature

• Junction temperature is the temperature of the silicon die in an integrated circuit

Silicon die

Junction

Temperature

PC Board

Ambient & Case Temperature

• This is not the same as the case (or package) temperature or the ambient (or air) temperature

Ambient

Temperature

Case

Temperature

Silicon die

Junction

Temperature

PC Board

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 equilibrium

Tambient = Tcase = Tjunction

Ambient

Temperature

Case

Temperature

Silicon die

Junction

Temperature

PC Board

Junction, Case, and Ambient Temperatures

• Next, the system is turned on

• The silicon die heats up due to the absorbed power being dissipated as heat

Tambient = Tcase< Tjunction

Ambient

Temperature

Case

Temperature

Silicon die

Junction

Temperature

PC Board

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

Ambient

Temperature

Case

Temperature

Silicon die

Junction

Temperature

PC Board

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 case

Tambient,original < Tambient < Tcase< Tjunction

Ambient

Temperature

Case

Temperature

Silicon die

Junction

Temperature

PC Board

Junction, Case, and Ambient Temperatures

• Therefore, under almost all conditions:

Tambient,original < Tambient < Tcase < Tjunction

Ambient

Temperature

Case

Temperature

Silicon die

Junction

Temperature

PC Board

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

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

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

Junction

Temperature

Ambient

Temperature

Silicon die

PC Board

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

Junction

Temperature

Ambient

Temperature

Silicon die

PC Board

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

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

+

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

+

V

-

R

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

• What is Power?

• What is Junction Temperature?

• What is Thermal Resistance?

• Electrical Parameters vs. Thermal Parameters

• Thermal Specifications

• Heatsinks

• DC Thermal Calculations

Thermal SpecificationsDatasheet Parameters

Maximum Junction Temperature

Tj,max = 150C

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?

• 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

RthJC vs. RthJAWhat 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. RthJACase Temperature Difference

• Recall: T = PDRth

PD = 1.5W

Silicon Die (Junction)

RthJC

1.1C/W

T

Die Attach Material

T = PDRthJC = (1.5W)(1.1C/W)

T = Tjunction – Tcase = 1.65C

RthJC vs. RthJA

• 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

• Recall: T = PDRth

PD = 1.5W

Silicon Die (Junction)

RthJC

1.1C/W

T

Die Attach Material

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

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

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

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 Area

RthCA ~ 80C/W

2 x Case Area

RthCA ~ 40C/W

4 x Case Area

RthCA ~ 20C/W

In General:

Heatsinks

The larger the surface area,

the lower the RthCA of a

heatsink

### Surface Mount Heatsinks (TO-252 DPAK)

FR-4 PCB

1 oz Copper

RthJA

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

DC Thermal CalculationMOSFET or Driver

DC Thermal CalculationMOSFET or Driver

• Conditions: Tambient = 85C, Iload = 5A

• Power Dissipation

PD = I2R = (5A)2(24m) = 0.6W

• Thermal Resistance (with 6cm2 Copper)

RthJA = 55C/W

• Junction Temperature

Tjunction = Tambient + PDRthJA

Tjunction = 85C + (0.6W)(55C/W) = 118C

• Conditions: Tambient = 85C, Iload = 5A

• Conditions: Tambient = 85C, Iload = 5A

• Power Dissipation

PD = I2R = (5A)2(24m) = 0.6W

• Conditions: Tambient = 85C, Iload = 5A

• Power Dissipation

PD = 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 Dissipation

PD = VI = (14V – 5V)(100mA) = 0.9W

• Thermal Resistance (with 6cm2 Copper)

RthJA = 55C/W

• Junction Temperature

Tjunction = Tambient + PDRthJA

Tjunction = 85C + (0.9W)(55C/W) = 134.5C

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

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