Intro Power Diss Thermal Res

Bridging Theory in PracticeTransferring Technical Knowledgeto Practical Applications

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

• 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?

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:

Junction Temperature

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

PC Board

JunctionTemperature

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

PC Board

JunctionTemperature

CaseTemperature

AmbientTemperature

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

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

JunctionTemperature

CaseTemperature

AmbientTemperature

• 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

JunctionTemperature

CaseTemperature

AmbientTemperature

• 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

JunctionTemperature

CaseTemperature

AmbientTemperature

• Therefore, under almost all conditions:

Tambient,original < Tambient < Tcase < Tjunction

PC Board

JunctionTemperature

CaseTemperature

AmbientTemperature

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 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

die Junction

TemperatureAmbientTemperature

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

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

Electrical & Thermal Parameters

Electrical Parameters

V = I R

R = Resistance ()

V = Potential Difference (V)

I = Current (A)

Thermal Parameters

Electrical Parameters Thermal Parameters

V = I R

R = Resistance ()

I = Current (A)

Rth = Thermal Resistance (C/W)

V = I R

R = Resistance ()

I = Current (A)

T = Temperature Difference (C)

V = I R

R = Resistance ()

I = Current (A)

T = Temperature Difference (C)

PD = Power Dissipated (W)

V = I R

R = Resistance ()

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

V = VoltageI = CurrentA = Area

d = Thickness = Electrical Conductivity

R = Resistance ()

th Rth

T = Temperature DifferencePD = Power Dissipated

A = Aread = Thickness

th = Thermal Conductivity

V = Voltage DifferenceI = CurrentA = Area

R = Resistance ()

T = Temperature DifferencePD = Power Dissipated

A = Aread = Thickness

th = Thermal ConductivityRth = Thermal Resistance (C/W)

thth A

th Rth

V = Voltage DifferenceI = CurrentA = Area

R = Resistance ()

Electrical Circuits Thermal Circuits

I = 10AR = 1

V = IR

V = (10A)(1) = 10V10V Potential Difference

Electrical Circuits vs. Thermal Circuits

I = 10AR = 1

V = IR

PD = 10WRth = 1C/W

I = 10AR = 1

V = IR

PD = 10WRth = 1C/W

T = PDRth

T = (10W)(1C/W) = 10C10C Temperature Difference

I = 10AR = 1

V = IR

PD = 10WRth = 1C/W

T = PDRth

T = (10W)(1C/W) = 10C10C Temperature Difference

Thermal SpecificationsDatasheet Parameters

Maximum Junction TemperatureTj,max = 150C

Thermal Resistance Junction to AmbientRthJA = 80K/W = 80C/W

Thermal Resistance Junction to CaseRthJC = 1.1K/W = 1.1C/W

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)

• Recall: T = PDRth

PD = 1.5W

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)

• Recall: T = PDRth PD = 1.5W

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

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)

FR-4 PCB1 oz Copper

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

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

DC Thermal CalculationVoltage Regulator

• Conditions: Tambient = 85C, VIN = 14V, VOUT = 5V, IOUT = 100mA

• Power DissipationPD = VI = (14V – 5V)(100mA) = 0.9W

• Junction TemperatureTjunction = Tambient + PDRthJA

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

Thank You!www.btipnow.com

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