Bridging Theory in Practice

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

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

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

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 Lead frame 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 Lead frame 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

+ 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 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

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 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 Lead frame (Case)

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:

Bridging Theory in Practice