1. Bridging Theory in Practice Transferring Technical Knowledge to Practical Applications

2. Introduction to Power Dissipation and Thermal Resistance

4. 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

5. 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

6. 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

7. What is Power? Work is the result of a power applied for a given amount of time Work = Power * Time

8. 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

9. 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 = I 2 R

10. 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 = I 2 R

11. Electrical Power P = VI P = I 2 R The important things you must remember here:

12. 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

13. Junction Temperature Junction temperature is the temperature of the silicon die in an integrated circuit PC Board Silicon die Junction Temperature Lead frame

14. This is not the same as the case (or package) temperature or the ambient (or air) temperature PC Board Silicon die Junction Temperature Case Temperature Ambient Temperature Lead frame Ambient & Case Temperature

15. 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 T ambient = T case = T junction PC Board Lead frame Silicon die Junction Temperature Case Temperature Ambient Temperature

16. Next, the system is turned on The silicon die heats up due to the absorbed power being dissipated as heat T ambient = T case < T junction PC Board Lead frame Silicon die Junction Temperature Case Temperature Ambient Temperature Junction, Case, and Ambient Temperatures

17. Some of the heat is transferred to the package (case) The case heats up, but not as much as the silicon die T ambient < T case < T junction PC Board Lead frame Silicon die Junction Temperature Case Temperature Ambient Temperature Junction, Case, and Ambient Temperatures

18. 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 T ambient,original < T ambient < T case < T junction PC Board Lead frame Silicon die Junction Temperature Case Temperature Ambient Temperature Junction, Case, and Ambient Temperatures

19. Therefore, under almost all conditions: T ambient,original < T ambient < T case < T junction PC Board Lead frame Silicon die Junction Temperature Case Temperature Ambient Temperature Junction, Case, and Ambient Temperatures

20. 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

21. 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

22. 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

23. 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

24. Why Is Thermal Resistance Important? If the thermal resistance is LOW, heat flows easily from an integrated circuit to the ambient air T ambient  T junction PC Board Silicon die Junction Temperature Ambient Temperature Lead frame

25. Why Is Thermal Resistance Important? If the thermal resistance is HIGH, heat does not flow well from an integrated circuit to the ambient air T ambient << T junction PC Board Lead frame Silicon die Junction Temperature Ambient Temperature

26. 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

27. 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

28. Electrical & Thermal Parameters Electrical Parameters I RV + - V = I R R = Resistance (  ) V = Potential Difference (V) I = Current (A) Thermal Parameters + -

29. Electrical ParametersThermal Parameters I RV + - V = I R R = Resistance (  ) V = Potential Difference (V) I = Current (A) R th = Thermal Resistance (C/W) + - R th Electrical & Thermal Parameters

30. Electrical ParametersThermal Parameters I RV + - V = I R R = Resistance (  ) V = Potential Difference (V) I = Current (A) R th = Thermal Resistance (C/W)  T = Temperature Difference (C) + - R th TT Electrical & Thermal Parameters

31. Electrical ParametersThermal Parameters I RV + - V = I R R = Resistance (  ) V = Potential Difference (V) I = Current (A) R th = Thermal Resistance (C/W)  T = Temperature Difference (C) P D = Power Dissipated (W) PDPD R th TT + - Electrical & Thermal Parameters

32. Electrical ParametersThermal Parameters I RV + - V = I R R = Resistance (  ) V = Potential Difference (V) I = Current (A)  T = P D R th R th = Thermal Resistance (K/W)  T = Temperature Difference (K) P D = Power Dissipated (W) PDPD R th TT + - Electrical & Thermal Parameters

33. Electrical Resistance vs. Thermal Resistance Electrical ResistanceThermal Resistance I R V + -

34. Electrical Resistance vs. Thermal Resistance Electrical ResistanceThermal Resistance V = Voltage I = Current A = Area d = Thickness  = Electrical Conductivity R = Resistance (  ) I A } d  R V + -

35. Electrical Resistance vs. Thermal Resistance Electrical ResistanceThermal Resistance V = Voltage I = Current A = Area d = Thickness  = Electrical Conductivity R = Resistance (  ) I A } d  R V + -

36. Electrical Resistance vs. Thermal Resistance Electrical ResistanceThermal Resistance I R V + - PDPD R th TT + - V = Voltage I = Current A = Area d = Thickness  = Electrical Conductivity R = Resistance (  )

37. Electrical Resistance vs. Thermal Resistance Electrical ResistanceThermal Resistance I A } d  R V + - PDPD A th R th TT + -  T = Temperature Difference P D = Power Dissipated A = Area d = Thickness th = Thermal Conductivity V = Voltage Difference I = Current A = Area d = Thickness  = Electrical Conductivity R = Resistance (  )

38. Electrical Resistance vs. Thermal Resistance Electrical ResistanceThermal Resistance I A } d  R  T = Temperature Difference P D = Power Dissipated A = Area d = Thickness th = Thermal Conductivity R th = Thermal Resistance (C/W) V + - PDPD A } d th R th TT + - V = Voltage Difference I = Current A = Area d = Thickness  = Electrical Conductivity R = Resistance (  )

39. Electrical CircuitsThermal Circuits I R VV + - PDPD R th TT + - I = 10A R = 1  V = IR  V = (10A)(1  ) = 10V 10V Potential Difference Electrical Circuits vs. Thermal Circuits

40. Electrical CircuitsThermal Circuits I R VV + - PDPD R th TT + - I = 10A R = 1   V = IR  V = (10A)(1  ) = 10V 10V Potential Difference P D = 10W R th = 1C/W Electrical Circuits vs. Thermal Circuits

41. Electrical CircuitsThermal Circuits I R VV + - PDPD R th TT + - I = 10A R = 1   V = IR  V = (10A)(1  ) = 10V 10V Potential Difference P D = 10W R th = 1C/W  T = P D R th  T = (10W)(1C/W) = 10C 10C Temperature Difference Electrical Circuits vs. Thermal Circuits

42. Electrical CircuitsThermal Circuits I R VV + - PDPD R th TT + - I = 10A R = 1   V = IR  V = (10A)(1  ) = 10V 10V Potential Difference P D = 10W R th = 1C/W  T = P D R th  T = (10W)(1C/W) = 10C 10C Temperature Difference Electrical Circuits vs. Thermal Circuits

43. 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

44. Thermal Specifications Datasheet Parameters Maximum Junction Temperature T j,max = 150C

45. Thermal Specifications Datasheet Parameters Thermal Resistance Junction to Ambient R thJA = 80K/W = 80C/W

46. Thermal Specifications Datasheet Parameters Thermal Resistance Junction to Ambient R thJA = 80K/W = 80C/W

47. Thermal Specifications Datasheet Parameters Thermal Resistance Junction to Case R thJC = 1.1K/W = 1.1C/W

48. Thermal Specifications Datasheet Parameters Why is R thJC << R thJA ?

49. R thJC vs. R thJA 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)

50. R thJC vs. R thJA What is the package case? The “case” is the most thermally conductive point of the integrated circuit package – where the lead frame is exposed:

51. R thJC vs. R thJA Case Temperature Difference Silicon Die (Junction) Die Attach Material Lead frame (Case) TT Recall:  T = P D R th P D = 1.5W R thJC 1.1C/W  T = P D R thJC = (1.5W)(1.1C/W)  T = T junction – T case = 1.65C

52. 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 R thJC vs. R thJA

53. R thCA = R thJA – R thJC R thJC vs. R thJA Silicon Die (Junction) Die Attach Material Lead frame (Case) TT Recall:  T = P D R th P D = 1.5W R thJC 1.1C/W R thCA = R thJA – R thJC R thCA = 80C/W – 1.1C/W R thCA = 78.9C/W  T = P D R thCA = (1.5W)(78.9C/W) = 118.35C

54. R thJC vs. R thJA In Summary:  T Junction-Case = 1.65C  T Case-Ambient = 118.35C  T Junction-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

55. 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

56. Heatsinks Since heat escapes from the surface of the case, increasing the case surface area will reduce R thCA To a first order, this is similar to using parallel electrical resistors Original Case Area R thCA ~ 80C/W 2 x Case Area R thCA ~ 40C/W 4 x Case Area R thCA ~ 20C/W

57. In General: The larger the surface area, the lower the R thCA of a heatsink Heatsinks

58. Surface Mount Heatsinks (TO-252 DPAK) R thJA FR-4 PCB 1 oz Copper

59. 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

60. DC Thermal Calculation MOSFET or Driver

61. Conditions: T ambient = 85C, I load = 5A Power Dissipation P D = I 2 R = (5A) 2 (24m  ) = 0.6W Thermal Resistance (with 6cm 2 Copper) R thJA = 55C/W Junction Temperature T junction = T ambient + P D R thJA T junction = 85C + (0.6W)(55C/W) = 118C DC Thermal Calculation MOSFET or Driver Conditions: T ambient = 85C, I load = 5A Power Dissipation P D = I 2 R = (5A) 2 (24m  ) = 0.6W Conditions: T ambient = 85C, I load = 5A Power Dissipation P D = I 2 R = (5A) 2 (24m  ) = 0.6W Thermal Resistance (with 6cm 2 Copper) R thJA = 55C/W

62. DC Thermal Calculation Voltage Regulator

63. Conditions: T ambient = 85C, V IN = 14V, V OUT = 5V, I OUT = 100mA Power Dissipation P D = VI = (14V – 5V)(100mA) = 0.9W Thermal Resistance (with 6cm 2 Copper) R thJA = 55C/W Junction Temperature T junction = T ambient + P D R thJA T junction = 85C + (0.9W)(55C/W) = 134.5C

64. 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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