POWER ELECTRONIC 1 EET 307/4

1. POWER ELECTRONIC 1 EET 307/4 THERMAL MANAGEMENT

2. Introduction • In the early days when the cost of power semiconductors was high the cost of heat removal was perhaps not so important, however as device costs have reduced and package sizes have become smaller a global approach to thermal management is now appropriate. • Surface area is sufficient for the device to release heat to the ambient.

3. Efficiency • Power processing being the objective of power electronics it is not surprising that power is one of the most important electrical quantities.

4. Efficiency • Power losses occur in resistive elements (intended components and parasitic) and in the power semiconductors where • Power Loss eHeateTemperature rise

5. Thermal Characterization - Power Semiconductors • Individual power semiconductors although capable of controlling large amounts of power have low 'heat' capability. • Devices are very thin and consist of a number of layers hence it is not possible to differentiate between the regions. • It is generally assumed that all the losses in the device are converted into heat which is dissipated at junctions producing a uniform laterally distributed temperature across the junction area resulting in the importance of Junction Temperature as a critical rating. • Device current carrying capability is limited by the permissible maximum junction temperature and the current density through the active silicon wafer. • Junction temperature affects device current and voltage capability. Device parameters deteriorate if the maximum junction temperature is exceeded and can contribute to failure mechanisms. Manufacturers provide upper and lower operating and storage temperatures.

6. Thermal Characterization - Power Semiconductors • The upper operational temperature limit is to contain excessive temperature rise due to current. • The upper storage temperature may be greater than the operating temperature and is based on 'no electrical connection' and is limited by the reliability and stability of device characteristics. • Although it is the device junction temperature that is the limiting value, device characteristics are often related to a measurable reference that is usually the case or mounting base. • The lower temperature limits are set at levels to avoid fracture of the semiconductor material due to differences in thermal expansion and contraction of the semiconductor material and the various other materials that connect the semiconductor die to the external 'world'.

7. Heat Propagation HEAT PROPAGATION CONVECTION CONDUCTION RADIATION

8. Heat Propagation 1- CONVECTION: Heat removal by transferring energy through a fluid that could be mix of hot and cold air and by natural or forced convection. h, the convection efficiency, is complex and includes the effects of fluid viscosity, volume , thermal resistivity & geometry.

9. Heat Removal 2- CONDUCTION: Heat transfer by contact . Liquid cooling techniques are mainly by conduction

10. Heat Removal 3- RADIATION: Flow of heat in the infra red section of the e.m. spectrum. At extreme altitude radiation is predominant. emissivity : s, Stefan Boltzmann constant

11. Heat Removal • Thermal energy at the junction as a result of power dissipation requires the device to have a capability of transferring this heat to the outside 'world' referred to as the Ambient. • Ambient is defined as the mass surrounding a material or device. • Ambient temperature (TA) is defined as the temperature surrounding the device but not influenced by the device heat dissipation.

12. Fourier's Heat transfer Law(modified for thermal equilibrium ) (conduction)

13. Heat Removal • Thermal ability of the device requires heat dissipation at a rate > the heat generated. • Combining previous yields equation D1.3 that describes the ability of a device to effect the transfer of thermal energy by conduction in terms of the device material thermal conductivity , high conductivity being desirable. (D1.3) is the device thermal conductance.

14. Thermal-Electrical Analogy Junction - Case Thermal Resistance • thermal resistance from junction to case (Rth,j-c) • The dimension of thermal resistance °C / W (some data sheets use K / W) Thermal resistance, or effective thermal resistance, can be defined as the temperature rise of a designated junction above the reference point per unit power dissipation, under conditions of thermal equilibrium.

15. Thermal-Electrical Analogy

16. Tjunction Rth,j-c Tcase Thermal-Electrical Analogy An equivalent thermal circuit Low thermal resistance (high thermal conductivity is desirable)

17. TJ PD Rth,j-c PD TC Rth,j-a Rth,c-a PD Thermal-Electrical Analogy Junction - Ambient thermal resistance • The thermal resistance from case to ambient, RthCA , is the ability of the device alone to transfer heat from the case to the ambient.

18. Tjunction PD Rth,j-c Tsink Tcase Rth,s-a Rth,c-a PD Tambient Thermal-Electrical Analogy Heat Exchangers Mounting the semiconductor on a heat sink reduces the effective value of RthCA by providing a low parallel thermal resistance path, RthS the heat flow divides, as would current flow, with the majority taking the lower thermal resistance path via the heatsink <<

19. Thermal-Electrical Analogy Heat Exchangers

20. Thermal-Electrical Analogy Thermal Analysis - Steady State " A semiconductor device is only as good as its heat exchanger"

21. TJ PD Rth,j-c PD TC Rth,j-a Rth,c-a PD TA Thermal-Electrical Analogy • The thermal performance of the device and heat exchanger can be represented by equations DEVICE HEATSINK

22. Power Dissipation versus TC Device: • The graphical representation of the equation • there are however 2 constraints that must be applied • negative PD or TC > TJmax is not possible • the graph implies that as • TC becomes more negative • PD can continue to increase • there is a maximum limit, PDmax, to device power dissipation based on device TJmax and Rth,j-c .

23. Power Dissipation versus TC HeatExchanger:

24. PDop 0 TCop TA Power Dissipation versus TC Device + HeatExchanger: finding PD(op) , the permissible operational power dissipation, and TC(op) ,the operational case temperature. Superimposing permits a 'load line' approach to

25. R = 0 th,s-a P o Dmax slope = W/ C P D 0 T C T T A Jmax Power Derating Curve • Device power capability reduces as the case temperature increases at a rate dependent on the device thermal resistance & maximum junction temperature. The slope of the curve is the power derating in W/ oC

26. L= large BY229 R =10 W E=100 V NTD12N10 Tamb= 50oC EXAMPLE – DC-DC Converter Thermal management of the NTD 12N10 mosfet and the BY229 rectifier in the dc-dc converter is provided by a ‘common’ heat sink of thermal resistance 3 oC / W.

27. Tjn,rectifier L= large Tjn,mosfet BY229 Rth,j-c(mosfet) R =10 W Rth,j-c(rectifier) E=100 V NTD12N10 Tamb= 50 oC Tsink Rth,s-a(heatsink) Tambient

28. RECTIFIER

29. BY229 POWER LOSS 3.75 W 2.4 A

30. NTD12N10 MOSFET ON-STATE RESISTANCE

31. NTD12N10 MOSFET ON-STATE POWER LOSS /DISSIPATION curve calculate calculate

32. Pmosfet Prectifier TEMPERATURE MOSFET&RECTIFIER POWER LOSS CURVES

33. SELECT Tjn,mosfet as ref Ptot= 8.75 W Pmosfet Pmos= 5 W Prect= 3.75 W Prectifier Tjn,mos= 100oC Tjn,rectt,= 94.25oC Tamb,= 50oC Tsink,mosfet= Tsink,rectifier Tsink,= 86.75oC

34. REPEAT THE EXERCISE STARTING WITH Tjn rect Prev2

35. TJ PD Rth,j-c PD TC Rth,j-a Rth,s-a PD TA Thermal Representation – AC-DC HW Power Switch Rth,c-s Heat Sink Ts PD

36. iin(t) iT1 T1 T4 Vout iT4 Iout R T2 T3 iT3 iT2 Thermal Representation – AC-DC FW

37. Thermal Representation – AC-AC

38. Tjn,rectifier L= large Tjn,mosfet BY229 Rth,j-c(mosfet) R =10 W Rth,j-c(rectifier) E=100 V NTD12N10 Tamb= 50 oC Tsink Rth,s-a(heatsink) Tambient Thermal Representation – DC-DC