Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
THERMAL MANAGEMENT Electronic equipment power density is on the increase and can lead to greater failure rates hence there is a need for effective thermal management that is introduced early in the design cycle rather than as an after thought. Failure to do so will compromise the design and the possible trade-offs between the semiconductor devices and the cooling mechanism. It can sometimes be an advantage to use a higher rated semiconductor device and in return reduce the size and cost of heat exchangers. 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 for efficient cooling to prolong equipment and component life and increase reliability. 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. 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'. Heat removal Convection, conduction and radiation are the three modes of heat propagation although some may be more predominant than others dependent on the system and its mode of operation. CONDUCTION: Heat transfer by contact. 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. 1 EET307: POWER ELECTRONICS Prof R T KENNEDY Ambient temperatue(T ) is defined as the temperature surrounding A THERMAL MANAGEMENT -intro the device but not influenced by 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 Temperatue(TA) is defined as the temperature surrounding the device but not influenced by the device heat dissipation. Junction - Case Thermal Resistance There is an analogy between electrical and thermal conduction that allows the use of circuit theory in thermal analysis and it is therefore not surprising that, like the use of electrical resistance, heat removal is discussed in terms of the device material thermal resistivity th . This introduces device thermal resistance from junction to case, Rth,j-c , as given by Rth, j c th l A The dimension of thermal resistance 0C / 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. Rth, j c ELECTRICAL voltage potential difference current power conductivity resistivity V V I P T P TJ Tc PD ANALOGY T PD Q th th THERMAL temperature temperature difference power dissipated heat thermal conductivity thermal resistivity Low thermal resistance (high thermal conductivity is desirable) TJ An equivalent thermal circuit PD RthJC TC EET307: POWER ELECTRONICS THERMAL MANAGEMENT -intro 2 Prof R T KENNEDY Junction - Ambient Thermal Resistance Having conducted the heat from the junction to the case reasonably efficiently due to the comparatively low junction-case thermal resistance it must now be transferred to the ambient. The thermal resistance from case to ambient, RthCA , is the ability of the device alone to transfer heat from the case to the ambient. A typical device equivalent thermal circuit TJ PD Rth,j-c PD TC Rth,j-a Rth,ca PD Heat Exchangers To overcome the inability of the power semiconductor device to transfer heat efficiently to the ambient it must be helped. The problem to be overcome is Rth,c-a the high thermal resistance from case to ambient. Mounting the semiconductor on a heat sink reduces the effective value of Rth,c-a by providing a low parallel thermal resistance path, Rth,s-a TJ PD Rth,j-c the heat flow divides, as would current flow, with the majority taking the lower thermal resistance path via the heatsink. PD PD TC Rth,s-a Rth,c-a PD EET307: POWER ELECTRONICS THERMAL MANAGEMENT -intro 3 Prof R T KENNEDY Thermal Analysis - Steady State " A semiconductor device is only as good as its heat exchanger" Thermal - Electrical Analogy A simplified version in which all heat transfer from the device case / base / substrate is represented by a single element Rth,s-a that includes thermal washers and contact is shown The thermal performance of the device and heat exchanger can be represented by equations TJ PD DEVICE TJ TC Rth, j c PD PD TC TJ Rth, j c Rth, j c Rth,j-c PD TC Rth,j-a HEATSINK Rth,c-a TC T A PD EET307: Rth,s a PD TC Rth,s a PD TA TA Rth,s a POWER ELECTRONICS THERMAL MANAGEMENT -intro 4 Prof R T KENNEDY Power Dissipation versus TC A numerical solution is relatively straightforward, however if the equations are translated into graphs, which being straight lines are easy to deal with, then a design aid is created TJ Device: RthJC The graphical representation of the equation slope = P D 1 RthJC there are however 2 constraints that must be applied 0 w negative PD or TC > TJmax is not possible T w the graph implies that as TC becomes more negative PD can continue to increase P T Jmax C Dmax P D there is a maximum limit, PDmax ,to device power dissipation based on device TJmax and Rth,j-c . 0 T Heat Exchanger: the graphical representation of the equation C slope = P D T Jmax 1 R thSA 0 Device + Heat exchanger T -TA Superimposing permits a 'load line' approach to finding PD(op) , the permissible operational power dissipation, and TC(op) ,the operational case temperature. R T A C thSA P D P Dmax PDop T C 0 TA Power Derating Curve TCop T Jmax 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 R =0 th,s-a P Dmax 0 slope = W / C P D 0 T A EET307: POWER ELECTRONICS THERMAL MANAGEMENT -intro T C 5 T Jmax Prof R T KENNEDY