如何计算散热器的散热功率

如何计算散热器的散热功率Calculati on CornerEstimati ng Parallel Plate-Fi n Heat Si nk Thermal Resista neeRobert E. Simons , Associate Editor, IBM CorporationAs no ted previously in this colu mn, the trend of in creas ing electro nic module power is maki ng it more and more difficult to cool electro nic packages with air. As a result there are an in creas ing nu mber of applicati ons that require the use of forced con vect ion air-cooled heat sinks to con trol module temperature. An example of a widely used type of heat si nk is the parallel plate con figurati on show n in Figure 1.—tfFigure 1. Parallel plate fin heat sink configuration.In order to select the appropriate heat sink, the thermal designer must first determ ine the maximumallowable heat sink thermal resista nee. To do this it is necessary to know the maximum allowable module casetemperature, T case, the module power dissipation, P resista nee at the module-to-heat si nk in terface, R allowable temperature at the heat sink attachment by moq and the thermal int. The maximumsurface, T base, isgivenwhere T air-in , is the temperature of the cooli ng air at the in let to the heat sinkpassages. At this point many thermal engineers will start looking at heat sink ven dor catalogs (or more likely today start searchi ng ven dors on the in ternet) to find a heat sink that will fit in the allowable space and provide a heat sink thermal resista nee, R hs , less tha n R max at some specified flow rate. In some cases, it may be useful to do a sizing to estimate R hs for various plate-fi n heat sink desig ns to determ ine if a feasible desig n con figurati on is possible. The rema in der of this article will provide the basic equations to do this. The thermal resistance of the heat sink is give n bywhere h is the conv ective heat tran sfer coefficie nt, A base is the exposed base surface area between fins, N fin is the number of fins, 丨仙 is the fin efficie ncy, and A fin is the surface area per fin tak ing into acco unt both sides of the fin.To proceed further it is n ecessary to establish the maximum allowable heat sink volume in terms of width, W, height, H, and len gth in the flow direct ion, L. It is also n ecessary to specify a fin thick ness, t fin . Using these parameters the gap, b, between the fins maybe determined fromThe exposed base surface area may the n be determ ined fromr ba ^nmd R m tThe maximum allowable heat si nk resista nee, R ma 马is the n give n by h 门' bdse + tin r lfin 八 finand the heat tran sfer area per fin fromAt this point it is necessary to specify the air flow rate either in terms of the average velocity, V, between the fins or a volumetric flow rate,G. If a volumetric flow rate is used, the corresp onding air velocity betwee n the fins isTo determine the heat transfer coefficient acting upon the fins, anequation developed by Teertstra et al. [1] relating Nusselt number, Nu, to Reyno Ids nu mber, Re, and Pr nu mber, Pr, maybe employed. This equatio n isThe Pran dtl nu mber iswhere - is the dynamic viscosity of air, c P the specific heat of air atcon sta nt pressure, and k is the thermal con ductivity of air. The Reyno Ids nu mber used in (8) is a modified cha nnel Reyno Ids nu mber defi ned asRc ・ P V「b上(HOp Lwhere 1 is the density of air. Equation (8) is based upon a composite model spanning the developing to fully developed laminar flow regimes and wasFor purposes of illustrati on these equati ons were used to estimate heat sinkthermal resistance for a 50 x 50 mm aluminum heat sink. The effect of increasing the fin height and the number of fins is shown in Figure 2 for a con sta nt air velocity and in Figure 3 for a con sta nt volumetric flow rate. In both cases it maybe seen that there are limits to how much heat sink thermal resistance may be reduced by either increasing fin len gth or addi ng more fins. Of course to determ ine how a heat sinkwill actually perform in a specific application it is necessary to determine validated by the authors [1] by comparing with numerical simulations over a broad range of the modified cha nnel Reyno Ids nu mber (0.26 < Re < 175) and with someexperimental data as well. Using the Nusselt number obtained in (8) the heat transfer coefficient is given byNote: Kfin should be K. 20051017where k 仙 is the thermal conductivity of the heat sink material. Theefficie ncy of the fins may be calculated using"tin ianh(m + Hf)where m is give n by iniUsing these equations it is possible to estimate heat sink thermal performa nee in terms of the thermal resista nee from the temperature at the base of the fins to the temperature of the air entering the fin passages. It maybe no ted that the relati on ship for Nusselt nu mber (8) in cludes the effect of the temperature rise in the air as it flows through the fin passages. To obta in the total thermal resista nee, R °t , to the base of the heat si nk it is n ecessary to add in the thermal con duct ion resista nce across the base of the heat sink. For uniform heat flow into the base R tot is give n by(14)the air velocity or volumetric flow rate that can be delivered throughthe heat sink. To do this it is necessary to estimate the heat sink pressure drop characteristics and match them to the fan or blower to be used. This is a topic for con sideratio n in a future article.20IttlSFigure 3. Effect of fin height and number of fins on heat sink thermal3 resista nee at a volumetric air flow rate of 0.0024 m /s (5 CFM).References 1. Teertstra, P., Yovanovich, M.M., and Culham, J.R., "AnalyticalForced Convection Modeling of Plate Fin Heat Sinks," Proceedings of 15th IEEE :n Hi IU 2 0 Number of fins o Figure 2. Effect of fin height and number of fins on heat sink thermalresista nee at an air velocity of 2.5 m/s (492 fpm).tin tnnii t ..1■§ ■ 5 #Semi-Therm Symposium, pp. 34-41, 1999.如何计算散热器的散热功率计算角估计平行板翅式散热器的热阻罗伯特 E 西蒙斯，副主编，IBM 公司正如以前在本专栏中，增加电力电子模块的趋势正在使越来越多的困难与空气冷却电子封装。