by ahmed zaghlol, ph.d., p.eng. applications engineering manager
r-theta inc. 6220 kestrel road mississauga, ontario l5t 1y9 tel. : (905) 795-0077 ext. 247 fax : (905) 795-2508 e-mail: [email protected] http://www.r-theta.com
heat sinks are used with semiconductor devices to provide extra area for heat dissipation. power electronic designers require quick and accurate heat sink solutions. with the advent of the internet, and realizing the potential of providing interactive design capability on the web, r-theta has introduced r-tools; a completely interactive on-line thermal design tool for heat sinks.
the r-tools mathematical engine is located on a web server at r-theta inc. r-tools simulation can be run on an internet browser, which is capable of utilizing java applets. r-tools thermal modeling is based on a set of analytical models for conduction heat transfer in the solid elements coupled with natural and forced convection heat transfer models in the cooling airflow. the conduction heat transfer model in the baseplate of the heat sink is based on the steady state solution of the laplace equation for general rectangular geometry.
the solution is based on a general three-dimensional fourier series solution, which satisfies the conduction equation in the base plate. for the forced convection air-cooled fins, an analytical model is used to predict the average heat transfer rate. the model used is a composite solution based on the limiting cases of fully developed and developing flow between parallel plates. because the r-tools is analytically based, the solution is achieved within a few seconds, a very short time compared to the several hours required for a full cfd simulation.
r-tools provides a method for quickly and accurately testing various heat sink configurations. the use of analytically based design tools allows the user to perform the thermal design of the heat sink concurrent with the optimization of the electrical and manufacturing elements prior to any prototype or testing. this approach results in reduction in design time and better reliability in the finished product.
user experiences
mr. vijay earanky, a design engineer for k.b. electronics ltd. in bedford nova scotia, canada is using r-tools extensively for the thermal design and the selection of heat sinks for power conversion equipment. mr. earanky said, "r-tools is a very useful tool which can predict and display the temperature field on the base plate accurately and quickly. hence, the user can locate the hot spots visually on the screen which will lead to an accurate prediction of the junction temperature". he adds that he prefers using r-tools to select r-theta heat sinks because it predicts the total thermal resistance of the heat sinks by calculating the actual spreading resistance in the base plate which is typically different than the assumed coverage in most of the printed catalogues.
dr. pamela dugdale who is working in the packaging r&d group at international rectifier in the u.k. said, "r-tools is quite useful when you are considering putting more than one device on a heat sink, in particular when customers require this in applications using high power devices."
mr. angelo fiorelli is working as a design engineer in spectrum control, inc. erie, pa. he is in charge of the thermal management design for emi filter and lvco (low voltage cut off) circuit products (see figure 1).
mr. fiorelli said that he accomplishes this task using r-tools software package on the r-theta web site. he said, "i was able to select numerous heat sink sizes from your catalogue that fit my enclosure size and predict the thermal profile of the fet's as they are mounted on the heat sink. the modeling software turned a difficult task into a simple one since i was limited to heat sinks that would fit the fixed enclosure dimensions required by my end customer."
mr. fiorelli performed experimental testing with an r-theta heat sink he selected for his project. he found that the predicted temperature using an r-tools thermal model of 78°c was very close to the measured value of the r-theta heat sink base plate (see figure 2).
he concluded, "i have passed this information on to our pcb vendor and other engineers at spectrum so that they can utilize this modeling tool".
references
lemczyk, culham and yovanovich, 1991 "thermal analysis of microelectronic packages" presented at the asme winter annual meeting december 1-6, 1991, atlanta, ga. teertestra, culham, yuvanovich and lemczyk, 1999.
figure 1: filter positioned approximately 6 inches above test bench. oriented for maximum airflow.
figure 2: temperature measurements for power semiconductors and r-theta heat sink.
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