Tool Eases Modeling of Power MOSFETs

Advances in materials, processes and packaging have enabled designers to achieve unprecedented levels of miniaturization. These designs can realize very high power densities. However, removing heat loss from a compact design is a significant challenge and calls for careful attention to thermal behavior.

As power MOSFETs are the main heat-producing components in power converters, they become vulnerable to a variety of thermal stresses that must be analyzed using thermal simulation. MOSFET designers and users require dedicated software that is easy to use, yet includes powerful modeling and simulation capabilities. In addition, short design cycles require software to execute rapidly while maintaining high accuracy.

Available on Vishay's website, ThermaSim is a free, easy-to-use thermal simulation tool that can be used in conjunction with Vishay's online library of MOSFETs. ThermaSim provides thermal analyses results in a matter of minutes, with custom adaptations available upon customer request. A buck converter design example illustrates how to perform ThermaSim simulations, and comparison of simulated data with actual measurements verifies the accuracy of the tool's results.

Underlying Technology

ThermaSim differs from existing tools in its approach to thermal modeling. Two-dimensional RC-network-based behavioral models are often insufficient because of the difficulties in maintaining boundary-condition independence and combining different levels of modeling. Simulations employing dimensional analysis in 2.5 dimensions have the same limitations. Only 3-D models can avoid this problem.

These models can be found in tools such as ANSYS that are based on finite-element analysis (FEA). The versatility of such tools offers the possibility of solving more than one type of problem such as electronic, heat-transfer and mechanical issues; however, the result of this complexity is that only specialists can use such modeling. On the other hand, package-dedicated tools such as Flopack, Flotherm, Icepak and ISE simplify the creation of models and meshing. Depending on their application domains, the advantages of such tools are specific to the level of the package, pc board or housing, but not on all levels simultaneously.

Designers using MOSFETs must account for the 3-D thermal behavior of these components. Once they have all the data necessary for their application, it becomes relatively straightforward to create complex models and make them available for potential users. The main difficulty is linked to the confidentiality of the data necessary for the creation of models.

This difficulty is overcome with ThermaSim because it was developed specifically to simulate Vishay's MOSFETs. The underlying technology is called REBECA-3D, a tool based on the boundary element method (BEM), which reduces the long computational times — a result of the geometric scale factor linked to electronic components — of classic numerical methods. The BEM solves conduction equations, including equations for nonlinear material in steady-state applications without internal meshing; yet, the thermal performance of the system design is ensured.

In other cases, transient high-power excursions make model and boundary condition definitions critical, and internal meshing becomes necessary. Even in this situation, ThermaSim can still provide thermal analysis results in a matter of minutes. However, there are limitations on the pulse width, period and run time.

DC-DC Converter Thermal Simulation

ThermaSim can be used to analyze the thermal performance of Vishay Siliconix's Si7390DP and Si7336ADP power MOSFETs in a synchronous buck dc-to-dc converter application (Fig. 1). Before getting started, it's important to note some details about this simulation example.

Typically in a dc-dc converter, major power dissipation occurs in three components: high-side and low-side power MOSFETs (Q1 and Q2), and an output inductor (L1). The electrical data collected from lab measurements that relate to thermal analysis are:

P1 = 0.52 W (average power dissipation in Q1, a Si7390DP)

P2 = 0.52 W (average power dissipation in Q2, a Si7336ADP)

P3 = 0.80 W (average power dissipation in L1, the output inductor).

The mechanical details of the four-layer pc board are: length (X) = 54 mm, width (Y) = 40 mm and thickness (T) = 1.5 mm. The top layer of the pc board is copper, with a thickness of 0.076 mm (2 oz) and estimated area coverage of 80%. The bottom layer is also copper, with the same thickness but estimated area coverage of only 60%.

The pc board has two internal conducting layers. Both are 0.035 mm (1.5 oz) of copper with estimated area coverage of 100%. These internal conducting layers and the top and bottom layers are isolated from each other with three internal insulation layers composed of 0.46-mm FR-4 epoxy glass.

Physical layout data is needed for the thermal simulation. The coordinates of the three power-dissipating components are X = 23.18 mm and Y = 18.09 mm for Q1; X = 23.18 mm and Y = 12.38 mm for Q2; and X = 40 mm and Y = 13.65 mm for L1. The dimensions of the copper pads for each component are X = 8 mm and Y = 5.15 mm for the Q1 pad; X = 8 mm and Y = 5.25 mm for the Q2 pad; and X = 25 mm and Y = 25 mm for the L1 pad.

Additional information is needed to account for the ambient thermal environment. For this simulation, the ambient temperature will be 22°C. Also, the pc board will be oriented horizontally with respect to gravity, with the component side facing up.

Putting ThermaSim to Use

To analyze the thermal performance of the Si7390DP and Si7336ADP using ThermaSim, go to www.vishay.com/mosfets/thermasim. First-time users will need to register on Vishay's website and establish a user name and password. Once logged in, the user is taken to the “ThermaSim Thermal Modeling Application” page.

The first step is to determine the components used in the simulation. This can be accomplished under the “Components” tab (Fig. 2). In the “Vishay products” section on this page, the Si7390DP can be selected by using the “Search” box, by browsing by package family or by scrolling through the selection list. In the “Power spec.” section, select the “Steady State” radio button and enter 0.52 as the power value in the adjacent box. Next, click on “Add” inside the “Vishay products” section. The “Component Selection List” should then be updated with the Si7390DP.

Repeat this process for the Si7336ADP and the output inductor. For the output inductor, refer to this component specifically as “inductor,” because the tool does not permit spaces in the component names. Use 6, 6 and 3 for size “lx,” size “ly” and size “lz,” and use 10, 4500 and 1250 for the “conductivity,” “density” and “specific heat,” respectively. When this process has been completed, the “Component Selection List” should match Fig. 2. Once this has been confirmed, either click “Next” at the bottom of the page or select the “PCB” tab to enter data for the pc board.

The second step is to enter the pc-board values in the appropriate sections of the “PCB” tab (Fig. 3) using the previously mentioned design parameters. Enter 54 for size X and 40 for size Y, and then click “Apply.” The graphic on the right side of the web page changes accordingly. Click on the “Top Layer” tab. Select “Copper 80%” for the material and enter 0.076 for the thickness.

Next, select the “Internal layers” tab and enter the parameters for the first internal insulation layer. The material is FR-4 epoxy glass and the thickness is 0.46 mm. Click “Add” and the “Internal layers” list box will update along with the adjacent graphic. While still in the “Internal layers” tab, add the first conducting layer. The material is copper 100% and the thickness is 0.035 mm (1.5 oz). Click “Add” and the “Internal layers” list box will update along with the adjacent graphic.

Using copy and paste commands, add the additional insulation and conducting layers. This should result in identical layers of epoxy for Layers 1, 3 and 5, and identical layers of copper for Layers 2 and 4. Next, click on the “Bottom layer” tab. Select “Copper 80%” for the material and enter 0.076 for the thickness. Once this has been accomplished, either click “Next” at the bottom of the page or select the “Position” tab at the top of the browser to go to the next step.

In this third step, the position of the power MOSFETs and output inductor are determined. Highlight each component in the “Component Selection List” and enter the coordinates of each power-dissipating component in the “Component positions” section. For the Si7390DP, enter 23.18 for X and 18.09 for Y, and then click “Apply.” For the Si7336ADP, enter 23.18 for X and 12.38 for Y, and then click “Apply.” For the “inductor,” enter 40 for X and 13.65 for Y, and then click “Apply.” Repeat this process in the “Pad size” section. For the Si7390DP, enter 8 for X and 5.15 for Y. For the Si7336ADP, enter 8 for X and 5.25 for Y. For the “inductor,” enter 25 for X and 25 for Y. The “Component Selection List” and adjacent graphic will update after the coordinates have been entered. When this process has been completed, click “Next” at the bottom of the page or select the “System” tab.

In the fourth step, the system-level thermal parameters are selected. Under the “Infinite environment” section, enter 22 in the “ambient temp” field. Select the “free convection” radio button, and select “-z gravity” for gravity orientation. In the “Simulation settings” section, select the “steady state” radio button. To go to the next step, click “Next” at the bottom of the page or select the “Setup” tab at the top of the browser.

The simulation setup is completed in step five. In the “Simulation settings” section, select “high (numerical accuracy = 95%).” Under the “Output data selection” section in a data window, the pc board and each individual component are listed together with their respective assigned data parameters. To select or deselect parameters for each component, choose the component in the data window, then check or uncheck the box associated with the parameter to be selected or deselected, and click “Apply.” After all the desired changes have been made, enter the user e-mail address and save the configuration for future use. This completes the simulation setup step. Click on “Start Simulation” at the bottom of the page or select the “Run” tab at the top of the browser to go to the final step.

The sixth and final step is to run the simulation. A new window opens at the beginning of this step. Check the input values for errors in the “Simulation checking” summary list. If all values were entered correctly, click “Send run” at the bottom of the page. If there are input errors, hit cancel to make corrections on the appropriate page by selecting the corresponding tab at the top of the browser.

After clicking on “Send run,” a new window opens to confirm that the simulation has been submitted and that the results will be e-mailed. Click on “Close” to return to the design, or exit the browser to close the tool.

For assistance in using the tool, click on the “? Help” button to access an online guide. You can also click on the “? E-mail” button to send suggestions, ask a question or report a problem.

Thermal Simulation Results

The results e-mail message that the user receives contains a summary of the design input data and two files: a PDF file and a text file that cover simulation results. The first page of the PDF file is a summary of the simulation input data, followed by the thermal conductivity range applicable in the design. The “Simulation Output” starts with “Global Output Results,” followed by sections for each component. The PDF file has both data tables and color pictures with temperature spectrums. Fig. 4 shows the simulated temperature spectrum for the Si7390DP. The simulation results for the Si7336ADP are shown in the same format. The contents of the text file produced in this simulation example are shown in the “Comprehensive Simulation Results” box.

Measured Results

Lab measurements for temperature rise were completed using FLIR Systems' ThermaCam. Fig. 5 shows the temperature spectrum for the components. For the Si7390DP, the difference between the simulated and measured steady-state temperatures was 69.11°C 2 62.10°C, or 7.01°C. For the Si7336ADP, the simulated steady-state temperature was 69.36°C, while the measured result was 62.10°C, a difference of 7.26°C. For both devices, this represents a positive difference of approximately 11% between the simulated and measured temperatures.

The difference between the simulation results and the lab measurements can be attributed to two significant factors. First, the pc board has seven metal terminals and a thick ground-jumper wire (Fig. 1). All of these metals help to remove heat from the board and result in lower MOSFET temperature. Second, the simulation tool cannot account for this metal on the pc board, which serves as a small heatsink. Therefore, the simulation results are on the higher side.

Positive differences between the simulated and measured results indirectly provide a positive thermal design margin, which is desirable in most situations. Thus, it is seen that the thermal simulation results obtained by using ThermaSim are acceptable for estimating thermal performance of Vishay power MOSFETs and for deriving a safe thermal design.

Future Improvements

The ThermaSim online tool can help analyze pc-board-mounted MOSFETs easily and efficiently. But the simulation tool does have limited capabilities in the transient thermal analysis of very fast-rising repetitive power pulses. In the current version of ThermaSim, the cycle transient ratio cannot be less than 0.01 and the time step cannot be less than 0.01 sec. Improvements in these capabilities are planned for future versions where die-level simulations will be iterated for repetitive pulses. This should enable simulations for power pulses in the microsecond range.

Comprehensive Simulation Results

After inputting all the necessary data in Vishay's ThermaSim, the user receives an e-mail containing a summary of the design input data, as well as PDF and text files that review the results of the simulation in detail. Following are the results contained in the text file for the Si7390DP and Si7336ADP simulation examples outlined in the article.

Minimum system temperature = 60.87°C

Maximum system temperature = 75.06°C

T_{ot flux} PCB = 1.68 W, T_min PCB = 60.87°C, T_max PCB = 69.87°C

Number of components = 3

SI7390DP T_min = 65.47°C

SI7390DP T_max = 69.85°C

SI7390DP T_die = 69.59°C

SI7390DP T_top = 69.11°C

SI7390DP T_bot = 69.23°C

SI7390DP Phi to PCB = 0.49 W

SI7336ADP T_min = 65.54°C

SI7336ADP T_max = 69.83°C