Improved MOSFET Model Achieves Higher Accuracy

Jan 1, 2007 12:00 PM
By Scott Pearson, Modeling Engineer, Sylvie Tran, Product Modeling Engineer, and Steven Sapp, Produc

Simulation Results

With a few minor exceptions, remarkable agreement has been demonstrated when product characterization and simulated MOSFET behavior are compared. For example, Fig. 4, Fig. 5, Fig. 6, and Fig. 7 plot simulated results versus measured data for the FDB8441, a 40-V n-channel trench MOSFET. In this case, the transfer characteristics curve shows only inconsequential deviation from the data in the weak inversion and subthreshold regions when simulated at high temperature. This is due to a discrepancy in the drain-source leakage current at the elevated temperature condition for which the BSIM3 MOSFET model does not account.

The capacitance simulation also reveals a difference in CRSS compared to product data (Fig. 7). On the other hand, the gate charge simulation does accurately represent the product data. When used to simulate a dc-dc switching converter application, the model produced comparable switching characteristics to the application waveforms.

Use of SPICE Model

Using a standard MOSFET symbol makes for a quick and simple way to simulate various MOSFET SPICE models. There are symbol files on the Fairchild website for use with the OrCAD Capture and Schematic tools. The symbol for a standard three-terminal MOSFET is shown in Fig. 8 and can be accessed at www.fairchildsemi.com.

The symbol file should be downloaded and saved in the directory where modeled library files are located. From an open schematic, select the icon or menu item to place a new part. The “Place Part” window is shown in Fig. 9.

In this window, select “Add Library.” Browse for and open the symbol file that was downloaded and saved to the working directory. From this new library, select the symbol “Fairchild MOS Std” and place it into the schematic. Once the MOSFET symbol has been placed, the model name will need to be changed. Double click on “Fairchild MOSFET” on the symbol just placed and enter the model name to be simulated. The final step is to add the library to the simulation profile. Within the “Capture” window, open a simulation profile and select the “Configuration Files” tab and category “Library.” Browse to locate the library file containing the model to be simulated. Next select the “Add to Design” button to make the model ready for simulation.

If a different model is to be used at a later point in time, not all of these steps are required. From the schematic, simply change the name of the MOSFET model. Then add the library file to the simulation profile if it has not been previously added.

Alternatively, a library can be added globally to “Capture.” When adding the library file to the simulation profile, select the “Add as Global” button. This library file will then be available for all designs in “Capture.”

Model Scaling

We have demonstrated a greatly simplified, exceptionally accurate trench power MOSFET model using a single BSIM3 device at its root. The resulting model can easily be scaled to simplify model development and can enable performance variation simulation. These new models are available for download from the Fairchild Semiconductor website at http://webdc.transim.com/fairchild/index.html.

References

1. Cordonnier, C. E.; Rossel, P.; Maimouni, R.; Tranduc, H.; Allain, D.; and Napieralskadouard, M., “Spice Model for TMOS Power MOSFETs,” Motorola Semiconductor, AN1043/D, 1989. www.onsemi.com/pub/Collateral/AN1043-D.PDF.

2. Liu, William, “MOSFET Models for SPICE Simulation including BSIM3v3 and BSIM4,” John Wiley & Sons Inc., 2001. Chapter 3, pp. 103-104.

Click here for the enhanced PDF version of this article

Want to use this article? Click here for options!
© 2007 Penton Media, Inc.