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VISHAY SILICONIX

Power MOSFETs

Application Note 834

Estimating Junction Temperature by Top Surface Temperature in Power MOSFETs
By Satoru Sawahata

Abstract
The thermal data provided on MOSFET datasheets is usually limited to thermal impedance between junction-to-lead and junction-to-ambient. While Vishay Siliconix provides the Web-based ThermaSimTM simulation tool for more accurate thermal simulation based on customized conditions, sometimes design engineers simply do not have time to run the simulation. However, measuring the top surface temperature is relatively easy, and a simple
Si4800BDY: SO-8 Single-Die, Bonding-Wired Package

way of estimating the MOSFET junction temperature based on this top surface measurement would give designers quite a useful tool. Just such a methodology is presented by this application note. To study the relationship between junction temperature and top surface temperature, we ran ThermaSim under specific conditions. To get better agreement with the datasheet, MOSFETs should be mounted on 1-in by 1-in square FR-4 board.

Figure 1 shows the ThermaSim results of the Si4800BDY when dissipating 0.5 W. The results show a die temperature of + 80.95 C and a top temperature of + 77 C.
MIN. TEMP. + 62.15 C MAX. TEMP. + 81.01 C DIE. TEMP. + 80.95 C TOP TEMP. + 77.00 C BOTTOM TEMP. + 67.16 C FLUX TO PCB 0.47 W

APPLICATION NOTE

Figure 1 - ThermaSim Output Result

Document Number: 69993 Revision: 29-Feb-08

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