6

Rev. A, March 2004

©2004 Fairchild Semiconductor Corporation

Assume a situation where the drain-source voltage of a Power MOSFET applied to a switching

power supply exceeds its maximum rating specified in the datasheet during delay time to pro-

tection activation while conducting short circuit test. The specific conditions are as follows:

transient thermal resistance becomes

If a 5kW power loss is assumed during avalanche, the resulting junction temperature rise will

be

This is an additional junction temperature rise caused by avalanche. Therefore, the system

designer should first calculate the junction temperature of a normal operation, and then add

the above value to obtain the transient junction temperature during avalanche. This tempera-

ture should be kept below the maximum allowable junction temperature with some safety mar-

gin according to the designer’s choice.

5. Conclusions

The system designers are frequently forced to determine the applicability of a Power MOSFET

to their application. This can be done by using the avalanche mode analysis and/or junction

temperature analysis which are very practical.

References

[1] “Single Pulse Unclamped Inductive Switching: A Rating System”, Fairchild Application Note

AN-7514

[2] “A Combined Single Pulse and Repetitive UIS Rating System”, Fairchild Application Note

AN-7515

[3] Rudy Severns, 1984, “Safe Operating Area and Thermal Design for MOSPOWER

Transistors”, MOSPOWER Applications Handbook, Siliconix

()

0.01

()

10.01

–

() ×

+

×

=

()

–

+

0.00274

=

∆T

5kW

0.00274

°CW

⁄

×

13.7

°C

==