9034 Datasheet, PDF (3/7 Pages) Fairchild Semiconductor – Power MOSFET Avalanche Guideline

4. Junction Temperature Analysis
Generally, breakdown of the Power MOSFET seldom occurs even if the drain-source voltage
exceeds the absolute maximum rating. The BV_DSS of the Power MOSFET has a positive tem-
perature coefficient as shown in Figure 4. It reaches about 990V at 120°C in this example.
Therefore, a greater voltage is required to cause device breakdown at higher temperature. In
many cases, the ambient temperature during the Power MOSFET operation is over 25°C and
the power loss causes the junction temperature of the Power MOSFET to rise above the ambi-
ent temperature.
1.2
1.1
1.0
0.9
0.8
-100
※ Notes :
1. V_GS = 0 V
2. I_D = 250 µ A
-50
0
50
100
150
200
T_J, Junction Temperature [^oC]
Figure 4. Normalized BV_DSS vs Tj, FQA11N90C
Also, note that the BV_DSS in Figure 4 is measured at 250µA of the drain current. In a real
breakdown, the drain current reaches a much higher level and the breakdown voltage is even
higher than the above value.
Figure 5. Waveforms from Switching Power Supply,
600V rated MOSFET
©2004 Fairchild Semiconductor Corporation
3
Rev. A, March 2004