The relationship between junction temperature and the SOA (Safe Operating Area) is shown below.
Tj(max) = rth(j-c) × Po + Ta
From Fig. 1, when the pulse width is 10 ms, The transient thermal resistance is read as:
rth(j-c) = 9 ℃ / W
The peak power that provides the maximum junction temperature (Tj(max)) is calculated as:
150 ℃＝ 9 ℃ / W＊Po + 25 ℃
Po = 125 / 9 = 14 W
For example, when VCE = 80 V, the allowable current when VCE = 20 V is calculated as:
14 / 20 = 0.7 A
Therefore, in the SOA graph shown in Fig. 2, the region limited by transient thermal resistance is the area bound by a blue dashed line. Bipolar transistors must be used within the region bound by this dashed line so that their junction temperature will not exceed the specified Tj(max).
The SOA region limited by transient thermal resistance is indicated as a thermal breakdown limitation.