The relationship between junction temperature
and the SOA is shown below.
Tj(max) = rth(j-c) x Po + Ta
From Figure 1, when the pulse width is 10 ms,
the transient thermal resistance is read as:
rth(j-c) = 0.92°C/W
The peak power that provides
the maximum junction temperature (Tj(max))
is calculated as:
150°C＝0.92°C/W＊Po + 25°C
Po = 125 / 0.92 = 135.8 W
For example, when VCE = 80 V,
the collector current is calculated as:
135.8 / 80 = 1.7 A
Therefore, in the SOA graph shown in Figure 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.