# Transient thermal resistance vs. safe operating area (SOA)

Thermal resistance is calculated as a rate of change of temperature caused by a single pulse application with respect to power dissipation. Transient thermal resistance is a response of a device to a single pulse expressed in terms of the pulse duration.

The thermally limited region of the SOA is calculated based on transient thermal resistance.

Transient thermal resistance is used to calculate a rise in junction temperature caused by the application of instantaneous pulse power. It can also be used to estimate a rise in junction temperature caused by the application of continuous pulse power.

For details, see the Bipolar Transistor Application Note: Thermal Stability and Thermal Design.

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＊P+ 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