# 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

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.

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