# Junction temperature calculation on dual mosfet packages

When calculating the junction temperature $$\T_J\$$ one should use the power dissipated by 1 transistor or the total power dissipated by the package.

Example:

A package with 2 independent mosfet, for example FDS89161 which is a Duan N-Channel mosfet on SOIC-8 package, the datasheet shows a $$\R_{\theta JC} = 40\$$ K/W

In general, junction temperature can be calculated with: $$\T_J= P\times R_{\theta JC} +T_C \$$

• Power dissipated by each transistor $$\P_{TRT1}=P_{TRT2}=P_{TRT} = \$$0.5$W • Power dissipated by the package $$\P_{PKG} = \$$1$ W

• Case temperature $$\T_C=90\$$ ºC

• Thermal resistance junction to case $$\R_{\theta JC} = 40\$$ K/W

In case of single transistor power:

$$\T_J= 0.5\times 40 +90 = 110\$$ ºC

This means that in the odd case of having $$\P_{TRT1}=1\$$ W and $$\P_{TRT2}=0\$$ W, then $$\T_{J1}=130\$$ ºC and $$\T_{J2}=90\$$ºC, but intuition suggest that the junction temperature should not be very different since both junctions are part of the same substrate.

In case of package power:

$$\T_J= 1\times 40 +90 = 130\$$ ºC

This means that in the odd case of having $$\P_{TRT1}=1\$$ W and $$\P_{TRT2}=0\$$ W, then $$\T_{J1}=T_{J2}=T_J=\$$130ºC, but intuition would suggest that $$\T_{J1}\$$ should be higher than $$\T_{J2}\$$

Which one is the right assumption?