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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?

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1 Answer 1

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

Good question and, the data sheet isn't very forthcoming but, they do say that this "dual device" has applications as a synchronous rectifier or as a primary switch for a bridge topology and, in both these applications, only one MOSFET will be activated at any one point in time.

For this reason alone I have to suggest that it is the joint power of both transistors that determines the junction temperature.

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  • \$\begingroup\$ Do you mean it depends on the application? What about a bidirectional switch with 2 back to back mosfets, in this case both mosfets are activated and circulating the same current? Total package power is still the way to go? \$\endgroup\$
    – Raul M.
    Commented Dec 1, 2022 at 21:34
  • \$\begingroup\$ Because the data sheet recommends the types of applications I listed in my answer, it assumes the user (you or me) will never have both MOSFETs activated hence, although they appear to be giving the junction temperature for one MOSFET, in applications where both MOSFETs are active at the same time, it is the combined power that determines the junction temperature. @RaulM. \$\endgroup\$
    – Andy aka
    Commented Dec 1, 2022 at 21:39

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