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Junction to case thermal resistance (Rjc) is a key parameter when characterizing a package. It can be expressed as the temperature difference between the two locations over the heat dissipation.

I am confused as in where this 'case' is supposed to be. Most of these packages have a copper plating on the back, which is an effective thermal passage. The 'case temperature' there is much higher and much closer to the junction temperature. If the 'case temperature' is measured on the front side on the plastic, the measurement is likely lower.

I was considering using a case temperature measurement and estimated heat loss to back track junction temperature with the help of Rjc, but I am not sure which case temperature is more valid in this calculation. Maybe an average works better?

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  • \$\begingroup\$ If thermal paths remove heat, temperature at pin connected to pad on back should reflect case temperature. \$\endgroup\$ Commented Nov 14, 2022 at 5:16
  • \$\begingroup\$ @StainlessSteelRat I see. I believe for some leaded transistors, the gate pin/lead is connected to the plating. You're suggesting I take the measurement at the gate pin? \$\endgroup\$
    – FlakR
    Commented Nov 14, 2022 at 7:41

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The datasheet will define where the 'case' is, if there is significant heat being produced. It's usually the obvious place where you would mount it to a heatsink.

Typical places are

  • TO220 - the underside of the bolt-down lug
  • 'PowerPad' devices - the power pad
  • Lead cooled DIL devices - the point on one or more specified leads where they neck down from wide to narrow
  • Lead cooled diodes - the 1N5401 figure 1 gives thermal information from which RθJC can be derived for the leads 1/4", 3/8", and 1/2" long, to a heatsink.

Add one or more specific devices that confuse you to your question, and we will try to find where it's defined in the datasheet for you.

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  • \$\begingroup\$ TO220 is relevant to me. Thanks for your answer. "the underside of the bolt-down lug" essentially just means the lower part of the plating on the back? I am also working with TO247. Maybe the idea is similar as 220? \$\endgroup\$
    – FlakR
    Commented Nov 14, 2022 at 7:43
  • \$\begingroup\$ As the lug is highly thermally conductive, the top of the mounting lug will be essentially the same temperature as the underside \$\endgroup\$
    – Neil_UK
    Commented Nov 14, 2022 at 10:32
  • \$\begingroup\$ I am also working with this d3k outline (4 leads in line), and I am trying to find a proper location to measure case temperature \$\endgroup\$
    – FlakR
    Commented Nov 14, 2022 at 20:42
  • \$\begingroup\$ @HYQ so maybe you know what device you're using in that package, but I don't. Add the specific device part number to your question, preferrably a link to its datasheet, and I'll have a look. As the package has a mounting hole, it's obviously intended to be bolted to a heatsink, so the reference position will probably be on the mounting face. Depending on the lead frame used, so the specific part, the manfacturer might have helpfully specified a point on one of the leads, as being representative of the junction temperature. But we won't know until we read the data sheet. \$\endgroup\$
    – Neil_UK
    Commented Nov 15, 2022 at 5:10

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