See in the picture below. On a separate note, why does some manufacturer specify both junction-to-case and junction-to-lead resistance?

enter image description here

On page 4 of this datasheet: https://www.infineon.com/dgdl/Infineon-IDW10G65C5-DS-v02_02-en.pdf?fileId=db3a304339dcf4b10139fc597c6800fc

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    \$\begingroup\$ Can you link the datasheet from which you copied this image, please. \$\endgroup\$ Jun 18, 2022 at 3:31
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    \$\begingroup\$ Why would you not specify both? Junction to case is only relevant for heatsinks and Junction to ambient is only relevant without heatsinks. \$\endgroup\$
    – DKNguyen
    Jun 18, 2022 at 4:14
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    \$\begingroup\$ It's not junction-to-lead, it's junction-to-ambient for the leaded version of the component. \$\endgroup\$
    – Neil_UK
    Jun 18, 2022 at 11:28
  • \$\begingroup\$ @Neil_UK The component only comes in one package, a 3-lead TO-247. There's no leadless version as far as I can tell. And if you're talking about leaded as in containing lead, it looks like it only comes in lead-free, so it's not that either. \$\endgroup\$
    – Hearth
    Jun 18, 2022 at 16:03
  • \$\begingroup\$ @Hearth The device can be bought in other packages, e.g. as IDK10G65C5. I don't know why they decided to have separate datasheets though. I'm pretty sure this is a remnant from a common datasheet that existed at some point. \$\endgroup\$
    – asdfex
    Jun 18, 2022 at 17:59

2 Answers 2


Rthj-c is the thermal resistance from the junction to the component body/case and it remains unchanged irrespective of the external thermal arrangement that the designer would make.

Thermal model of a TO package with a heat sink

(Image source: Infineon datasheet - Recommendations for board assembly of Infineon transistor outline type packages)

Rthj-a indicates the thermal resistance from junction to air, including all interfaces from component case to air (thermal adhesive, heatsink etc.,).

But, Rthj-a specified in the datasheet refers to the thermal resistance from junction to air, when the component is not attached to a heat sink or any other heat dissipating material, but left open in air. Hence the higher value of Rthja.

This Infineon application note explains in detail all the specifications of Infineon SiC diodes. You may refer to section 3 to understand more about the thermal specification of the device.

Disclosure: I work for Infineon.

  • \$\begingroup\$ Infineon_employee - Welcome :) (a) Please read the tour, the help center and when you have time, the Stack Exchange (SE) FAQ to see details of the site rules & etiquette, which is different from typical forums. (b) When someone is affiliated with anything mentioned in an answer (link, image, text etc.) then the affiliation must be disclosed in the text - implying it in the username is not enough. I added that for you this time. (c) When someone includes any content (text, image, photo etc.) copied or closely reworded etc. [...] \$\endgroup\$
    – SamGibson
    Jul 23, 2022 at 13:14
  • \$\begingroup\$ [...] from elsewhere, then the original source must be referenced. I found the image source and did that for you this time. || In future, please comply with the SE site rules including (i) disclosure of your affiliation (see the example I added in this answer) if you mention, include, or refer to Infineon or related companies in any way in an answer, to avoid any possible accusations of astroturfing / undisclosed promotion; and (ii) include a reference link for any material that you include in an answer, which was copied from elsewhere. Thanks and, again, welcome :) \$\endgroup\$
    – SamGibson
    Jul 23, 2022 at 13:15

I was unable to easily find out Infineon's test conditions for that datasheet parameter. Perhaps it is covered in some ancient standard. There does seem to be a lot of variation in quoted numbers for a similar overmolded TO-247-3 case (from 40°C/W to 62°C/W). Obviously there are some big differences in the measurement method and/or the leadframe itself. Generally, junction-to-ambient numbers are highly variable depending on actual conditions (especially for SMD parts) and the datasheet numbers should be verified in your application. Junction-to-case is a much more reliable number.

The "leaded" may be a vestigial option copied from datasheets that cover both SMD and leaded versions of semiconductors. For example this Infineon datasheet for a SiC MOSFET in the similar (one more lead) "PG-TO247-4-3" case.

enter image description here


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