How to use RthJA from datasheet ?

urbanelwiz

Considering TO-247 package for below question.

When heatsink is used, we use RthJC and Rth for TIM and Heatsink and the total would be the RthJA in this case. We can use this to estimate Tj with calculation and also measure temperature at Tc, T_TIM, or T_heatsink and estimate Tj using appropriate Rth.

When heatsink is not used, datasheet RthJA needs to be considered and the heat is spread through the pins and resin package. In this case RthJA can be used to estimate Tj with calculation.

1. However while testing, where do we measure temperature and any of these locations allow us to use datasheet RthJA? (metal case or plastic case ?)
2. For the datasheet RthJA, is this provided for a specific PCB specification ? (board thickness, Cu Oz, etc…)

Thank you!

Forum_Moderator

Thank you for your post, it has been approved and we will respond as soon as possible.

TBhatia

Hello urbanelwiz,

Thank you for your questions. Please refer to PRD-05652: Mounting Recommendations and Thermal Measurement for Wolfspeed® SiC Power Devices in Through-Hole Packages Application Note section 6 to understand more about measuring and estimating the junction temperature for pour Through-Hole devices.

Since you do not have a heatsink connected to the device, you have access to measure the Case Leadframe temperature directly and can estimate the junction temperature using:

where Tc is the case temperature you measure and Rth,JC is from the datasheet.

The heat transfer through the lead and the PCB is negligible and is not considered for the datasheet Rth,JA. Let me know if you have further questions!

TBhatia

Hi, I hope that this answered your question. I will close this discussion for now but if you have a follow up question, please "Start a New Discussion" and we would be glad to support you further.