Wolfpack thermal resistance

KNJ

could you share how to define the thermal resistance Rth_JH for Wolfpack module? because there is multiple SiC MOSFET dies in the module, is there any thermal resistance deviation between dies on different locations of inner module?

CAB011M12FM3 :

BrettSparkman

Hello KNJ,

The thermal resistances provided in Wolfpack datasheets are evaluated based on measured data.

An Rth,jh measurement is performed on a Siemens T3Ster system for multiple modules.

Junction temperatures are measured using a thermally sensitive electrical parameter (TSEP).

A calibration phase is performed where the coldplate forces a temperature and then the TSEP is measured. This provides high accuracy of junction temperature measurements.

After calibration, power is then applied to the switch positions, and the device currents and voltages are measured using Kelvin connections where possible.

The temperature under each die position is also measured with an NTC typically 2mm from the coldplate surface.

From these measurements, the junction temperatures, heatsink temperatures, and power dissipations are known values. Thus, Rth,jh can be calculated as: Rth,jh = (Tj – Tntc) / P.

Upon reviewing the results, the highest average Rth,jh per switch position is identified to reduce uncertainty from switch positions that may have higher temperatures due to thermal crowding and module variation.

This largest averaged value is reported for the datasheet Rth,jh.

The same switch position's die that had the Rth,jh closest to the largest averaged value is then identified from the data set, and Zth curves are then then generated from this specific die's data.

So yes, some individual dies' Rth,jh may be slightly higher than reported.

However, the reported value is based upon the worst-performing switch position and is provided as a typical value such that the module is not wholly limited by the single worst-performing die.

As the design process should incorporate some thermal margin, this helps to mitigate the risk of exceeding the rated maximum junction temperature.

A coldplate with liquid cooling is utilized when measuring these systems on the T3Ster system.

The measured Rth,jh may be influenced by performance of the coldplate or heatsink.

For WolfPACK modules, the reported Rth,jh also includes the thermal impedance of the TIM layer.

The TIM is applied per the WolfPACK TIM Application Note, Document CPWR-AN42:

https://assets.wolfspeed.com/uploads/2021/11/cpwr_an42_fm3_tim_application_note.pdf

Wolfspeed is presently using Timtronics Grey Ice 4060; using a lower performance TIM may result in a reduction in thermal performance compared to datasheet values.

Please let me know if you have any further questions!

Thanks,

Brett

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.

KNJ

thank you very much!