Difference of thermal resistance between SOT-223 and TO-252

[YuTa_4942221]

I saw the data that the Tj temperature rise of SOT-223 and TO-252 (DPAK) does not change significantly depending on the conditions.
However, when comparing the thermal resistance of each of the same Rds(on) devices, TO-252 is 4.0 [℃ / W] and TO-252 is 18.2 [℃ / W], which is very high.
Please tell me the reason why the thermal resistances are so different.

[SoU_4942226 · Accepted solution]

As shown in the attached document (P.4), the definition of the temperature measurement point differs between TO-252 (DPAK) and SOT-223.
Therefore, the unit of thermal resistance is different.
DPAK: Rthjc Junction-Case
SOT-223: Rthjs Junction-Solder Point

You should confirm the measurement position first, then measure Tj.