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How could I caculator thermal junction temperature for TPS54426?

Other Parts Discussed in Thread: TPS54426

Test condition:

1. Measure the thermal couple under surface temperature, Tc is 58.1°C

2. TPS54426 the = 51.3(°C/W)

3. Pd=0.85 Watt.

4. This package have the Thermal pad. (We have to mount a PCB).

5. Not include Thermal pad condition =101.7°C

 

Question:

How could I evaluation of? (Need consider the Thermal pad condition)

Q1.How to use this formula (Tj=Pd*Rjc+Tc) to calculate?

(Need consider Thermal pad containing proportional relationship both with Tc.)

Q2.Why the  Rjc>Rja  from the TPS54426 specification?

( formula Rja=Rjc+Rca, normally the Rja > Rjc)