1.
Thermal measurements methodology is described in this document:
https://www.nxp.com/docs/en/application-note/AN1955.pdf
The temperatures during measurements was not 225'C and 25'C. Datasheet provides actual case temperature during measurement (63' or 66'C).
Die and case temperatures was measured by IR sensor and thermocouple. Power dissipation was measured by the DC power supply. The Rθjc value has been calculated from the measured values.
MRFE6VP61K25HR5 is a very high power device. It can't be used without heat sink. Thus, junction-to-environment resistance is not a useful parameter for this transistor and it is not measured and not provided in the datasheet.
2.
Power derating factor is a reverse value for Rθjc. 1/0.15['C/W] = 6.67[W/'C]
3.
The same measurements was performed as for Rθjc. However, the pulsed input signal was used. Average DC power was used to calculate thermal impedance.
4.
We have no specific reasons. Typically, RF transistors are designed for specific applications (continuous or pulsed). If a transistor is designed for wide range of pulse widths than we specify thermal impedances for different widths or provide a graph.
For example, see Figure 12:
https://www.nxp.com/docs/en/data-sheet/MMRF1006H.pdf
By the way, the thermal impedance graphs for MRFE6VP61K25HR5 are expected to be the same.
5.
These are the same transistors but supplied in different rail packages.
Have a great day,
Pavel
NXP TIC
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