LTspice, a simple thermal simulation.

Thermal simulation in an electric circuit simulator is really quite simple.
Thermal resistance becomes electrical resistance, and thermal capacitance becomes electrical capacitance.
Heat sources in watts become current sources in amps,
Temperature becomes voltage
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A Simulation model for thermal in LTspice.

Calculations for the thermal simulation (text).
The thermal model for a T0-220 device on a small heatsink (circuit).