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Ladders of RC-cell are usually used for compact thermal representation of power modules or assemblies. A large number of cells are considered in order to balance the intrinsic poor dynamic accuracy of the representation. However the number is limited by convergence problems in simulation. The RC-cells representation is also questionable in terms of relation to physical quantities. These limitations may be overcome using the so-called diffusive representation to build systematically a low-order but accurate thermal model of any assembly, and including thermal couplings. The paper intends to demonstrate how to build practically a diffusive model and assert its validation by experiment and through confrontation to finite-element simulations. A compact diffusive model comes as a state space model and main be easily implemented in circuit simulator.