This paper deals with the problem of measurement of the losses in Power GaN chips due to almost impossibility to measure the current under the penalty of adding parasitic inductances incompatible with the switching speeds of these components. For this purpose, this paper investigates the steady-state inverse heat conduction model (IHCM) developed from thermal simulations. The main objective is to derive the power dissipation in a GaN chip mounted on or embedded in a PCB from the surface temperature measured by infrared thermography or thermal sensors. In this problem, the power devices losses are the solutions of the inverse problem of the temperature-to-power mapping. IHCM here is based on simple observations from real PCB structures with GaN chip embedded in PCB substrates. The error of the resultant power map with thermal noises is minimized by Least-Square Optimization, which transforms the direct inverse problem into a constrained optimization problem.