Background: The article provides a detailed analytical description of the turn-off process in a high-voltage power MOSFET, with nonlinear junction capacity. With the use of a gate circuit with a low resistance and considering the internal parasitic parameters of the switch, the phenomena occurring during switching process inside the semiconductor structure are crucial to correct determine the value of the energy dissipated in the transistor. A full explanation of these phenomena is quite difficult because of the limited experimental measurements inside the transistor. Material and methods: To explain, simulation studies in Pspice for modelling switching transients was developed. To estimate switching losses an original thermovision method based on the measurement of heat dissipated in the power semiconductor switches has been used. Results: The results indicate very low values of energy Eoff dissipated in the turn-off process of the transistors under consideration, even at a drain current of approx. 20A. Measurement results shows the scale of the error of measurements of energy dissipated in the MOSFET turn-off transient when using oscilloscope measurements. Conclusions: The results provided in the paper confirm good metrological properties of the proposed by authors thermovision method employed in the study for the determination of energy losses in converters with very high energy efficiency.