This work aims to estimate the Metal-Oxide-Semiconductor Field-Effect Transistor’s (MOSFET) junction temperature, guessing in real time the dynamic drain-source resistance during MOSFET operation in LLC (Inductor-Inductor-Capacitor) converters usually adopted in Switched-Mode Power Supplies. The task is accomplished using an on-state voltage measurement circuit that allows clamping of the high-voltage drain-source voltage during the off-state. The results of this method have been compared with the temperature accurately measured using a co-packed die as a thermal sensor.