A new high power (25MW) amplifier system based on Gate Turn-off Thyristors (GTO's) has been procured to be used as a power amplifier in the control system of the vertical position of the plasma in the JET experiment. The new amplifier is characterised by a nominal duty cycle of 30s every 600s. The power dissipation in each GTO can reach peaks of almost 8.1kW for short periods. The switching frequency of each GTO and the current during the execution of a JET pulse are somewhat unpredictable. It was therefore felt necessary to provide the power devices with a thermal protection which has to be reasonably accurate in order not to unnecessarily limit the performances of the amplifier. At the same time it should be reasonably simple and cost effective. An on-line simulation/calculation of the junction temperature was therefore studied and adopted: the conduction losses, the turn-on and turn-off losses are taken into account; the direction of the output current together with the knowledge of which GTO's are in the ON status determine if the current is flowing in the GTO's or in the freewheeling diodes. A detailed description of the model used and of the hardware realisation of the simulation is given in the paper.