This work concerns a parasitic N+PNN+transistor appearing in a power MOSFET. 2D Medici simulations point out a close correlation between the static characteristic and the transient values of both the current and the reverse bias of the transistor irradiated by a flash-X ray. The static characteristic points out a steady-state where the transistor behaves as a resistor. If the dose rate is sufficient, it can bring the transistor to the same steady-state. In both cases, this steady-state is reached after the same physical process: conduction of the emitter-base junction, shift of the electric field towards the NN+junction and highly avalanching conditions in the device.