A new model for plasma behaviour in Filippov type plasma focus (PF) systems has been described and used. This model is based on the so-called slug model and Sing Lee's model for Mather type PF devices. Using the model, the discharge current and its derivative as a function of time, and the pinch time and the maximum discharge current as a function of pressure, have been predicted. At the end, the predicted data are compared with the experimental data obtained through a Filippov type PF facility with a nominal maximum energy of 90 kJ.