A compact/SPICE model for normally-off 650 V GaN power switches is presented. The industry standard physics-based ASM-HEMT model is used as the base for modeling the GaN HEMT in the cascode configured power switch. Accurate models for current-voltage (I-V) and capacitance-voltage (C-V) characteristics have been obtained. It is found that the bias-dependent two-dimensional electron gas under the field-plate regions are very important for modelling the complex capacitances of the cascode switch. Field-plates have been modeled with the ASM-HEMT formulations, and accurate models for input, output, and reverse transfer capacitance of the switch is obtained. The compact model for the full switch is created by synthesizing the MOSFET and HEMT models and optimizing model parameters.