Modeling of the cascode switching structure used in Flyback converter for achieving fast startup in the deeply dimmed phase-cut LED driver is presented in this draft. The cascode structure’s inherent instability and oscillation issue is modeled and analyzed quantitatively. Three solutions are proposed to stabilize the structure and suppress the unstable voltage oscillation. Solutions are studied using the proposed model for design robustness. And this model can be further applied to the popular new high voltage (for example 650V) cascode GaN FET technology. Experimental results of a 20W phase-cut dimmable LED driver are demonstrated to verify the proposed modeling method and solutions.