How do I use linearitzation for the graph or system for nonlinear systems?

Iam modelling a Welding power supply. In the attachemnts, I have shown the equvivalent ciruit. The plot in the attachments shows the relation between I_o(Yaxis) and D is Duty Cycle(Xaxis). I need draw bode plot to the transfer fuction I_o (output) and Duty cyle as input. But the transfer function i obtained is I_o= (V_s*D)/([1+(R_s/R_L *D^2 )]*R_L ). Where D is non linear.

Iam using PLECS tool box in Matlab 2012a