You have some work to do
- find a relation between your current i1(t) which is the output of your system and the voltage u(t) wich is the input of your system
- because you are using a switch meas that u(t) is a step signal u(t)=12 step(t)
- It's easier to do it with Laplace transform U(p)=12/p then find H(p)=I1(p)/U(p) which is your transfer function
- when 1 2 and 3 are done look this example
%H(p)=10/(2p+3)
N=10;D=[2 3]
model=tf(N,D)
Bode(model)
% is your frequency response with logarithm scale. this requiert a control system toolbox. you can also use freqresp
