Transfer function in simulink model

I have a TF, (1-exp(tauD*s)*exp(tauC/tauD))/(tauC*s+1).I want to model the TF using simulink and simulate the tf with the tauD,tauC which are time varying. but using the TF command,a=100;b=100;H1=1/(b*s+1);H2=exp(-a/b)*exp(-a*s)/(b*s+1);H=H1-H2;step(H) yeilds a different kind of response in the simulink.

Need some help here.