Thread Subject: PWM transfer function

Hi guys,
I built a PWM converter model with Simulink/ Simpower system blockset. Now need to do some mathematical analysis on it. I've got the converter transfer function, but was wondering how to obtain the PWM transfer function. Is there any general expression for PWM transfer function that by numerically substituting the parameters I can get the TF with numerical values?
It seems Matlab has a toolbox called "System Identification toolbox" which can help to get some system transfer functions, my PWM was built with block diagrams, so how to use this toolbox to obtain the transfer function of PWM?

Thank you very much!

Yes... Vout = PWM_DC.Vin
where Vin is the input voltage, Vout is the output voltage and PWM_DC is the duty cycle for the PWM Pulse. So if the period is 1 microsecond and PWM is high for 0.1 microsecond, duty cycle is 0.1