[pwm in simulik]

Hi,
i am designing a closed loop capacitive accelrometer in Simulink, in which iam planning to use a pulse width modulator in the feedback loop. as my whole circuit in simulink is a mathematical approximation of the physical model.... iam kind of confused how to use the PWM block given in the SImulink library directly( since it is a time domain block, if iam correct!!) so i was planning to impliment the PWm block by using its Transfer function, so that the whole circuit would be in one form. but to my ignorance... i dont know how to write a transfer function for a PWM block. So can you please help me out with this.
Thank you in advance
Vishu