Somi,

  If you have access, you might want to try looking at Chapter 3 of
"Switch-Mode Power Supplies" by Christophe Basso. Chapter 3 provides
detailed discussion of the controller transfer function. The topic is
also covered in "SMPS simulation with SPICE3" by Steve Sandler
as I remember.

                                                               Best Regards,

                                                                  Sheldon

More than that, since the opamp has no dc feedback, the ref. voltage will be amplified by a factor of 1E4---1E5 (dc gain). Even if Vref=0 there will be still an offset problem due to lack of resistive feedback - unless, the circuit is part of an overall stabilizing feedback loop.

This is an error amplifier used in controllers.  Here is how to derive the transfer function:

First, bring R2, C1 and C2 under one component, Z2.  R2 is in series with C2 and both in parallel with C1.

Z2 = ((R2+(sC2)^-1)^-1+sC1)^-1

The amplifier becomes a differential amplifier in AC, with the components in the non-inverting input removed.  The transfer function is:

V_COMP=REFERENCE*(1+Z2/R1)-V_OUT*Z2/R1

The differential amplifier transfer function is explained here:
http://masteringelectronicsdesign.com/the-differential-amplifier-transfer-functi....