here is something, also remember that your drawn error amp in top post drives its output till its inputs are equal in voltage...the circuit (smps) is set up so that when that happens (both opamp inputs same) you are in regulation.
Remember opamp with negative feedback..you expect to see the inputs the same voltage.
- - - Updated - - -
look at the attached ltspice sim of error amp regulating linear regulator..see how it works
- - - Updated - - -
it might be linear regulator but principle is same as error amp in smps, except in smps, the error amp output determines the duty cycle