A typical application for the UC2842L in an off-line flyback converter is shown in Figure 8-1. The UC2842L uses an inner current control loop that contains a small current sense resistor which senses the primary inductor current ramp.  This current sense resistor transforms the inductor current waveform to a voltage signal that is input directly into the primary side PWM comparator.  This inner loop determines the response to input voltage changes.  An outer voltage control loop involves comparing a portion of the output voltage to a reference voltage at the input of an error amplifier.  When used in an off-line isolated application, the voltage feedback of the isolated output is accomplished using a secondary-side error amplifier and adjustable voltage reference, such as the TL431. The error signal crosses the primary to secondary isolation boundary using an opto-isolator whose collector is connected to the VREF pin and the emitter is connected to FB.  The outer voltage control loop determines the response to load changes.

Figure 8-1 Typical Flyback Application Circuit