The voltage sampling feedback circuit is shown in figure 4.5, which is mainly composed of components such as optocoupler PC817 and voltage stabilizer TL431. TL431 is used as a reference and feedback error amplifier to sample the output and generate the corresponding error voltage. The error voltage is converted to an error current by optocoupler PC817, coupled to the primary, and converted to a voltage as input to the VFB port of the control chip UC3843